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Achnanthidium enigmaticum E.Morales & Manoylov sp. nov. Figs 43 –59 (LM), 60–65 (SEM) Description:— Frustules rectangular in girdle view (not shown here). Valves linear to elliptic with rostrate apices (Figs 43 –59). Length: 17.0–54.4 µm, width: 4.1–5.2 µm, stria density 23–26 (up to 29 toward apices) in 10 µm in RV, and 23–26 (up to 34 toward apices) in 10 µm in SV. Hymenes, girdle bands and plastids not determined. RV with lanceolate, raised axial area (more in internal Fig. 61 than in external view Fig. 62), broad at valve ends (Figs 60–63), becoming enlarged in the central area where it assumes a circular or rhomboid shape (Figs 43 –53). Central area with slightly raised central nodule in external view (Figs 60, 61), clearly raised in internal view (Figs 62 and 63), and with shortened striae on both sides (Figs 60–63). Virgae wider than striae becoming slender toward the valve apices, lower with respect to axial area and at same level as striae in external and internal views (Fig. 61). Raphe filiform and straight, externally with proximal ends expanded and opposite to each other (Fig. 60, white arrows), and distal fissures expanded, close-hooked in shape and strongly deflected toward same direction, surfacing gradually at valve face/mantle junction, within the striation region (Fig. 61, white arrows). Internally, proximal raphe ends curved and deflected in opposite directions (Fig. 63, white arrow), distal raphe ends terminate in low helictoglossae located at a distance from valve apices (Fig. 63, black arrow). Striae varying from parallel at valve central area, only sometimes becoming slightly convex toward the apices, short striae on both sides of central area present (Figs 60–63). Areolae visible under LM, round, elliptic or slit-like (Figs 43 –53). A single row of elliptic areolae present on valve mantle, more slit-like in shape and interrupted by clear area at valve apex (Figs 60, 63). A clear area runs around the edge of the face/mantle junction (Figs 60–61). SV with sunken lanceolate axial area (only observed in external view), broad at valve ends (seen as an expansion as the distal end of striation is reached) (Figs 64, 65). Virgae wider than striae, becoming slender toward valve apices, but in general wider than those in RV at the same region (compare Figs 63 and 65). Striae at same level as virgae. Central area with evenly spaced striae, rarely shortened (Figs 64, 65). Type locality:— U.S.A. Texas, Uvalde County, Sabinal River, 29.491 N, 99.493 W, NAWQA August 7 th, 1996 (holotype slide ANSP GC 102382 a = Fig. 48) Etymology:— The species epithet refers to the fact that this diatom has remained undiscovered despite the rather long history of studies of algae, particularly diatoms, in Texas. Ecology for both taxa:— While A. lucectorii sp. nov. was found in 3.1% relative abundance in a count of 600 valves, A. enigmaticum sp. nov. was only found in a relative abundance of Encyonema neogracile Krammer (1997a: 177, with a relative abundance of 26.8%), Delicata delicatula (Kützing) Krammer (2003: 113, 20.5%), Encyonopsis microcephala (Grunow) Krammer (1997b: 91, 11.8%), Ulnaria ulna (Nitzsch) Compère (2001: 100, 7.8%), Achnanthidium caledonicum (Lange-Bertalot) Lange-Bertalot (1999: 277, 5.3%), Navicula cryptotenella Lange-Bertalot (1985: 62, 3.0%) and other species in relative abundances A. caledonicum and N. cryptotenella, the type locality could have a calcareous, alkaline influence (Van Dam et al. 1994, Hofman et al. 2011). K. Manoylov recently found, in periphyton collections from May 2022, A. lucectorii sp. nov. in Roy Creek, Austin, TX (relative abundance A. enigmaticum sp. nov. in Hamilton Creek and Deadmans Creek, close to Austin, TX (relative abundance, Published as part of Morales, Eduardo A. & Manoylov, Kalina M., 2023, Achnanthidium lucectorii sp. nov. and A. enigmaticum sp. nov., novel diatoms (Bacillariophyta, Achnanthidiaceae) from Texas, U. S. A., pp. 1-20 in Phytotaxa 592 (1) on pages 6-8, DOI: 10.11646/phytotaxa.592.1.1, http://zenodo.org/record/7835613, {"references":["Krammer, K. (1997 a) Die cymbelloiden Diatomeen. Eine Monographie der weltweit bekannten Taxa. Teil 1. Allgemeines und Encyonema Part. Bibliotheca Diatomologica 36: 1 - 382.","Krammer, K. (2003) Cymbopleura, Delicata, Navicymbula, Gomphocymbellopsis, Afrocymbella. Diatoms of Europe, Diatoms of the European Inland waters and comparable habitats 4: 1 - 529.","Krammer, K. (1997 b) Die cymbelloiden Diatomeen. Eine Monographie der weltweit bekannten Taxa. Teil 2. Encyonema Part., Encyonopsis und Cymbellopsis. Bibliotheca Diatomologica 37: 1 - 469.","Compere, P. (2001) Ulnaria (Kutzing) Compere, a new genus name for Fragilaria subgen. Alterasynedra Lange-Bertalot with comments on the typification of Synedra Ehrenberg. In: Jahn, R., Kociolek, J. P., Witkowski, A. & Compere, P. (Eds.) Lange-Bertalot Festschrift. Studies on diatoms dedicated to Prof. Dr. Dr. h. c. Horst Lange-Bertalot on the occasion of his 65 th birthday, A. R. G. Gantner Verlag K. G., Ruggell, pp. 97 - 101.","Lange-Bertalot, H. (1999) Neue Kombinationen von Taxa aus Achnanthes Bory (sensu lato). Iconographia Diatomologica 6: 270 - 283.","Van Dam, H., Mertens, A. & Sinkeldam, J. (1994) A coded checklist and ecological indicator values of freshwater diatoms from the Netherlands. Netherlands Journal of Aquatic Ecology 28: 117 - 133. https: // doi. org / 10.1007 / BF 02334251"]}

Achnanthidium enigmaticum E.Morales & Manoylov sp. nov. Figs 43 –59 (LM), 60–65 (SEM) Description:— Frustules rectangular in girdle view (not shown here). Valves linear to elliptic with rostrate apices (Figs 43 –59). Length: 17.0–54.4 µm, width: 4.1–5.2 µm, stria density 23–26 (up to 29 toward apices) in 10 µm in RV, and 23–26 (up to 34 toward apices) in 10 µm in SV. Hymenes, girdle bands and plastids not determined. RV with lanceolate, raised axial area (more in internal Fig. 61 than in external view Fig. 62), broad at valve ends (Figs 60–63), becoming enlarged in the central area where it assumes a circular or rhomboid shape (Figs 43 –53). Central area with slightly raised central nodule in external view (Figs 60, 61), clearly raised in internal view (Figs 62 and 63), and with shortened striae on both sides (Figs 60–63). Virgae wider than striae becoming slender toward the valve apices, lower with respect to axial area and at same level as striae in external and internal views (Fig. 61). Raphe filiform and straight, externally with proximal ends expanded and opposite to each other (Fig. 60, white arrows), and distal fissures expanded, close-hooked in shape and strongly deflected toward same direction, surfacing gradually at valve face/mantle junction, within the striation region (Fig. 61, white arrows). Internally, proximal raphe ends curved and deflected in opposite directions (Fig. 63, white arrow), distal raphe ends terminate in low helictoglossae located at a distance from valve apices (Fig. 63, black arrow). Striae varying from parallel at valve central area, only sometimes becoming slightly convex toward the apices, short striae on both sides of central area present (Figs 60–63). Areolae visible under LM, round, elliptic or slit-like (Figs 43 –53). A single row of elliptic areolae present on valve mantle, more slit-like in shape and interrupted by clear area at valve apex (Figs 60, 63). A clear area runs around the edge of the face/mantle junction (Figs 60–61). SV with sunken lanceolate axial area (only observed in external view), broad at valve ends (seen as an expansion as the distal end of striation is reached) (Figs 64, 65). Virgae wider than striae, becoming slender toward valve apices, but in general wider than those in RV at the same region (compare Figs 63 and 65). Striae at same level as virgae. Central area with evenly spaced striae, rarely shortened (Figs 64, 65). Type locality:— U.S.A. Texas, Uvalde County, Sabinal River, 29.491 N, 99.493 W, NAWQA August 7 th, 1996 (holotype slide ANSP GC 102382 a = Fig. 48) Etymology:— The species epithet refers to the fact that this diatom has remained undiscovered despite the rather long history of studies of algae, particularly diatoms, in Texas. Ecology for both taxa:— While A. lucectorii sp. nov. was found in 3.1% relative abundance in a count of 600 valves, A. enigmaticum sp. nov. was only found in a relative abundance of Encyonema neogracile Krammer (1997a: 177, with a relative abundance of 26.8%), Delicata delicatula (Kützing) Krammer (2003: 113, 20.5%), Encyonopsis microcephala (Grunow) Krammer (1997b: 91, 11.8%), Ulnaria ulna (Nitzsch) Compère (2001: 100, 7.8%), Achnanthidium caledonicum (Lange-Bertalot) Lange-Bertalot (1999: 277, 5.3%), Navicula cryptotenella Lange-Bertalot (1985: 62, 3.0%) and other species in relative abundances Though the new taxa were in low abundances as to consider the Sabinal River their optimal habitat, the available physico-chemical for July 31, 1996 were: river discharge 3.7 L̔ s-1, water temperature 31 °C, electric conductance 476 µS̔ cm-1, dissolved oxygen 6.9 mg ̔L- 1, pH 7.8, total nitrogen 0.31 mg ̔L- 1, and orthophosphates Based on the autecology of the assemblage found in type material, the habitat of the new species can be characterized as an oligotrophic site with water of good quality, with low nutrients, low electric conductance and circumneutral pH, which corresponds to the actual parameters measured on site. Based on the presence of A. caledonicum and N. cryptotenella, the type locality could have a calcareous, alkaline influence (Van Dam et al. 1994, Hofman et al. 2011). K. Manoylov recently found, in periphyton collections from May 2022, A. lucectorii sp. nov. in Roy Creek, Austin, TX (relative abundance A. enigmaticum sp. nov. in Hamilton Creek and Deadmans Creek, close to Austin, TX (relative abundance

Two new species, Achnanthidium lucectorii sp. nov., belonging to the A. minutissimum complex (species with straight terminal raphe ends), and A. enigmaticum sp. nov., within the A. pyrenaicum complex (species with hooked terminal raphe fissures), are described from Texas, U.S.A. utilizing light and scanning electron microscopy. Achnanthidium lucectorii sp. nov. has a raphe valve with striae located toward the apices composed of polygonal areolae, a marked stauros, a non-expanded raphe fissure, with straight (non-dilated) proximal ends, and distal ends extending onto the valve mantle. Internally, its raphe valve is flat, the proximal ends of the raphe end in small capitate expansions, and the distal ends terminate in low helictoglossae that fuse with the apices by a low apical siliceous thickening. In turn, its sternum valve is unique in that it is mostly flat in inner view, except at the central nodule, which is slightly raised. In turn, A. enigmaticum sp. nov. externally has a raphe valve with raphe distal ends shaped like a closed hook, at these ends the raphe slit surfaces at the valve face/mantle junction; its axial area is raised in external and internal views, with the virgae and striae in a lower position. These and other traditionally considered features are compared in detail with taxa morphologically similar to each of the new species. The significance of these features is discussed based on a literature survey, and the implications of considering the valve surface topography and the external and internal details of the raphe and central area are assessed. The latter, in favor of a continued morphological exploration of Achnanthidium until molecular studies are appropriately integrated into current morphological approaches used in the taxonomy of the genus.

Achnanthidium lucectorii E.Morales & Manoylov sp. nov. Figs 1–36 (LM), 37–42 (SEM) Description:— Frustules apically sigmoidally arched (in an open V-shaped girdle view), producing a raphe valve with a concave valve center (Figs 22, 23). Valves elliptic with swollen central area and subcapitate apices (Figs 1–21, 24–36), less noticeable in smaller valves. Length: 10.2–35.7 µm, width: 3.1–4.3 µm, stria density 26–28 (up to 30 toward apices) in 10 µm in RV, and 26–27 (up to 32 toward apices) in 10 µm in SV, 1–4 areolae per stria on both valves. Hymenes, girdle bands and plastids not determined. RV with narrow, lanceolate, raised axial area (externally [Figs 37–39] and internally [Fig. 40]), tapering into a pointy end at valve apex, becoming enlarged in central area where it connects with the central stauros (Figs 37–40). Central stauros with bow-tie shaped fascia (rarely apically rhomboid in some larger valves) (Figs 1–21), with raised central nodule in external view (Figs 37–39, 41), flat in internal view (Fig. 40). Virgae wider than striae except at valve ends where they become slender and rod-shaped, straight or zig-zagged due to polygonal shape of areolae (Figs 38, 39). Virgae lower with respect to axial area and at same level as striae in internal and external views (Figs 37–40). Raphe uniformly filiform and straight, with proximal ends opposite to each other (Figs 37–39). Distal fissures straight and extending over striation regions at both poles (Fig. 39, white arrows). Internally, proximal raphe ends with a small capitate terminus (Fig. 40, white arrows) and distal ends terminating in low helictoglossae fused with a low thickening of valve apex (Fig. 40, black arrow). Striae varying from convex toward valve central area (striae frequently lacking or being shorter on central stauros) becoming extremely convex and noticeably denser toward apices (Figs 37–39, 41). Areolae round or elliptic toward the central sternum, becoming transapically elongated toward valve face/mantle transition, and densely packed and polygonal at valve extremes (Fig. 38, open arrows), usually with two large areolae on both sides of distal raphe end (Figs 37, 39). A single row of slit-like areolae present on valve mantle, stopping short of the very apex (unclearly seen in Fig. 40). A clear area runs around edge of the face/mantle junction (Figs 37–39). SV (Figs 41, 42) with wider, flat (only observed in internal view) lanceolate axial area, more or less broad at valve ends. Central nodule is only slightly raised internally. Virgae of same characteristics as in RV, but at same level as striae and axial area (only internal views available). Striae of same general characteristics as those of RV, except they are evenly spaced (some are short) at valve central area, they become less convex toward the valve apices, and single row of elongated striae on the valve mantle are continuous around valve apex. The clear area around edge of the face/mantle junction is less noticeable. Type:— U.S.A. Texas, Uvalde County, Sabinal River, 29.491 N, 99.493 W, NAWQA, August 7 th, 1996 (holotype slide ANSP GC 102382 a = Fig. 10) Etymology:— The species epithet honors our late colleague Luc Ector. Luc promoted a major advancement in the taxonomy of the genus Achnanthidium through a series of courses and workshops that were the foundation of the taxonomy used in the European Water Framework Directive. The special issue in Algological Studies (2011: 136/137), edited by Luc and the result of one of those workshops, presented critical analyses of type material and new species in Achnanthidium from around the world, a foundation for current studies in this genus.

Achnanthidium lucectorii E.Morales & Manoylov sp. nov. Figs 1–36 (LM), 37–42 (SEM) Description:— Frustules apically sigmoidally arched (in an open V-shaped girdle view), producing a raphe valve with a concave valve center (Figs 22, 23). Valves elliptic with swollen central area and subcapitate apices (Figs 1–21, 24–36), less noticeable in smaller valves. Length: 10.2–35.7 µm, width: 3.1–4.3 µm, stria density 26–28 (up to 30 toward apices) in 10 µm in RV, and 26–27 (up to 32 toward apices) in 10 µm in SV, 1–4 areolae per stria on both valves. Hymenes, girdle bands and plastids not determined. RV with narrow, lanceolate, raised axial area (externally [Figs 37–39] and internally [Fig. 40]), tapering into a pointy end at valve apex, becoming enlarged in central area where it connects with the central stauros (Figs 37–40). Central stauros with bow-tie shaped fascia (rarely apically rhomboid in some larger valves) (Figs 1–21), with raised central nodule in external view (Figs 37–39, 41), flat in internal view (Fig. 40). Virgae wider than striae except at valve ends where they become slender and rod-shaped, straight or zig-zagged due to polygonal shape of areolae (Figs 38, 39). Virgae lower with respect to axial area and at same level as striae in internal and external views (Figs 37–40). Raphe uniformly filiform and straight, with proximal ends opposite to each other (Figs 37–39). Distal fissures straight and extending over striation regions at both poles (Fig. 39, white arrows). Internally, proximal raphe ends with a small capitate terminus (Fig. 40, white arrows) and distal ends terminating in low helictoglossae fused with a low thickening of valve apex (Fig. 40, black arrow). Striae varying from convex toward valve central area (striae frequently lacking or being shorter on central stauros) becoming extremely convex and noticeably denser toward apices (Figs 37–39, 41). Areolae round or elliptic toward the central sternum, becoming transapically elongated toward valve face/mantle transition, and densely packed and polygonal at valve extremes (Fig. 38, open arrows), usually with two large areolae on both sides of distal raphe end (Figs 37, 39). A single row of slit-like areolae present on valve mantle, stopping short of the very apex (unclearly seen in Fig. 40). A clear area runs around edge of the face/mantle junction (Figs 37–39). SV (Figs 41, 42) with wider, flat (only observed in internal view) lanceolate axial area, more or less broad at valve ends. Central nodule is only slightly raised internally. Virgae of same characteristics as in RV, but at same level as striae and axial area (only internal views available). Striae of same general characteristics as those of RV, except they are evenly spaced (some are short) at valve central area, they become less convex toward the valve apices, and single row of elongated striae on the valve mantle are continuous around valve apex. The clear area around edge of the face/mantle junction is less noticeable. Type:— U.S.A. Texas, Uvalde County, Sabinal River, 29.491 N, 99.493 W, NAWQA, August 7 th, 1996 (holotype slide ANSP GC 102382 a = Fig. 10) Etymology:— The species epithet honors our late colleague Luc Ector. Luc promoted a major advancement in the taxonomy of the genus Achnanthidium through a series of courses and workshops that were the foundation of the taxonomy used in the European Water Framework Directive. The special issue in Algological Studies (2011: 136/137), edited by Luc and the result of one of those workshops, presented critical analyses of type material and new species in Achnanthidium from around the world, a foundation for current studies in this genus., Published as part of Morales, Eduardo A. & Manoylov, Kalina M., 2023, Achnanthidium lucectorii sp. nov. and A. enigmaticum sp. nov., novel diatoms (Bacillariophyta, Achnanthidiaceae) from Texas, U. S. A., pp. 1-20 in Phytotaxa 592 (1) on pages 3-6, DOI: 10.11646/phytotaxa.592.1.1, http://zenodo.org/record/7835613

A new monoraphid diatom species Achnanthidium gladiussp. nov. is described from Indonesia. The description is based on molecular data (18SV4), morphological analysis and comparison with similar species. According to molecular data, Achnanthidium gladius sp. nov. is closely related to Achnanthidium minutissimum. Morphologically, the new species differs from similar species by the absence of a fascia on raphe valve, cell size, and striae density and pattern. The new species is only known from the type locality in Indonesia. Comparison with close related species is given.

Achnanthidium peruvianum E.Morales & Ector in Morales et al. (2011: 121) (Figs 166–193) Synonym: Achnanthes kryophiloides M.H.Hohn in Patrick et al. (1966), p. 468, pl. 1, Figs 5, 6 Morphometric features:— 8.0–20.2 µm long; 2.3–4.3 µm wide; 28–34 striae in 10 µm; striae mainly composed of 4–5 areolae. Taxonomical remarks:— The species presents similar morphological features to another South American species called Achnanthidium sehuencoensis, which has almost the same valve outline. However, SEM analysis separated the two due to the deflected terminal raphe fissures, the non-expanded central raphe fissures, and lower stria density (20–24/10 µm, Morales et al. 2009) compared to A. peruvianum. Achnanthidium petersenii (Hustedt) C.E.Wetzel, Ector & Jüttner (in Jüttner et al. 2019: 66) has wider fascia on the raphe valve. However, on the rapheless valves, the axial area is moderately broad, and the central area broadly rhombic-lanceolate (Jüttner et al. 2019). In A. peruvianum, the axial area is narrow and linear, while the central area is undifferentiated with short striae (Morales et al. 2011). Distribution and ecological information:— Achnanthidium peruvianum was described from the Rondos River in Peru, South America (Hohn in Patrick et al. 1966). In our samples, the species was rare (abundance, Published as part of Costa, Lívia F., Wetzel, Carlos E., Maquardt, Gisele C., Zanon, Jaques E., Ector, Luc & Bicudo, Denise C., 2022, Taxonomy and ecology of Achnanthidium (Bacillariophyta, Achnanthidiaceae) from southeastern Brazil with the description of six new species, pp. 187-223 in Phytotaxa 575 (3) on page 207, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Morales, E. A., Ector, L., Fernandez, E., Novais, M. H., Hlubikova, D., Hamilton, P. B., Blanco, S., Vis, M. L. & Kociolek, J. P. (2011) The genus Achnanthidium Kutz. (Achnanthales, Bacillariophyceae) in Bolivian streams: a report of taxa found in recent investigations. Algological Studies 136 - 137: 89 - 130. https: // doi. org / 10.1127 / 1864 - 1318 / 2011 / 0136 - 0089","Morales, E. A., Fernandez, E. & Kociolek, J. P. (2009) Epilithic diatoms (Bacillariophyta) from cloud forest and alpine streams in Bolivia, South America 3: diatoms from Sehuencas, Carrasco National Park, Department of Cochabamba. Acta Botanica Croatica 68 (2): 263 - 283.","Juttner, I., Wetzel, C. E., Williams, D. M. & Ector, L. (2019) Investigations of the type materials of Achnanthes parallela J. R. Carter and Achnanthes petersenii Hustedt (Bacillariophyceae) with comments on the genus Rossithidium Round & Bukhtiyarova. Botany Letters 167 (1): 57 - 69. https: // doi. org / 10.1080 / 23818107.2019.1668297"]}

Achnanthidium peetersianum C.E.Wetzel, Jüttner & Ector in Wetzel et al. (2019: 345) (Figs 144–165) Morphometric features:— 7.6–12.3 µm long; 2.0–2.7 µm wide; 29–34 striae in 10 µm; striae mainly composed of 3–5 areolae. Taxonomical remarks:— Our population is in agreement with the type species, besides it has slightly narrower valves when compared to the French population (2.8–3.3 μm, Wetzel et al. 2019). This species has narrow valves with subcapitate to small capitate apices, similar to A. macrocephalum (Hustedt) Round & Bukhtiyarova (1996: 349), although the latter taxon presents broadly capitate apices. Despite being difficult to separate, a reevaluation of A. macrocephalum type material was recently published in the same study as A. peetersianum (Wetzel et al. 2019). Using SEM micrographs, the authors showed that A. peetersianum presented predominantly rounded areolae and had a higher density per stria, while A. macrocephalum striae are composed of 1–2 elongated areolae of different lengths. Moreover, the axial area of the rapheless valve of the latter taxon is expanded, widening towards the central area. Recently, Yu et al. (2019) described two small species that resemble ours: A. subtilissimum P.Yu, Q. -M.You & Q.-X.Wang (in Yu et al. 2019: 161) and A. limosua P.Yu, Q. -M.You & Q.-X.Wang (in Yu et al. 2019: 158). The first species is likely a synonym of A. macrocephalum, and the second presents visibly larger valves (3.2–4.0 µm) than A. peetersianum. Achnanthidium indistinctum also bears several similarities with A. peetersianum. They can be distinguished in girdle view by the apparently thickened striae in the first taxon, which is not visible in our specimens (see Figs 158, 159). Concerning the valve view, A. indistinctum presents rostrate apices and an almost nonexistent central area that never forms a fascia. On the other hand, A. peetersianum exhibits subcapitate to capitate apices and a small rounded central area that rarely forms a fascia. Notably, SEM analysis can also help separate both taxa due to the distal raphe ends straight in A. indistinctum and bent in our specimens. Despite the slightly narrower valves in the latter taxon (1.8–2.2 µm, Van de Vijver & Kopalová 2014), the metric features of these two species overlap. Achnanthidium reimeri (Camburn) Ponader & Potapova (2007: 235) has a similar valve outline with our population, however, it presents longer and wider valves and a rhomboid central area on the rapheless valve (Ponader & Potapova 2007). Distribution and ecological information:— Recently described from Le Sauvigny River in France, it was one of the most well-distributed species found in this study (40 % of occurrence). It is plausible that this taxon was previously identified in Brazilian publications as A. macrocephalum. In this study, A. peetersianum occurred in all of the sampled habitats. Its ecological preference was for slightly acidic (pH optimum of 6.6–6.7), electrolytic-poor (optimum of 28.2–49 μS∙ cm-1) and oligo- to mesotrophic waters (TP optimum of 14.9–27.1 μg∙L- 1, TN optimum of 374.4–519.4 μg∙L- 1). We also observed this species in enriched waters (TP> 50 μg∙L- 1)., Published as part of Costa, Lívia F., Wetzel, Carlos E., Maquardt, Gisele C., Zanon, Jaques E., Ector, Luc & Bicudo, Denise C., 2022, Taxonomy and ecology of Achnanthidium (Bacillariophyta, Achnanthidiaceae) from southeastern Brazil with the description of six new species, pp. 187-223 in Phytotaxa 575 (3) on pages 204-205, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Wetzel, C. E., Juttner, I., Gurung, S. & Ector, L. (2019) Analysis of the type material of Achnanthes minutissima var. macrocephala (Bacillariophyta) and description of two new small capitate Achnanthidium species from Europe and the Himalaya. Plant Ecology and Evolution 152 (2): 340 - 350. https: // doi. org / 10.5091 / plecevo. 2019.1628","Bukhtiyarova, L. & Round, F. E. (1996) Revision of the genus Achnanthes sensu lato. Psammothidium, a new genus based on A. marginulatum. Diatom Research 11: 1 - 30. https: // doi. org / 10.1080 / 0269249 X. 1996.9705361","Yu, P., You, Q. - M., Pang, W. - T., Cao, Y. & Wang, Q. - X. (2019) Five new Achnanthidiaceae species (Bacillariophyta) from Jiuzhai Valley, Sichuan Province, Southwestern China. Phytotaxa 405 (3): 147 - 170. https: // doi. org / 10.11646 / phytotaxa. 405.3.5","Van de Vijver, B. & Kopalova, K. (2014) Four Achnanthidium species (Bacillariophyta) formerly identified as Achnanthidium minutissimum from the Antarctic Region. European Journal of Taxonomy 79: 1 - 19. https: // doi. org / 10.5852 / ejt. 2014.79","Ponader, K. C. & Potapova, M. G. (2007) Diatoms from the genus Achnanthidium in flowing waters of the Appalachian Mountains (North America): Ecology, distribution and taxonomic notes. Limnologica 37 (3): 227 - 241. https: // doi. org / 10.1016 / j. limno. 2007.01.004"]}

Achnanthidium lusitanicum Novais & M.Morais in Novais et al. (2015: 129) (Figs 45–99) Morphometric features:— 6.5–16.5 µm long; 2.6–3.4 µm wide; 30–34 striae in 10 µm; striae mainly composed of 4–6 areolae. Taxonomical remarks:— Achnanthidium lusitanicum shares morphological similarities with A. eutrophilum (Lange-Bertalot) Lange-Bertalot (1999: 271) but our species has slender valves narrower than the latter taxon, which has rhombic-lanceolate to rhombic-elliptical frustules in small specimens and valve apices bluntly rounded to cuneate. Also, A. lusitanicum have a higher stria density (A. eutrophilum: 3.2–5.8 µm wide, striae 25–30/10 µm, Hlúbiková et al. 2011). Achnanthidium caravelense Novais & Ector (in Novais et al. 2011: 142) is similar to A. lusitanicum when analyzed by LM and SEM. However, it differs from A. lusitanicum mainly by the elliptic raphe valve with almost linear margins. The SEM images show that A. caravelense presents a lower number of areolae per striae (3–4, Novais et al. 2011). The valve outline of Achnanthidium palmeti Gassiole, Le Cohu & M.Coste (2013: 22) is very similar to A. lusitanicum, and the cell metrics overlap. However, some A. palmeti specimens have slightly wider valves (2.7–4.6 μm, Gassiole et al. 2013). The main character differing both species is the frustules that are arched with slightly recurved apices in A. palmeti and its terminal raphe fissures of the raphid valve are deflected towards the same side. Indeed, our species has straight raphe fissures and an almost flat valve face. In São Paulo state, Achnanthidium saprophilum (H.Kobayasi & Mayama) Round & Bukhtiyarova (1996: 349) shows valves with no inflation in the central portion as A. lusitanicum, and it also presents lower stria density (26–30 in 10 μm). Besides that, A. saprophilum showed presence of areolae with different shapes, a feature not observed in A. lusitanicum. In addition, a similar Brazilian population was cited as A. jackii by Bartozek et al. (2018). However, the cited species had linear-lanceolate valves with broadly rounded and rostrate apices, a central area always forming a fascia and lower stria density (28–30/10 µm, Van de Vijver et al. 2018). Notably, the central area of A. lusitanicum rarely produces a unilateral fascia, being often absent or small rounded with some shortened striae. Distribution and ecological information:— The taxon was found in all three habitats (occurrence of 10 %), but it was more often found in surface sediments from reservoirs. In the sediment assemblages, the ecological preferences of the taxa were for slightly alkaline (pH optimum of 7.3), medium cond. (optimum of 100.3 μS∙ cm-1) and eutrophic waters (TP optimum of 56.7 μg∙L- 1 and TN optimum of 1016.5 μg∙L- 1). It is worth mentioning that it also occurred in oligo- and mesotrophic waters.

The present study aims to expand the knowledge on Achnanthidium species occurring in tropical areas of southern Brazil. For this purpose, 345 samples from 179 sampling sites distributed throughout the state of São Paulo were analyzed for ‘monoraphid’ diatom diversity. This paper focuses on samples with a high abundance of Achnanthidium species. The permanent slides and stubs were observed using light and scanning electron microscopy. Morphometric analysis was performed to discriminate between similar species through the Elliptical Fourier Descriptors analysis. For this, valve outlines from Brazilian species were compared to those from type material available from the literature. The ecological optimum and tolerance was calculated for taxa that presented more than 10 % of occurrence per habitat. In total, 12 Achnanthidium species were observed. Six are described as new: Achnanthidium acutum sp. nov., A. brasiliense sp. nov., A. parvicapitatum sp. nov., A. paulistanum sp. nov., A. ritae sp. nov. and A. vandevijveri sp. nov. The morphometric analysis supported the differentiation between some taxa. Nine species, with occurrences of at least 10 % per habitat, had their ecological optimum and tolerance calculated. In general, these species prefer slightly acidic to neutral pH, medium electrolyte cond. and mesotrophic water conditions.

Achnanthidium brasiliense L.F.Costa & G.C.Marquardt sp. nov. (Figs 26–44) Description:— LM: Valves linear-lanceolate, with rostrate to subcapitate apices (Figs 26–38); 12.3–17.7 µm long, 2.5–2.9 µm wide. Raphe valve: axial area linear to linear-lanceolate, very narrow; central area small, rounded, almost absent, sometimes forming a rectangular fascia reaching one or both sides (Figs 26–33). Raphe filiform, straight. Transapical striae slightly radiate throughout the entire valve, becoming denser and more strongly radiate towards the apices (Figs 26–33); 26–30 in 10 µm. Rapheless valve: axial area narrow and linear, slightly widening towards the central area; central area absent or narrow lanceolate (Figs 34–38). Transapical striae slightly radiate throughout the entire valve, more spaced in the central area and sometimes lacking one stria unilaterally (Figs 34–38). Striae become denser towards the apices: 26–28 in 10 µm. SEM: Raphe terminating after the last stria in the border between the valve face and mantle (Figs 39, 40). Central and terminal raphe fissures straight and slightly expanded (Figs 39, 40). Striae mainly composed of 2–4 rounded to elongated areolae, sometimes slit-like shaped near the valve margin and becoming denser and wider near the distal ends (Figs 39–44). Mantle with one row of slit-like areolae (Figs 39, 44). Type:— BRAZIL. São Paulo state: São Paulo, Guarulhos, Tanque Grande reservoir, epiphython, 23° 22’ 29” S, 46° 27’ 31” W, S.A.Oliveira & D.C.Bicudo, 9th January 2014 (holotype SP-428935! = Fig. 34, isotype: BR-4615). Etymology:— The specific epithet brasiliense sp. nov. refers to the country of the type locality. Taxonomical remarks:— Achnanthidium brasiliense sp. nov. resembles A. minutissimum, however, an important feature in the separation of both taxa are the apices, which are always protracted in A. minutissimum. Observing the illustrations provided by studies of the type material (Novais et al. 2015, Marquardt et al. 2017) it is possible to notice the stronger curved valves in apices of A. minutissimum, showed by the difficulty in focusing on the apices under LM. Also, in SEM micrographs of A. minutissimum (Fig. 7B, C, E in Marquardt et al. 2017), the araphid valves present a strong curvature following the axial area, a feature not observed in our specimens. Regarding the morphometric features, A. brasiliense sp. nov. presents a lower stria density than the German A. minutissimum type material (30– 35/10 µm, Novais et al. 2015) and the type material population presented in Marquardt et al. (2017), which includes rapheless valves ranging from 30–34 striae in 10 µm in the middle. Although the number of striae in A. minutissimum is also denser near the distal ends, the appearance of areolae near the apices in the raphe valves of A. brasiliense sp. nov. change in size and shape, becoming larger. Achnanthidium minutissimum is a widely used name for a complex of morphologically similar species worldwide. The morphometric characteristics of all the species that belong to this complex and were observed in this study are summarized in Table 4. In addition, a quantitative shape analysis was carried out to separate A. brasiliense sp. nov. from A. minutisimum and its results are discussed below. Still concerning the similarities among A. brasiliense sp. nov. and other species, Novais et al. (2015) reanalyzed the type materials of two species resembling ours: A. microcephalum Kützing (1844: 75) and A. neocryptocephalum (Grunow in Van Heurck) Novais & Van de Vijver (in Novais et al. 2015: 116) ≡ A. minutissima var. cryptocephala Grunow (in Van Heurck 1880: 27). Achnanthidium brasiliense sp. nov. presents slender valves than A. microcephalum and the stria density is also a distinguishing feature (30–32 in 10 µm, Novais et al. 2015). Moreover, external terminal raphe fissures in A. microcephalum are slightly deflected in the same direction but straight in A. brasiliense sp. nov. Although the striae are also denser near the apices in both taxa, areolae become wider in A. brasiliense sp. nov. Regarding A. neocryptocephalum, their representatives always present a rectangular fascia in the raphe valve, a rare feature in A. brasiliense sp. nov. Achnanthidium jackii Rabenhorst (1861: 1003) (Rabenhorst 1861 –1862) described from “Quellwasser bei Salem” (Municipality of Baden-Württemberg state, southern Germany) was studied in full detail by Van de Vijver et al. (2018), has priority over A. neocryptocephalum, a new name for Achnanthes minutissima var. cryptocephala, thus is also a taxonomic synonym of A. jackii. Achnanthidium barbei Le Cohu & Pérès (in Pérès et al. 2014: 388) also belongs to the A. minutissimum -complex and presents a similar morphology. It never forms a fascia in the central area of the raphe valve and has protracted apices (Pérès et al. 2014). Distribution and ecological information:— In our dataset, A. brasiliense sp. nov. was the most frequent species (63 % of occurrence) observed in periphytic, planktonic and surface sediment assemblages (highest abundance: 72 % in SP469485). In all these habitats, the ecological preference of the species was for neutral waters (pH optimum of 6.8– 7.2), medium to high cond. (optimum of 64.3–148.7 μS∙ cm-1) and mesotrophic conditions (TP optimum of 25.5–46.4 μg∙L- 1 and TN optimum of 596–1303.8 μg∙L- 1). However, it also occurred in enriched waters (TP> 50 μg∙L- 1)., Published as part of Costa, Lívia F., Wetzel, Carlos E., Maquardt, Gisele C., Zanon, Jaques E., Ector, Luc & Bicudo, Denise C., 2022, Taxonomy and ecology of Achnanthidium (Bacillariophyta, Achnanthidiaceae) from southeastern Brazil with the description of six new species, pp. 187-223 in Phytotaxa 575 (3) on page 196, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Novais, M. H., Juttner, I., Van de Vijver, B., Morais, M. M., Hoffmann, L. & Ector, L. (2015) Morphological variability within the Achnanthidium minutissimum species complex (Bacillariophyta): comparison between the type material of Achnanthes minutissima and related taxa, and new freshwater Achnanthidium species from Portugal. Phytotaxa 224 (2): 101 - 139. https: // doi. org / 10.11646 / phytotaxa. 224.2.1","Marquardt, G. C., Costa, L. F., Bicudo, D. C., Bicudo, C. E. M., Blanco, S., Wetzel, C. E. & Ector, L. (2017) Type analysis of Achnanthidium minutissimum and A. catenatum and description of A. tropicocatenatum sp. nov. (Bacillariophyta), a common species in Brazilian reservoirs. Plant Ecology and Evolution 150 (3): 313 - 330. https: // doi. org / 10.5091 / plecevo. 2017.1325","Kutzing, F. T. (1844). Die kieselschaligen Bacillarien oder Diatomeen. W. Kohne, Nordhausen. https: // doi. org / 10.5962 / bhl. title. 64360","Van Heurck, H. (1880) Synopsis des Diatomees de Belgique. Atlas. Anvers, Ducaju et Cie, pls 1 - 30.","Rabenhorst, L. (1861 - 1862) Die Algen Europa's, Fortsetzung der Algen Sachsens, resp. Mittel-Europas. Decades I-CIX, numbers 1 - 1600 (or 1001 - 2600). Dresden.","Van de Vijver, B., Wetzel, C. E. & Ector, L. (2018) Analysis of the type material of Achnanthidium jackii Rabenhorst (Bacillariophyta, Achnanthidiaceae). Notulae Algarum 55: 1 - 4.","Peres, F., Le Cohu, R. & Delmont, D. (2014) Achnanthidium barbei sp. nov. and Achnanthidium costei sp. nov., two new diatoms from French rivers. Diatom Research 29 (4): 387 - 397. https: // doi. org / 10.1080 / 0269249 X. 2014.890956"]}

Achnanthidium acutum L.F.Costa, C.E.Wetzel & Ector sp. nov. (Figs 2–25) Description:—LM: Valves narrowly linear-elliptic, with acutely rounded to very slightly rostrate apices (Figs 2–20); 9.3–21.2 µm long, 2.2–3.2 µm wide. Raphe valve: axial area narrow and linear; central area rounded, sometimes forming a uni- or bilateral rectangular fascia, with no or one shortened stria on both sides of the valve (Figs 2–12). Raphe filiform straight. Transapical striae slightly radiate throughout the entire valve, more spaced in the central area, becoming denser and more strongly radiate towards the apices; 27–30 in 10 µm (Figs 2–12). Rapheless valve: axial area narrow and linear; central area absent, with 1–2 shortened striae on both sides of the valve (Figs 13–20). Transapical striae slightly radiate throughout the entire valve, more spaced in the central area, and becoming denser and more strongly radiate towards the apices; 28–30 in 10 µm (Figs 13–20). SEM: Raphe terminating after the last stria (Figs 21, 22). Central and terminal raphe fissures straight (Figs 21, 22). Striae mainly composed of 3–4 or 4–5 rounded to elongated areolae (Figs 21–25), sometimes slit-like near the valve margin (Figs 21–23). Mantle with one row of slit-like areolae (Figs 21–23). Type:— BRAZIL. São Paulo state: São Paulo, Rio Grande reservoir, epiphyton (Salvinia), 23° 45’ 59.46” S, 46° 30’ 35.7” W, S. Wengrat & D. C.Bicudo, 5th August 2009 (holotype SP-427898! = Fig. 17, isotype: BR-4614). Etymology:— The specific epithet refers to the acutely rounded apices format of the species. Taxonomical remarks:— Concerning the characteristic linear valve shape, A. lineare resembles A. acutum sp. nov. However, A. lineare differs from Brazilian populations mostly by the almost parallel margins and broadly rounded apices in the raphe valves, whereas A. acutum sp. nov. shows narrowly linear-elliptic valves, with acutely rounded to very slightly rostrate apices. Also, the always-rectangular fascia in the central area of the raphe valve in A. lineare contrasts to the nearly rounded central area of A. acutum sp. nov. when observed in LM. Additional remarks include the density of areolae per striae in 10 µm, higher in A. acutum sp. nov. (up to 5) when compared to A. lineare (1– 3). This latter feature can be useful especially regarding the rapheless valve differentiation. The separation of both species was corroborated through morphological analysis between them. The results are presented and discussed in the ‘Morphological patterns in valve shape from EFD analysis’ section. Van de Vijver et al. (2011) analyzed A. lineare type material and described two new species: A. pseudolineare Van de Vijver, Novais & Ector (in Van de Vijver et al. 2011: 186) and A. sublineare Van de Vijver, Jarlman & Ector (in Van de Vijver et al. 2011: 179). Both taxa are described as having a linear outline of the raphe valve. Furthermore, Achnanthidium pseudolineare clearly has a denser number of striae in 10 µm (35–40) compared to A. acutum sp. nov. (27–30). Similarly, A. sublineare has a higher number of striae in 10 µm (33–34), a wider and lanceolate axial area in the rapheless valve, 1–2 areolae per striae and narrower valves (1.5–2.1 µm wide). Besides, this taxon possesses the central area formed by a rectangular fascia in the raphe valve, comparable to those in A. lineare. Achnanthidium petuniabuktianum Pinseel, Van de Vijver & Kopalová (2015: 68) also presents linear valves with rounded apices, but in SEM images, it shows a distinct wide axial area and striae with just 1–2 areolae, differing from A. acutum sp. nov. Also, the terminal raphe fissures are bent to the same side in A. petuniabuktianum while straight in our species. In Brazilian waters, A. brasiliense sp. nov. can co-occurs with A. acutum sp. nov. but can be distinguished by its lanceolate valves with rostrate apices and the linear valves with acutely rounded apices of A. acutum sp. nov. From the Antarctic region, two species closely resemble A. acutum sp. nov. but differ by possessing distinctly protracted and rostrate apices. The first one, A. maritimo-antarcticum Van de Vijver & Kopalová (2014: 6), also has the central area of the rapheless valve in disagreement with A. acutum sp. nov. morphology: it is weakly elliptical, whereas in the Brazilian species a central area is absent. On the other hand, A. indistinctum Van de Vijver & Kopalová (2014: 4) received this name by presenting small frustules (length 8.5–13.0 μm, width 1.8–2.2 μm, Van de Vijver & Kopalová 2014), a feature that distinguish it from our species, since A. acutum sp. nov. has longer and wider valves. In addition, besides the higher stria density (30–35 in 10 μm) than Brazilian taxa, slit-like areolae are apparently never present in A. indistinctum, a feature observed in marginal areolae of some valves in our population. Distribution and ecological information:— Achnanthidium acutum sp. nov. was found in all three habitats (i.e. periphyton, sediment and plankton) and present in 23 % of the quantified samples, being more abundant in periphytic samples (highest abundance: 12 % in SP427898). Due to its wide distribution, the ecological optimum could be calculated. The ecological preference of the species was for neutral pH (optimum of 6.8–7.1), medium to high cond. (optimum of 76.7–167.3 μS∙ cm-1), and meso- to eutrophic waters (TP optimum of 39.0–77.8 μg∙L- 1 and TN optimum of 647.7–1627.9 μg∙L- 1)., Published as part of Costa, Lívia F., Wetzel, Carlos E., Maquardt, Gisele C., Zanon, Jaques E., Ector, Luc & Bicudo, Denise C., 2022, Taxonomy and ecology of Achnanthidium (Bacillariophyta, Achnanthidiaceae) from southeastern Brazil with the description of six new species, pp. 187-223 in Phytotaxa 575 (3) on pages 194-195, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Van de Vijver, B., Ector, L., Beltrami, M. E., de Haan, M., Falasco, E., Hlubikova, D., Jarlman, A., Kelly, M., Novais, M. H. & Wojtal, A. Z. (2011) A critical analysis of the type material of Achnanthidium lineare W. Sm. (Bacillariophyceae). Algological Studies 136 - 137: 167 - 191. https: // doi. org / 10.1127 / 1864 - 1318 / 2011 / 0136 - 0167","Pinseel, E., Van de Vijver, B. & Kopalova, K. (2015) Achnanthidium petuniabuktianum sp. nov. (Achnanthidiaceae, Bacillariophyta), a new representative of the A. pyrenaicum group from Spitsbergen (Svalbard Archipelago, High Arctic). Phytotaxa 226 (1): 63 - 74. https: // doi. org / 10.11646 / phytotaxa. 226.1.6","Van de Vijver, B. & Kopalova, K. (2014) Four Achnanthidium species (Bacillariophyta) formerly identified as Achnanthidium minutissimum from the Antarctic Region. European Journal of Taxonomy 79: 1 - 19. https: // doi. org / 10.5852 / ejt. 2014.79"]}

Achnanthidium tropicocatenatum G.C.Marquardt, C.E.Wetzel & Ector (Figs 238–275) Morphometric features:— 6.7–18.1 µm long; 2.3–3.1 µm wide; 36–42 striae in 10 µm; striae mainly composed of 4–6 areolae. Taxonomical remarks:— Achnanthidium tropicocatenatum and A. catenatum are similar. In the protologue of the first species, many characters were taken into account to provide the differences between them (e.g. SEM, LM, geometric morphometric analysis, and ecological preferences). Nevertheless, it is possible to distinguish both taxa by the less inflated central portion, fewer capitate apices and higher stria density in A. tropicocatenatum (28–30 striae/10 µm in A. catenatum, Bílý & Marvan 1959). Also, in A. catenatum, the girdle view is distinct, with the valve being strongly curved with the apices sharply pointed. In contrast, in A. tropicocatenatum, the valve is less arched with apices slightly curved (Marquardt et al. 2017). Marquardt et al. (2017) commented about the confusion made with two different populations in Novais et al. (2015), probably joining two different species in one taxon (A. lusitanicum in Novais et al. 2015). The first population from Janeiro de Baixo, Zêzere River (Tejo basin, Portugal), is considered the type material of the species and differs from A. tropicatenatum by its rostrate apices, or broadly rounded ones in smaller specimens. Distribution and ecological information:— The taxon was recently described despite its presence in Brazilian literature being misidentified as A. catenatum. Indeed, in our samples, the species A. tropicocatenatum had the highest frequency of occurrence (61 %). Its ecological optimum was calculated for all habitats. The species showed an ecological preference for slightly acid to neutral (pH optimum of 6.7–6.9), low to medium cond. (optimum of 53.7–94.3 μS∙ cm-1) and mesotrophic waters (TP optimum of 19.8–37.3 μg∙L- 1 and TN optimum of 504.2–665.8 μg∙L- 1). Achnanthidium tropicocatenatum was previously described from oligotrophic conditions in São Paulo state (Marquardt et al. 2017); however, it presented greater tolerance in our study area and was identified from oligo- to eutrophic conditions., Published as part of Costa, Lívia F., Wetzel, Carlos E., Maquardt, Gisele C., Zanon, Jaques E., Ector, Luc & Bicudo, Denise C., 2022, Taxonomy and ecology of Achnanthidium (Bacillariophyta, Achnanthidiaceae) from southeastern Brazil with the description of six new species, pp. 187-223 in Phytotaxa 575 (3) on page 210, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Bily, J. & Marvan, P. (1959) Achnanthes catenata sp. n. Preslia 31: 34 - 35.","Marquardt, G. C., Costa, L. F., Bicudo, D. C., Bicudo, C. E. M., Blanco, S., Wetzel, C. E. & Ector, L. (2017) Type analysis of Achnanthidium minutissimum and A. catenatum and description of A. tropicocatenatum sp. nov. (Bacillariophyta), a common species in Brazilian reservoirs. Plant Ecology and Evolution 150 (3): 313 - 330. https: // doi. org / 10.5091 / plecevo. 2017.1325","Novais, M. H., Juttner, I., Van de Vijver, B., Morais, M. M., Hoffmann, L. & Ector, L. (2015) Morphological variability within the Achnanthidium minutissimum species complex (Bacillariophyta): comparison between the type material of Achnanthes minutissima and related taxa, and new freshwater Achnanthidium species from Portugal. Phytotaxa 224 (2): 101 - 139. https: // doi. org / 10.11646 / phytotaxa. 224.2.1"]}

Achnanthidium peruvianum E.Morales & Ector in Morales et al. (2011: 121) (Figs 166–193) Synonym: Achnanthes kryophiloides M.H.Hohn in Patrick et al. (1966), p. 468, pl. 1, Figs 5, 6 Morphometric features:— 8.0–20.2 µm long; 2.3–4.3 µm wide; 28–34 striae in 10 µm; striae mainly composed of 4–5 areolae. Taxonomical remarks:— The species presents similar morphological features to another South American species called Achnanthidium sehuencoensis, which has almost the same valve outline. However, SEM analysis separated the two due to the deflected terminal raphe fissures, the non-expanded central raphe fissures, and lower stria density (20–24/10 µm, Morales et al. 2009) compared to A. peruvianum. Achnanthidium petersenii (Hustedt) C.E.Wetzel, Ector & Jüttner (in Jüttner et al. 2019: 66) has wider fascia on the raphe valve. However, on the rapheless valves, the axial area is moderately broad, and the central area broadly rhombic-lanceolate (Jüttner et al. 2019). In A. peruvianum, the axial area is narrow and linear, while the central area is undifferentiated with short striae (Morales et al. 2011). Distribution and ecological information:— Achnanthidium peruvianum was described from the Rondos River in Peru, South America (Hohn in Patrick et al. 1966). In our samples, the species was rare (abundance

Achnanthidium paulistanum L.F.Costa & D.Bicudo sp. nov. (Figs 117–143) Description:— LM: Valves narrowly linear-lanceolate, with subrostrate to rostrate apices (Figs 117–134); 8.4–15.0 µm long, 2.1–2.6 µm wide. Raphe valve: axial area narrow and linear; central area small rounded, almost absent, with 1–2 shortened striae on both sides of the valve, and never forming a fascia (Figs 117–126). Raphe filiform straight. Transapical striae slightly radiate throughout the entire valve, becoming denser and more strongly radiate towards the apices, 30–31 in 10 µm (Figs 117–126). Rapheless valve: axial area narrow and linear, slightly widening towards the central area; central area very small rounded or absent, with 1–2 shortened striae on both sides of the valve (Figs 127–134). Transapical striae slightly radiate throughout the entire valve, becoming denser towards the apices, 30–32 in 10 µm (Figs 127–134). Girdle view rectangularly arched, with apices recurved to the rapheless valve (Figs 135–137). SEM: Raphe terminating right after the last stria (Figs 138–140). Central raphe fissures close, straight and slightly expanded (Figs 138–140). Terminal raphe fissures short, deflected (Figs 138–140). Striae mainly composed of 3–5 rounded to elongated areolae, sometimes slit-like near the valve margin (Figs 138–143). Mantle with one row of slitlike areolae (Figs 139, 140). Type:— BRAZIL. São Paulo state: São Pedro, SP –304, km 127, periphyton from a small lake, A.A.J.Castro & C.E.M.Bicudo, 20th March 1990 (holotype SP-255724! = Fig. 118, isotype: BR-4617). Etymology: —The specific epithet refers to people born in the state of São Paulo (in Portuguese, the “ paulistas ”). Taxonomical remarks:— Achnanthidium paulistanum sp. nov. is characterized by a linear or narrowly lanceolate valve shape, with subrostrate apices. In contrast, A. brasiliense sp. nov. has linear-lanceolate valves and rostrate apices. Notably, the slightly deflected terminal raphe fissures are always present in A. paulistanum sp. nov. valves, a feature that was not observed in A. brasiliense sp. nov. specimens. The central area never forms a fascia and is almost absent in the new species. In this way, the main characters differing in both species are the terminal raphe fissures and the central area shape of the raphid valve. The deflected terminal raphe fissures differentiate the species from others, including A. indistinctum specimens with valve outline very similar to A. paulistanum sp. nov. It also can be distinguished by the girdle view: the Brazilian species has apices recurved to the rapheless valve and mantle striae almost indiscernible in LM, whereas the Antarctic one presents girdle view with apices straight and mantle striae thickened in LM. In valve view, A. indistinctum has slightly higher stria density and slightly narrower valves (1.8–2.2 µm wide, 30–35 striae in 10 µm, Van de Vijver & Kopalová 2014). Achnanthidium maritimo-antarcticum, another Antarctic species, differs by having parallel to slightly convex margins, appearing sometimes undulated, and clearly protracted apices, whereas A. paulistanum sp. nov. has convex to slightly convex margins and apices not clearly differentiate from the main body. In ultrastructure, A. maritimo-antarcticum possesses terminal raphe fissures straight to weakly curved (Van de Vijver & Kopalová 2014). Due to its linear shape, Achnanthidium acutum sp. nov. is morphologically close to this taxon, but A. paulistanum sp. nov. can be narrowly lanceolate. These species also differ with respect to the apices, which are acutely rounded in the first and subrostrate in the latter. Nevertheless, the slightly deflected terminal raphe fissures and the higher stria density in A. paulistanum sp. nov. can be used to separate both taxa. Distribution and ecological information:— The taxon was rare and occurred only in the epiphytic type sample., Published as part of Costa, Lívia F., Wetzel, Carlos E., Maquardt, Gisele C., Zanon, Jaques E., Ector, Luc & Bicudo, Denise C., 2022, Taxonomy and ecology of Achnanthidium (Bacillariophyta, Achnanthidiaceae) from southeastern Brazil with the description of six new species, pp. 187-223 in Phytotaxa 575 (3) on pages 203-204, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Van de Vijver, B. & Kopalova, K. (2014) Four Achnanthidium species (Bacillariophyta) formerly identified as Achnanthidium minutissimum from the Antarctic Region. European Journal of Taxonomy 79: 1 - 19. https: // doi. org / 10.5852 / ejt. 2014.79"]}

Achnanthidium vandevijveri L.F.Costa, Ector & C.E.Wetzel sp. nov. (Figs 276–316) Description:— LM: Valves 7.8–16.5 µm long, 3.0–3.8 µm wide. Raphe valve: valves linear-lanceolate to rhombiclanceolate, with subrostrate to rostrate apices (Figs 276–291). Axial area narrow and linear, central area absent or small, rounded (Figs 276–291). Raphe filiform and straight. Transapical striae slightly radiate throughout the entire valve, more spaced in the central area, and becoming denser and more strongly radiate towards the apices; 27–34 in 10 µm (Figs 276–291). Rapheless valve: valves linear-lanceolate, with rostrate to subcapitate apices (Figs 292–306). Axial area narrow and linear, slightly widening towards the central area; central area absent to very small rounded (Figs 292–306). Transapical striae slightly radiate throughout the entire valve, becoming denser towards the apices; 30–32 in 10 µm (Figs 292–306). SEM: Raphe prolonged, terminating after the last stria on the border between the valve face and mantle (Figs 307, 308, 311–313). Central raphe fissures close and straight (Figs 307, 308, 311–313). Terminal raphe fissures short, weakly deflected (Figs 307, 308, 311–313). Striae mainly composed of 3–5 rounded to elongated areolae, sometimes slit-like near the valve margin (Figs 307–316). Mantle with one row of slit-like areolae (Figs 307, 309, 314, 315). Type:— BRAZIL. São Paulo state: São Paulo, Garças Lake, periphyton, 23° 38’ 44.4” S, 46° 37’ 29.9” W, P.D.Almeida & D.C.Bicudo, 7th July 2014 (holotype SP-469485! = Fig. 282, isotype: BR-4620). Etymology: —We dedicate this species to Prof. Dr. Bart Van de Vijver (Meise Botanic Garden, Meise, Belgium). Taxonomical remarks: — The different valve outline between the raphe and rapheless valves make the taxon more easily distinguishable from others. The rhombic-lanceolate outline of the raphe valve resembles the taxon to A. eutrophilum. Besides the different rapheless valve outline, A. eutrophilum presents wider valves and slightly lower stria density (3.2–5.8 µm wide, stria density 25–30/10 µm, Hlúbiková et al. 2011). Using SEM it is possible to see other differences such as the prolonged terminal raphe ends after the last stria and the almost never slit-like areolae shape. In contrast, in A. vandevijver sp. nov., the terminal raphe fissures terminate right after the last stria, and the areolae are often slit-like near the margins of the valve. Despite the general similarity, Achnanthidium dolomiticum Cantonati & Lange-Bertalot (2006: 1185) presents its raphe valve with some singularities when compared with A. vandevijveri sp. nov. The Italian species has the central area forming a bilateral narrow fascia, a feature never present in the Brazilian one. In addition, the stria density is slightly higher (33–35 in 10 µm, Cantonati & Lange-Bertalot 2006) than our new species A. vandevijveri sp. nov. From our study area, the rapheless valve of A. vandevijveri sp. nov. is similar to valves of A. lusitanicum. The latter taxon differs mainly by the apices form in the raphe valve. While the rapheless valve has overlapping metric features, they can be distinguished by the visibly rounded central area in A. lusitanicum and the absence or presence of very small, rounded ones in A. vandevijveri sp. nov. Distribution and ecological information:— Achnanthidium vandevijveri sp. nov. was restricted to samples SP-469483 and SP-469485 from Garças Lake, a eutrophic urban lake (Bicudo et al. 2007, 2020) in the metropolitan area of São Paulo. It occurred in plankton, but mainly in periphyton (highest abundance: 15 %). The limnological conditions of the sampling site varied from acid to alkaline (pH 5.8–8.6), cond. was high (307–378 μS∙ cm-1), as were the nutrient concentrations in the environment (TP 97–107.1 μg∙L- 1, TN 1734.2–2757.9 μg∙L- 1)., Published as part of Costa, Lívia F., Wetzel, Carlos E., Maquardt, Gisele C., Zanon, Jaques E., Ector, Luc & Bicudo, Denise C., 2022, Taxonomy and ecology of Achnanthidium (Bacillariophyta, Achnanthidiaceae) from southeastern Brazil with the description of six new species, pp. 187-223 in Phytotaxa 575 (3) on pages 210-212, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Hlubikova, D., Ector, L. & Hoffmann, L. (2011) Examination of the type material of some diatom species related to Achnanthidium minutissimum (Kutz.) Czarn. (Bacillariophyceae). Algological Studies 136 / 137: 19 - 43. https: // doi. org / 10.1127 / 1864 - 1318 / 2011 / 0136 - 0019","Cantonati, M. & Lange-Bertalot, H. (2006) Achnanthidium dolomiticum sp. nov. (Bacillariophyta) from oligotrophic mountain springs and lakes fed by dolomite aquifers. Journal of Phycology 42 (6): 1184 - 1188. https: // doi. org / 10.1111 / j. 1529 - 8817.2006.00281. x","Bicudo, D. C., Fonseca, B. M., Bini, L. M., Crossetti, L. O., Bicudo, C. E. M. & Araujo-Jesus, T. (2007) Undesirable side-effects of water hyacinth control in a shallow tropical reservoir. Freshwater Biology 52 (6): 1120 - 1133. https: // doi. org / 10.1111 / j. 1365 - 2427.2007.01738. x","Bicudo, D. C., Zanon, J. E., Ferragut, C., Crossetti, L. O., Faustino, S. B. & Bicudo, C. E. M. (2020) Garcas Reservoir trophic state dynamics: a 20 - year synthesis. Hoehnea 47: e 722019. https: // doi. org / 10.1590 / 2236 - 8906 - 72 / 2019"]}

Achnanthidium saprophilum (H.Kobayashi & Mayama) Round & Bukhtiyarova (Figs 215–237) Morphometric features:— 6.0–15.3 µm long; 2.5–3.2 µm wide; 26–32 striae in 10 µm; striae mainly composed of 4–5 areolae. Taxonomical remarks:— The species can be confused with smaller specimens of A. tropicocatenatum, however, they differ by the subcapitate to capitate apices and higher stria density (36–45 in 10 µm) in A. tropicocatenatum. Achnanthidium peetersianum presents small capitate apices and has narrower valves (2.0–2.7 µm) than A. saprophilum. Also, externally terminal raphe fissures are slightly bent to the same side in A. peetersianum, at the same level as the striae on the valve face, whereas in our species external terminal raphe fissures are always straight and terminating after the last striae. Despite displaying different raphid valves with linear-lanceolate to rhombic-lanceolate in A. vandevijveri sp. nov., the rapheless valves present almost the same valve outline as A. saprophilum. However, the valves outline in A. vandevijveri sp. nov. are slightly wider and longer. Achnanthidium acsiae Wojtal, E.Morales, Van de Vijver & Ector (in Wojtal 2011: 226) has similar valve outline and metric features as A. saprophilum, however, the former has the central area of raphe valve often forming a bilateral fascia, a feature never observed in our population. Achnanthidium saprophilum rarely shows a unilateral fascia. Still, the stria density is higher than ours and they are composed of less areolae (28–36 striae in 10 µm, 3–4 areolae per stria, Wojtal et al. 2011). Achnanthidium pseudolineare differs from A. saprophilum by the rapheless valve, presenting linear valves with parallel margins and broadly rounded apices. The stria density is also different between the species, being higher in A. pseudolineare (32–40 in 10 µm, Van de Vijver et al. 2011). Achnanthidium costei Pérès & Le Cohu (in Pérès et al. 2014: 390) also has linear valves with parallel margins, but this outline is characteristic of the raphe valves (Pérès et al. 2014). Its rapheless valves shows another distinct feature: the axial area is wider and lanceolate. Achnanthidium saprophilum has both valves linear-lanceolate with convex margins, and the axial area straight on rapheless valves. Concerning Achnanthidium saprophilum, despite the number of areolae per striae in the isotype being higher (4–7, Hlúbiková et al. 2011) than ours (4–5), the areolae morphology of the valves from our population is variable, including apically elongated areolae, as described for A. saprophilum (Kobayasi & Mayama 1982). Distribution and ecological information:— The species occurred in planktonic, periphytic and surface sediment assemblages in low abundance (, Published as part of Costa, Lívia F., Wetzel, Carlos E., Maquardt, Gisele C., Zanon, Jaques E., Ector, Luc & Bicudo, Denise C., 2022, Taxonomy and ecology of Achnanthidium (Bacillariophyta, Achnanthidiaceae) from southeastern Brazil with the description of six new species, pp. 187-223 in Phytotaxa 575 (3) on pages 208-210, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Wojtal, A. Z., Ector, L., Van de Vijver, B., Morales, E. A., Blanco, S., Piatek, J. & Smieja, A. (2011) The Achnanthidium minutissimum complex (Bacillariophyceae) in southern Poland. Algological Studies 136 - 137: 211 - 238. https: // doi. org / 10.1127 / 1864 - 1318 / 2011 / 0136 - 0211","Van de Vijver, B., Ector, L., Beltrami, M. E., de Haan, M., Falasco, E., Hlubikova, D., Jarlman, A., Kelly, M., Novais, M. H. & Wojtal, A. Z. (2011) A critical analysis of the type material of Achnanthidium lineare W. Sm. (Bacillariophyceae). Algological Studies 136 - 137: 167 - 191. https: // doi. org / 10.1127 / 1864 - 1318 / 2011 / 0136 - 0167","Peres, F., Le Cohu, R. & Delmont, D. (2014) Achnanthidium barbei sp. nov. and Achnanthidium costei sp. nov., two new diatoms from French rivers. Diatom Research 29 (4): 387 - 397. https: // doi. org / 10.1080 / 0269249 X. 2014.890956","Hlubikova, D., Ector, L. & Hoffmann, L. (2011) Examination of the type material of some diatom species related to Achnanthidium minutissimum (Kutz.) Czarn. (Bacillariophyceae). Algological Studies 136 / 137: 19 - 43. https: // doi. org / 10.1127 / 1864 - 1318 / 2011 / 0136 - 0019","Kobayasi, H. & Mayama, S. (1982) Most pollution-tolerant diatoms of severely polluted rivers in the vicinity of Tokyo. Japanese Journal of Phycology 30: 188 - 196."]}

Achnanthidium ritae L.F.Costa sp. nov. (Figs 194–214) Description:— LM: Valves linear-elliptic, with non-protracted, broadly rounded apices (Figs 194–210); 5.3–10.3 µm long, 2.5–2.9 µm wide. Raphe valve: axial area narrow and linear, central area absent or small rounded, sometimes with one shortened stria on both sides of the valve (Figs 194–201). Raphe filiform and straight. Transapical striae slightly radiate throughout the entire valve, more spaced in the central area (Figs 194–201); 28–30 in 10 µm. Rapheless valve: axial area narrow and linear, slightly widening towards the central area; central area absent or narrow lanceolate (Figs 202–210). Transapical striae slightly radiate throughout the entire valve, more spaced in the central area; 26–28 in 10 µm (Figs 202–210). SEM: Raphe prolonged, terminating after the last stria (Fig. 211). Central raphe fissures close, straight and droplike expanded (Fig. 211). Terminal raphe fissures straight (Fig. 211). Striae mainly composed of 4–6 rounded to almost squared or elongated areolae (Figs 211–214). Mantle with one row of slit-like areolae (Figs 211–214). Type:— BRAZIL. São Paulo state: São Paulo, Billings Complex, Taquacetuba branch, epilithon, 23° 48’ 44.1” S, 46° 37’ 51.24” W, S.Wengrat & D.C.Bicudo, 7th August 2009 (holotype SP-427904! = Fig. 205, isotype: BR-4619). Etymology:— This species is dedicated to Rita Franco. Taxonomical remarks:— Achnanthidium duriense Novais & Ector is the most similar species, however, it differs by having rapheless valves clearly lanceolate, and also by the higher stria density (35 in 10 µm, Novais et al. 2015). Achnanthidium straubianum (Lange-Bertalot) Lange-Bertalot (1999: 273) presented wider valves in the protologue (3.5–4 µm wide, Lange-Bertalot & Metzeltin 1996), compared to the isotype that has narrower valves (2.6–3.5 µm, Hlúbiková et al. 2011). However, even in the isotype illustrations it is possible to observe wider and shorter valves (ratio length/width 2.0–2.6, measured from the Figs 73–103 in Hlúbiková et al. 2011) than the species described herein (2.2–3.9). Hlúbiková et al. (2011) stated the combination of length, width and striation pattern as a reliable criterion to separate A. straubianum from similar species. Another taxon resembling A. ritae sp. nov. is A. nanum (F.Meister) Novais & Jüttner (in Novais et al. 2015: 121), a species described from Switzerland (Meister 1935). Achnanthidium nanum presents a different valve outline in the illustrations of the protologue and type material reevaluated in Novais et al. (2015). It also has linear-lanceolate valves differing from the linear-elliptic valves of A. ritae sp. nov. Also, the species present different areolae shapes and the central area of our specimens is often absent while it is clearly visible in A. nanum. Two species recently described from Korea, Achnanthidium ovale M.Miao & B.-H.Kim (in Miao et al. 2020: 5) and Achnanthidium cavitatum M.Miao & B.-H.Kim (in Miao et al. 2020: 7), also can be compared to the Brazilian species. Achnanthidium ovale has a more elliptic valve outline, wider valves (3.8–4.1 µm) and a greater stria density (30–35 in 10 µm). The ultrastructural differences, such as the terminal raphe fissures strongly bent to the same side in the Korean taxon, are another notable feature (Miao et al. 2020). Achnanthidium cavitatum is similar in that it has linear-elliptic valves; however, the main difference from A. ritae sp. nov. is the wider axial area observed in the rapheless valve (Miao et al. 2020). Achnanthidium cavitatum also has slightly wider valves (3.0–3.5 µm, Miao et al. 2020) than our population. Furthermore, Achnanthidium rivulare Potapova & Ponader (2004: 36) and A. crassum (Hustedt) Ponader & Potapova (in Potapova & Ponader 2004: 38) differ from A. ritae sp. nov. in their linear-lanceolate axial area in the raphe valve, unilaterally deflected terminal raphe fissures and wider valves (2.6–4.4 µm and 3–4.5 µm respectively, Potapova & Ponader 2004). Distribution and ecological information:— Achnanthidium ritae sp. nov. was found in 8 % of the counted samples, mainly occurring in periphytic samples (highest abundance: 14 % in SP-427904) with an ecological preference for alkaline (pH optimum of 7.9), high cond. (optimum of 182.4 μS∙ cm-1) and mesotrophic water conditions (TP optimum of 41.1 μg∙L- 1 and TN optimum 1306.2 μg∙L- 1). It also presented a wide tolerance and was found to occur in oligo- to eutrophic waters., Published as part of Costa, Lívia F., Wetzel, Carlos E., Maquardt, Gisele C., Zanon, Jaques E., Ector, Luc & Bicudo, Denise C., 2022, Taxonomy and ecology of Achnanthidium (Bacillariophyta, Achnanthidiaceae) from southeastern Brazil with the description of six new species, pp. 187-223 in Phytotaxa 575 (3) on pages 207-208, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Novais, M. H., Juttner, I., Van de Vijver, B., Morais, M. M., Hoffmann, L. & Ector, L. (2015) Morphological variability within the Achnanthidium minutissimum species complex (Bacillariophyta): comparison between the type material of Achnanthes minutissima and related taxa, and new freshwater Achnanthidium species from Portugal. Phytotaxa 224 (2): 101 - 139. https: // doi. org / 10.11646 / phytotaxa. 224.2.1","Lange-Bertalot, H. (1999) Neue Kombinationen von Taxa aus Achnanthes Bory (sensu lato). In: Lange-Bertalot, H. (Ed.), Iconographia Diatomologica. Annotated Diatom Micrographs. Vol. 6. Phytogeography - Diversity - Taxonomy. Koeltz Scientific Books, Konigstein, Germany, pp. 276 - 289.","Lange-Bertalot, H. & Metzeltin, D. (1996) Oligotrophie-Indikatoren. 800 Taxa reprasentativ fur drei diverse Seen-Typen. Kalkreich - Oligodysroph - Schwach gepuffertes Weichwasser [Indicators of oligotrophy. 800 taxa representative of three ecologically distinct lake types. Carbonate buffered - Oligodystrophic - Weakly buffered soft water]. Iconographia Diatomologica 2: 1 - 390.","Hlubikova, D., Ector, L. & Hoffmann, L. (2011) Examination of the type material of some diatom species related to Achnanthidium minutissimum (Kutz.) Czarn. (Bacillariophyceae). Algological Studies 136 / 137: 19 - 43. https: // doi. org / 10.1127 / 1864 - 1318 / 2011 / 0136 - 0019","Meister, F. (1935) Seltene und neue Kieselalgen. Bericht der Schweizerischen Botanischen Gesellschaft 44: 87 - 108.","Miao, M., Li, Z., Hwang, E. - A., Kim, H. - K., Lee, H. & Kim, B. - H. (2020) Two new benthic diatoms of the genus Achnanthidium (Bacillariophyceae) from the Hangang River, Korea. Diversity 12 (7): 285. https: // doi. org / 10.3390 / d 12070285","Potapova, M. G. & Ponader, K. C. (2004) Two common North American diatoms, Achnanthidium rivulare sp. nov. and A. deflexum (Reimer) Kingston: morphology, ecology and comparison with related species. Diatom Research 19 (1): 33 - 57. https: // doi. org / 10.1080 / 0269249 X. 2004.9705606"]}

Achnanthidium sp. (Figs 317–327) LM observations:— Valves linear-elliptic to linear-lanceolate, with broadly rounded apices to subrostrate (Figs 317– 327); 7.4–11.6 µm long, 3.5–4.1 µm wide. Raphe valve: axial area narrow and linear, central area small rounded, with 1–2 shortened striae on both sides of the valve (Figs 317–321). Raphe filiform straight. Transapical striae slightly radiate throughout the entire valve, becoming denser and strongly radiate towards the apices; 24–28 in 10 µm (Figs 317–321). Rapheless valve: axial area narrow and linear, widening towards the central area, central area absent or lanceolate (Figs 322–327). Transapical striae radiate throughout the entire valve, becoming denser and strongly radiate towards the apices; 23–27 in 10 µm (Figs 322–327). Taxonomical remarks:— Achnanthidium modestiformis (Lange-Bertalot) Van de Vijver (in Van de Vijver et al. 2002: 17) presents similar morphometric features (12–17 µm long, 3.5–4 µm wide, stria density 24–25/10 µm, as Achnanthes modesta Manguin (in Bourrelly & Manguin 1954: 22). However, the Brazilian population has broadly rounded to rostrate apices, whereas they are more capitate in A. modestiformis. Moreover, Achnanthidium sp. has shorter valves. Another similar specimen cited in Brazilian literature is Achnanthes acares M.H. Hohn & Hellerman (1963: 273) (Schneck et al. 2008). The illustrations presented by Hohn & Hellerman (1963) concerning this taxon show a different central area, elliptical in the raphe valve and coarse striae resembling Planothidium. Krammer & Lange-Bertalot (1991) illustrated the holotype of this species using LM, however, they only showed the raphe valves of the specimens, which probably do not correspond to a monoraphid taxon. Additionally, another Brazilian population resembled ours, but it was not identified at the species level (Santos et al. 2011, Figs 21–24, as “ Achnanthidium sp. ”). More studies are necessary concerning the identity of this taxon. Distribution and ecological information:— Achnanthidium sp. was rare (occurrence of, Published as part of Costa, Lívia F., Wetzel, Carlos E., Maquardt, Gisele C., Zanon, Jaques E., Ector, Luc & Bicudo, Denise C., 2022, Taxonomy and ecology of Achnanthidium (Bacillariophyta, Achnanthidiaceae) from southeastern Brazil with the description of six new species, pp. 187-223 in Phytotaxa 575 (3) on pages 214-215, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Van de Vijver, B., Frenot, Y. & Beyens, L. (2002) Freshwater diatoms from Ile de la Possession (Crozet Archipelago, Subantarctica). Bibliotheca Diatomologica 46: 1 - 412.","Bourrelly, P. & Manguin, E. (1954) Contribution a la flore algale d'eau douce des Iles Kerguelen. Memoires de l'Institut Scientifique de Madagascar, series B 5: 5 - 58.","Hohn, M. H. & Hellerman, J. (1963) The taxonomy and structure of diatom populations from three Eastern North American rivers using three sampling methods. Transactions of the American Microscopical Society 82 (3): 250 - 329. https: // doi. org / 10.2307 / 3223932","Schneck, F., Torgan, L. C. & Schwarzbold, A. (2008) Diatomaceas epiliticas em riacho de altitude no sul do Brasil. Rodriguesia 59 (2): 325 - 338. https: // doi. org / 10.1590 / 2175 - 7860200859205","Krammer, K. & Lange-Bertalot, H. (1991) Bacillariophyceae 4. Teil: Achnanthaceae, Kritische Erganzungen zu Navicula (Lineolatae) und Gomphonema. Gesamtliteraturverzeichnis Teil 1 - 4. In: Ettl, H., Gartner, G., Gerloff, J., Heynig, H. & Mollenhauer, D. (Eds.) Susswasserflora von Mitteleuropa Vol. 2 / 4. Gustav Fischer Verlag, Stuttgart-Jena, pp. 1 - 437.","Santos, E. M., Tremarin, P. I. & Ludwig, T. A. V. (2011) Diatomaceas perifiticas em Potamogeton polygonus Cham. & Schltdl.: citacoes pioneiras para o estado do Parana [Periphytic diatoms on Potamogeton polygonus Cham. & Schltdl.: first records from Parana State]. Biota Neotropica 11 (3): 303 - 315. https: // doi. org / 10.1590 / S 1676 - 06032011000300025"]}

Achnanthidium brasiliense L.F.Costa & G.C.Marquardt sp. nov. (Figs 26–44) Description:— LM: Valves linear-lanceolate, with rostrate to subcapitate apices (Figs 26–38); 12.3–17.7 µm long, 2.5–2.9 µm wide. Raphe valve: axial area linear to linear-lanceolate, very narrow; central area small, rounded, almost absent, sometimes forming a rectangular fascia reaching one or both sides (Figs 26–33). Raphe filiform, straight. Transapical striae slightly radiate throughout the entire valve, becoming denser and more strongly radiate towards the apices (Figs 26–33); 26–30 in 10 µm. Rapheless valve: axial area narrow and linear, slightly widening towards the central area; central area absent or narrow lanceolate (Figs 34–38). Transapical striae slightly radiate throughout the entire valve, more spaced in the central area and sometimes lacking one stria unilaterally (Figs 34–38). Striae become denser towards the apices: 26–28 in 10 µm. SEM: Raphe terminating after the last stria in the border between the valve face and mantle (Figs 39, 40). Central and terminal raphe fissures straight and slightly expanded (Figs 39, 40). Striae mainly composed of 2–4 rounded to elongated areolae, sometimes slit-like shaped near the valve margin and becoming denser and wider near the distal ends (Figs 39–44). Mantle with one row of slit-like areolae (Figs 39, 44). Type:— BRAZIL. São Paulo state: São Paulo, Guarulhos, Tanque Grande reservoir, epiphython, 23° 22’ 29” S, 46° 27’ 31” W, S.A.Oliveira & D.C.Bicudo, 9th January 2014 (holotype SP-428935! = Fig. 34, isotype: BR-4615). Etymology:— The specific epithet brasiliense sp. nov. refers to the country of the type locality. Taxonomical remarks:— Achnanthidium brasiliense sp. nov. resembles A. minutissimum, however, an important feature in the separation of both taxa are the apices, which are always protracted in A. minutissimum. Observing the illustrations provided by studies of the type material (Novais et al. 2015, Marquardt et al. 2017) it is possible to notice the stronger curved valves in apices of A. minutissimum, showed by the difficulty in focusing on the apices under LM. Also, in SEM micrographs of A. minutissimum (Fig. 7B, C, E in Marquardt et al. 2017), the araphid valves present a strong curvature following the axial area, a feature not observed in our specimens. Regarding the morphometric features, A. brasiliense sp. nov. presents a lower stria density than the German A. minutissimum type material (30– 35/10 µm, Novais et al. 2015) and the type material population presented in Marquardt et al. (2017), which includes rapheless valves ranging from 30–34 striae in 10 µm in the middle. Although the number of striae in A. minutissimum is also denser near the distal ends, the appearance of areolae near the apices in the raphe valves of A. brasiliense sp. nov. change in size and shape, becoming larger. Achnanthidium minutissimum is a widely used name for a complex of morphologically similar species worldwide. The morphometric characteristics of all the species that belong to this complex and were observed in this study are summarized in Table 4. In addition, a quantitative shape analysis was carried out to separate A. brasiliense sp. nov. from A. minutisimum and its results are discussed below. Still concerning the similarities among A. brasiliense sp. nov. and other species, Novais et al. (2015) reanalyzed the type materials of two species resembling ours: A. microcephalum Kützing (1844: 75) and A. neocryptocephalum (Grunow in Van Heurck) Novais & Van de Vijver (in Novais et al. 2015: 116) ≡ A. minutissima var. cryptocephala Grunow (in Van Heurck 1880: 27). Achnanthidium brasiliense sp. nov. presents slender valves than A. microcephalum and the stria density is also a distinguishing feature (30–32 in 10 µm, Novais et al. 2015). Moreover, external terminal raphe fissures in A. microcephalum are slightly deflected in the same direction but straight in A. brasiliense sp. nov. Although the striae are also denser near the apices in both taxa, areolae become wider in A. brasiliense sp. nov. Regarding A. neocryptocephalum, their representatives always present a rectangular fascia in the raphe valve, a rare feature in A. brasiliense sp. nov. Achnanthidium jackii Rabenhorst (1861: 1003) (Rabenhorst 1861 –1862) described from “Quellwasser bei Salem” (Municipality of Baden-Württemberg state, southern Germany) was studied in full detail by Van de Vijver et al. (2018), has priority over A. neocryptocephalum, a new name for Achnanthes minutissima var. cryptocephala, thus is also a taxonomic synonym of A. jackii. Achnanthidium barbei Le Cohu & Pérès (in Pérès et al. 2014: 388) also belongs to the A. minutissimum -complex and presents a similar morphology. It never forms a fascia in the central area of the raphe valve and has protracted apices (Pérès et al. 2014). Distribution and ecological information:— In our dataset, A. brasiliense sp. nov. was the most frequent species (63 % of occurrence) observed in periphytic, planktonic and surface sediment assemblages (highest abundance: 72 % in SP469485). In all these habitats, the ecological preference of the species was for neutral waters (pH optimum of 6.8– 7.2), medium to high cond. (optimum of 64.3–148.7 μS∙ cm-1) and mesotrophic conditions (TP optimum of 25.5–46.4 μg∙L- 1 and TN optimum of 596–1303.8 μg∙L- 1). However, it also occurred in enriched waters (TP> 50 μg∙L- 1).

Achnanthidium parvicapitatum L.F.Costa, Ector & C.E.Wetzel sp. nov. (Figs 100–116) Description:— LM: Valves linear-lanceolate, with subtle shoulders and subcapitate to small capitate apices (Figs 100– 111); 18.2–22.7 µm long, 2.7–3.0 µm wide. Raphe valve: axial area narrow and linear, central area rounded, with 2–4 shortened striae on both sides of the valve, rarely forming a rectangular fascia (Figs 100–105). Raphe filiform straight. Transapical striae slightly radiate throughout the entire valve, more spaced in the central area, and becoming denser and more strongly radiate towards the apices (Figs 100–105); 29–33 in 10 µm. Rapheless valve: axial area narrow and linear, widening towards the central area; central area absent or narrow lanceolate, with 1–2 shortened striae on both sides of the valve (Figs 106–111). Transapical striae slightly radiate throughout the entire valve, more spaced in the central area, and becoming denser towards the apices; 28–32 in 10 µm. Girdle view rectangularly arched, with pointed apices recurved to the rapheless valve (Fig. 112). SEM: Raphe prolonged after the striae, terminating on the border between the valve face and mantle (Fig. 113). Central and terminal raphe fissures straight (Fig. 113). Striae mainly composed of 3–4 or 4–5 rounded to elongated areolae, sometimes slit-like near the valve margin (Figs 113, 115). Mantle with one row of slit-like areolae (Fig. 113). Presence of two small punctuations, spaced and apically arranged on central area of rapheless valve of some longer valves (Fig. 115). Type:— BRAZIL. São Paulo state: Votorantim, Santa Helena reservoir, epilithon, 23º 34’ 58.56” S, 47º 25’ 50.52” W, E.C. R. Bartozek & D.C.Bicudo, 27th February 2014 (holotype SP-469524! = Fig. 108, isotype: BR-4616). Etymology:— The specific epithet refers to the shape of the apices, which are small and capitate. Taxonomical remarks:— Achnanthidium sieminskae Witkowski, Kulikovskiy & Riaux-Gobin (2012: 65) described from marine environments of the Kerguelen Archipelago, Austral Islands (Witkowski et al. 2012) resemble our population. The Antarctic taxon has linear to narrowly linear-lanceolate valves with almost parallel to slightly convex margins (Van de Vijver & Kopalová 2014), whereas A. parvicapitatum sp. nov. has more lanceolate valves with clearly convex margins, with the central portion evidently larger than the constriction before the apices, and sometimes larger than the apices. The apices in A. sieminskae can also be rostrate (Van de Vijver & Kopalová 2014), differing from our populations. Achnanthidium caledonicum (Lange-Bertalot) Lange-Bertalot (1999: 277) can be confused with A. parvicapitatum sp. nov. The main difference between these two species is the broadly capitate apices in A. caledonicum, while the Brazilian species present small capitate to subcapitate specimens. Another difference is that the valves of A. caledonicum are linear with parallel margins or sometimes inflated just at the centre, while A. parvicapitatum sp. nov. is linear-lanceolate and narrower. Previously, one Austrian specimen, in a mixed population, closely resembling A. parvicapitatum sp. nov. was illustrated in Lange-Bertalot & Krammer (1989, pl. 55, Fig. 4) and identified as Achnanthes minutissima var. scotica (J.R.Carter) Lange-Bertalot (in Lange-Bertalot & Krammer 1989: 106; currently considered a synonym of Achnanthidium caledonicum). Specimens incorrectly identified as A. caledonicum are very similar (Krammer & Lange-Bertalot 2004, pl. 34, Figs 4–6) and an investigation of those specimens is currently being carried out. Furthermore, similar German specimens were also identified as Achnanthidium cf. caledonicum (pl. 24, Figs 58–60 in Lange-Bertalot et al. 2017). Achnanthidium neomicrocephalum Lange-Bertalot & F.Staab (in Krammer & Lange-Bertalot 2004: 431) bears some similarities with A. parvicapitatum sp. nov., such as linear-lanceolate valves with capitate to subcapitate apices. Its slender valve shape with longer, narrower valves (22–38 µm long, 2.5–2.8 µm wide, Krammer & Lange-Bertalot 2004) allows their separation, even though there are some overlaps in size ranges reported (21.6–25.3 μm long, 2.5–2.8 μm wide, by Silva-Lehmkuhl et al. 2019). Another distinction between the taxa is the subtle shoulders observed in the Brazilian populations but not present in A. neomicrocephalum. Recently described from China, Achnanthidium longissimum P.Yu, Q. -M.You & Kociolek (in Yu et al. 2018: 340) was compared in its protologue to A. caledonicum and A. neomicrocephalum. Despite the similar valve outline to A. parvicapitatum sp. nov., A. longissimum has capitate apices, and the raphe’s terminal fissures are strongly bent towards the same side of the valve. This feature is clearly visible in LM images. Achnanthidium longissimum also has larger valves and a reduced stria density (36–48 μm long, 4.0–4.5 μm wide, 22–25 striae in 10 μm, Yu et al. 2018) when compared to the new Brazilian taxon. Another similar species to A. parvicapitatum sp. nov. is Achnanthidium digitatum Pinseel, Vanormelingen, P.B.Hamilton & Van de Vijver (in Pinseel et al. 2017: 271). However, A. digitatum has smaller, narrower valves and a higher stria density (8.6–19.1 μm long, 1.8–2.3 μm wide, 31–36 striae in 10 µm, Pinseel et al. 2017). Additionally, the valves of A. digitatum are more linear, with margins almost parallel, and the striae are mostly composed of two areolae, an observation that is different from the population described here. In our study area, A. parvicapitatum sp. nov. co-occurred with two taxa that are sometimes difficult to separate using LM: A. brasiliense sp. nov. and A. tropicocatenatum. In general, smaller specimens are difficult to discern, however, A. parvicapitatum sp. nov. is usually slender and has less lanceolate valve margins than A. brasiliense sp. nov. They also have slightly distinct apices, while the latter taxon presents more rostrate than subcapitate apices. Concerning A. tropicocatenatum, it is smaller and presents visibly inflated valves in the median portion. The girdle view is also slightly arched, but the apices in A. tropicocatenatum are more strongly and sharply recurved to the rapheless valve. Distribution and ecological information:— Achnanthidium parvicapitatum sp. nov. was distributed in all the habitats, occurring in 20 % of the counted samples (highest abundance: 36 % in SP-428935). In all the communities, the species showed ecological preferences for slightly acid (pH optimum of 6.6–6.8), low to medium cond. (optimum of 56–74 μS∙ cm-1), and oligo- to mesotrophic water conditions (TP optimum of 14.1–22.5 μg∙L- 1 and TN optimum of 321.4–468.2 μg∙L- 1)., Published as part of Costa, Lívia F., Wetzel, Carlos E., Maquardt, Gisele C., Zanon, Jaques E., Ector, Luc & Bicudo, Denise C., 2022, Taxonomy and ecology of Achnanthidium (Bacillariophyta, Achnanthidiaceae) from southeastern Brazil with the description of six new species, pp. 187-223 in Phytotaxa 575 (3) on pages 201-202, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242, {"references":["Witkowski, A., Kulikovskiy, M. & Riaux-Gobin, C. (2012) Achnanthidium sieminskae, a new diatom species from the Kerguelen Archipelago (Austral Islands). In: Wolowski, K., Kaczmarska, I., Ehrman, J. M. & Wojtal, A. Z. (Eds.) Current Advances in Algal Taxonomy and its Applications. W. Szafer Institute of Botany, Polish Academy of Sciences, Krakow, pp. 61 - 68.","Van de Vijver, B. & Kopalova, K. (2014) Four Achnanthidium species (Bacillariophyta) formerly identified as Achnanthidium minutissimum from the Antarctic Region. European Journal of Taxonomy 79: 1 - 19. https: // doi. org / 10.5852 / ejt. 2014.79","Lange-Bertalot, H. (1999) Neue Kombinationen von Taxa aus Achnanthes Bory (sensu lato). In: Lange-Bertalot, H. (Ed.), Iconographia Diatomologica. Annotated Diatom Micrographs. Vol. 6. Phytogeography - Diversity - Taxonomy. Koeltz Scientific Books, Konigstein, Germany, pp. 276 - 289.","Lange-Bertalot, H. & Krammer, K. (1989) Achnanthes eine Monographie der Gattung mit Definition der Gattung Cocconeis und Nachtragen zu den Naviculaceae. Bibliotheca Diatomologica 18: 1 - 393.","Krammer, K. & Lange-Bertalot, H. (2004) Bacillariophyceae 4. Teil: Achnanthaceae, Kritische Erganzungen zu Achnanthes s. l., Navicula s. str., Gomphonema. Gesamtliteraturverzeichnis Teil 1 - 4. In: Ettl, H., Gartner, G., Gerloff, J., Heynig, H. & Mollenhauer, D. (Eds.) Susswasserflora von Mitteleuropa Vol. 2 / 4. Spektrum Akademischer Verlag, Heidelberg-Berlin, pp. 1 - 468.","Lange-Bertalot, H., Hofmann, G., Werum, M. & Cantonati, M. (2017) Freshwater benthic diatoms of Central Europe: over 800 common species used in ecological assessment. English edition with updated taxonomy and added species. In: Cantonati, M., Kelly, M. G. & Lange-Bertalot, H. (Eds.) Koeltz Botanical Books, Schmitten-Oberreifenberg, pp. 1 - 942.","Silva-Lehmkuhl, A. M., Tremarin, P. I., Vercellino, I. S. & Ludwig, T. A. V. (2019) Periphytic diatoms from an oligotrophic lentic system, Piraquara I reservoir, Parana state, Brazil. Biota Neotropica 19 (2): e 20180568. https: // doi. org / 10.1590 / 1676 - 0611 - bn- 2018 - 0568","Yu, P., Kociolek, J. P., You, Q. & Wang, Q. (2018) Achnanthidium longissimum sp. nov. (Bacillariophyta), a new diatom species from Jiuzhai Valley, Southwestern China. Diatom Research 33 (3): 339 - 348. https: // doi. org / 10.1080 / 0269249 X. 2018.1545704","Pinseel, E., Vanormelingen, P., Hamilton, P. B., Vyverman, W., Van de Vijver, B. & Kopalova, K. (2017) Molecular and morphological characterization of the Achnanthidium minutissimum complex (Bacillariophyta) in Petuniabukta (Spitsbergen, High Arctic) including the description of A. digitatum sp. nov. European Journal of Phycology 52 (3): 264 - 280. https: // doi. org / 10.1080 / 09670262.2017.1283540"]}

Achnanthidium acutum L.F.Costa, C.E.Wetzel & Ector sp. nov. (Figs 2–25) Description:—LM: Valves narrowly linear-elliptic, with acutely rounded to very slightly rostrate apices (Figs 2–20); 9.3–21.2 µm long, 2.2–3.2 µm wide. Raphe valve: axial area narrow and linear; central area rounded, sometimes forming a uni- or bilateral rectangular fascia, with no or one shortened stria on both sides of the valve (Figs 2–12). Raphe filiform straight. Transapical striae slightly radiate throughout the entire valve, more spaced in the central area, becoming denser and more strongly radiate towards the apices; 27–30 in 10 µm (Figs 2–12). Rapheless valve: axial area narrow and linear; central area absent, with 1–2 shortened striae on both sides of the valve (Figs 13–20). Transapical striae slightly radiate throughout the entire valve, more spaced in the central area, and becoming denser and more strongly radiate towards the apices; 28–30 in 10 µm (Figs 13–20). SEM: Raphe terminating after the last stria (Figs 21, 22). Central and terminal raphe fissures straight (Figs 21, 22). Striae mainly composed of 3–4 or 4–5 rounded to elongated areolae (Figs 21–25), sometimes slit-like near the valve margin (Figs 21–23). Mantle with one row of slit-like areolae (Figs 21–23). Type:— BRAZIL. São Paulo state: São Paulo, Rio Grande reservoir, epiphyton (Salvinia), 23° 45’ 59.46” S, 46° 30’ 35.7” W, S. Wengrat & D. C.Bicudo, 5th August 2009 (holotype SP-427898! = Fig. 17, isotype: BR-4614). Etymology:— The specific epithet refers to the acutely rounded apices format of the species. Taxonomical remarks:— Concerning the characteristic linear valve shape, A. lineare resembles A. acutum sp. nov. However, A. lineare differs from Brazilian populations mostly by the almost parallel margins and broadly rounded apices in the raphe valves, whereas A. acutum sp. nov. shows narrowly linear-elliptic valves, with acutely rounded to very slightly rostrate apices. Also, the always-rectangular fascia in the central area of the raphe valve in A. lineare contrasts to the nearly rounded central area of A. acutum sp. nov. when observed in LM. Additional remarks include the density of areolae per striae in 10 µm, higher in A. acutum sp. nov. (up to 5) when compared to A. lineare (1– 3). This latter feature can be useful especially regarding the rapheless valve differentiation. The separation of both species was corroborated through morphological analysis between them. The results are presented and discussed in the ‘Morphological patterns in valve shape from EFD analysis’ section. Van de Vijver et al. (2011) analyzed A. lineare type material and described two new species: A. pseudolineare Van de Vijver, Novais & Ector (in Van de Vijver et al. 2011: 186) and A. sublineare Van de Vijver, Jarlman & Ector (in Van de Vijver et al. 2011: 179). Both taxa are described as having a linear outline of the raphe valve. Furthermore, Achnanthidium pseudolineare clearly has a denser number of striae in 10 µm (35–40) compared to A. acutum sp. nov. (27–30). Similarly, A. sublineare has a higher number of striae in 10 µm (33–34), a wider and lanceolate axial area in the rapheless valve, 1–2 areolae per striae and narrower valves (1.5–2.1 µm wide). Besides, this taxon possesses the central area formed by a rectangular fascia in the raphe valve, comparable to those in A. lineare. Achnanthidium petuniabuktianum Pinseel, Van de Vijver & Kopalová (2015: 68) also presents linear valves with rounded apices, but in SEM images, it shows a distinct wide axial area and striae with just 1–2 areolae, differing from A. acutum sp. nov. Also, the terminal raphe fissures are bent to the same side in A. petuniabuktianum while straight in our species. In Brazilian waters, A. brasiliense sp. nov. can co-occurs with A. acutum sp. nov. but can be distinguished by its lanceolate valves with rostrate apices and the linear valves with acutely rounded apices of A. acutum sp. nov. From the Antarctic region, two species closely resemble A. acutum sp. nov. but differ by possessing distinctly protracted and rostrate apices. The first one, A. maritimo-antarcticum Van de Vijver & Kopalová (2014: 6), also has the central area of the rapheless valve in disagreement with A. acutum sp. nov. morphology: it is weakly elliptical, whereas in the Brazilian species a central area is absent. On the other hand, A. indistinctum Van de Vijver & Kopalová (2014: 4) received this name by presenting small frustules (length 8.5–13.0 μm, width 1.8–2.2 μm, Van de Vijver & Kopalová 2014), a feature that distinguish it from our species, since A. acutum sp. nov. has longer and wider valves. In addition, besides the higher stria density (30–35 in 10 μm) than Brazilian taxa, slit-like areolae are apparently never present in A. indistinctum, a feature observed in marginal areolae of some valves in our population. Distribution and ecological information:— Achnanthidium acutum sp. nov. was found in all three habitats (i.e. periphyton, sediment and plankton) and present in 23 % of the quantified samples, being more abundant in periphytic samples (highest abundance: 12 % in SP427898). Due to its wide distribution, the ecological optimum could be calculated. The ecological preference of the species was for neutral pH (optimum of 6.8–7.1), medium to high cond. (optimum of 76.7–167.3 μS∙ cm-1), and meso- to eutrophic waters (TP optimum of 39.0–77.8 μg∙L- 1 and TN optimum of 647.7–1627.9 μg∙L- 1).

Achnanthidium peetersianum C.E.Wetzel, Jüttner & Ector in Wetzel et al. (2019: 345) (Figs 144–165) Morphometric features:— 7.6–12.3 µm long; 2.0–2.7 µm wide; 29–34 striae in 10 µm; striae mainly composed of 3–5 areolae. Taxonomical remarks:— Our population is in agreement with the type species, besides it has slightly narrower valves when compared to the French population (2.8–3.3 μm, Wetzel et al. 2019). This species has narrow valves with subcapitate to small capitate apices, similar to A. macrocephalum (Hustedt) Round & Bukhtiyarova (1996: 349), although the latter taxon presents broadly capitate apices. Despite being difficult to separate, a reevaluation of A. macrocephalum type material was recently published in the same study as A. peetersianum (Wetzel et al. 2019). Using SEM micrographs, the authors showed that A. peetersianum presented predominantly rounded areolae and had a higher density per stria, while A. macrocephalum striae are composed of 1–2 elongated areolae of different lengths. Moreover, the axial area of the rapheless valve of the latter taxon is expanded, widening towards the central area. Recently, Yu et al. (2019) described two small species that resemble ours: A. subtilissimum P.Yu, Q. -M.You & Q.-X.Wang (in Yu et al. 2019: 161) and A. limosua P.Yu, Q. -M.You & Q.-X.Wang (in Yu et al. 2019: 158). The first species is likely a synonym of A. macrocephalum, and the second presents visibly larger valves (3.2–4.0 µm) than A. peetersianum. Achnanthidium indistinctum also bears several similarities with A. peetersianum. They can be distinguished in girdle view by the apparently thickened striae in the first taxon, which is not visible in our specimens (see Figs 158, 159). Concerning the valve view, A. indistinctum presents rostrate apices and an almost nonexistent central area that never forms a fascia. On the other hand, A. peetersianum exhibits subcapitate to capitate apices and a small rounded central area that rarely forms a fascia. Notably, SEM analysis can also help separate both taxa due to the distal raphe ends straight in A. indistinctum and bent in our specimens. Despite the slightly narrower valves in the latter taxon (1.8–2.2 µm, Van de Vijver & Kopalová 2014), the metric features of these two species overlap. Achnanthidium reimeri (Camburn) Ponader & Potapova (2007: 235) has a similar valve outline with our population, however, it presents longer and wider valves and a rhomboid central area on the rapheless valve (Ponader & Potapova 2007). Distribution and ecological information:— Recently described from Le Sauvigny River in France, it was one of the most well-distributed species found in this study (40 % of occurrence). It is plausible that this taxon was previously identified in Brazilian publications as A. macrocephalum. In this study, A. peetersianum occurred in all of the sampled habitats. Its ecological preference was for slightly acidic (pH optimum of 6.6–6.7), electrolytic-poor (optimum of 28.2–49 μS∙ cm-1) and oligo- to mesotrophic waters (TP optimum of 14.9–27.1 μg∙L- 1, TN optimum of 374.4–519.4 μg∙L- 1). We also observed this species in enriched waters (TP> 50 μg∙L- 1).

Achnanthidium minutissimum (Kütz.) Czarn. (Fig. 14) Memoirs of the California Academy of Sciences 17: 157 (Czarnecki 1994). — Basionym: Achnanthes minutissima Kütz., Linnaea 8 (5): 578, pl. 16, fig. 54 (Kützing 1833); Kützing, 50, pl. 4, fig. 54 (Kützing 1834); Kützing, Dec. VIII, no. 75 (Kützing 1834), in Kützing, Algarum Aquae Dulcis Germanicum, Decades I-XVI, numbers 1-160. Halis Saxonum, 1833-1836. DESCRIPTION LM (Fig. 14 A-AW) Frustules slightly arched, with convex RL, concave RV. Valves linear-lanceolate. Margins tapering gradually towards short protracted rounded, rostrate poles. Valve dimensions (n = 46): length 8.5-15 µm, width 2.5-3.5 µm. Central area on raphe valve varies from small rounded to a narrow rectangular fascia on one or both sides. Striae more widely spaced in valve centre. Raphe filiform, straight, central pores slightly widened, terminating close to margin of central area. Central area on rapheless valve indistinct or narrow elliptical. Axial area narrow linear on both valves, slightly widening towards valve centre. Striae radiate throughout on both valves, 30-35 in 10 µm (RV and RL), becoming denser at poles., Published as part of Jüttner, Ingrid, Hamilton, Paul B., Wetzel, Carlos E., De, Bart Van, Vijver, King, Lydia, Kelly, Martyn G., Williams, David M. & Ector, Luc, 2022, A study of the morphology and distribution of four Achnanthidium Kütz. species (Bacillariophyta), implications for ecological status assessment, and description of two new European species, pp. 147-176 in Cryptogamie, Algologie 20 (10) on page 155, DOI: 10.5252/cryptogamie-algologie2022v43a10, http://zenodo.org/record/7819378, {"references":["CZARNECKI D. B. 1994. - The freshwater diatom culture collection at Loras College, Dubuque, Iowa, in KOCIOLEK J. P. (ed.), Proceedings of the 11 th International Diatom Symposium, San Francisco, 12 - 17 August 1990. Memoirs of the California Academy of Sciences 17: 155 - 174.","KUTZING F. T. 1833. - Synopsis Diatomacearum oder Versuch einer systematischen Zusammenstellung der Diatomeen. Linnaea 8 (5): 529 - 620."]}

Achnanthidium neomicrocephalum Lange-Bert. & F.Staab (Figs 9-13) Süsswasserflora von Mitteleuropa, Bacillariophyceae. 4. Teil Achnanthaceae: 431, pl. 89, figs 1-13 (Krammer & Lange-Bertalot 2004). DESCRIPTION LM (Fig. 9 A-DZ) Frustules slightly arched, with convex RL, concave RV.Valves linear-lanceolate. Margins slightly tapering towards small protracted, capitate poles. Valve margins slightly inflated in valve centre, sometimes more strongly on one side. Valve dimensions (n = 31): length 12-30.5 µm, width at valve centre 2-2.5 µm. On RV, central area small rounded, bordered by 1-2 slightly shortened, more widely spaced striae on each side, or small transverse fascia. Raphe filiform, straight, central pores terminating close to margin of central area. Central area on rapheless valve indistinct. Axial area narrow, linear on both valves, slightly widening towards valve centre. Striae radiate throughout on both valves, 30-32 in 10 µm (RV and RL). SEM (Figs 10 A-H; 11A-F; 12A-H; 13A-D) Raphe located on slightly raised sternum, terminal raphe fissures straight terminating at valve/mantle junction. Internally, central raphe endings shortly deflected to opposite sides. Striae composed of 2-3 areolae (RV and RL). Areolae round or oval adjacent to axial area, sometimes slit-like adjacent to valve margin. Row of transapically elongated areolae on mantle separated from areolae on valve face by hyaline area.

Achnanthidium lacuslustense sp. nov. (Figs 17; 18) HOLOTYPE. — Germany. Lustsee, 14.IV.2013, I. Jüttner (holo-, NMW [NMW.C.2013.010.2013]; iso-, BM, BR, CANA). TYPE LOCALITY. — Germany. Upper Bavaria, Osterseen area, Lustsee, 47°48’33.509”N, 11°17’36.959”E. HABITAT. — Epiphytic and epipsammic in lake littoral. ETYMOLOGY. — The species was named after the location where it was found, the Lustsee in Upper Bavaria, Germany. REGISTRATION. — http://phycobank.org/103380 DESCRIPTION LM (Fig. 17 A-M) Frustules with strongly concave RV and convex RL with recurved poles.Valves lanceolate,margins tapering gradually towards large capitate poles.Width of poles slightly less or as wide as the maximum width at valve centre. Valve dimensions (n = 14): length 15.5-29.0 µm, width at valve centre 3-4 µm.Central area on RV a moderately broad fascia.Raphe filiform, straight.Central pores slightly expanded, terminating at margin of central area.Central area on RL rectangular, bordered by 2 short striae on each side. Axial area narrow linear on both valves. Striae radiate throughout on both valves, 28-30 in 10 µm (RV), 30 in 10 µm (RL). SEM (Fig. 18 A-H) Raphe located on raised sternum, central raphe endings slightly expanded, tear drop-shaped, terminal raphe fissures straight with tear drop-shaped ends, terminating at valve/ mantle junction. Internally, central raphe endings very slightly deflected to opposite sides, terminating on side of central nod - ule, terminal raphe endings form small helictoglossae. Striae straight on RV, composed of 3-4, rarely 2, areolae. Areolae round adjacent to axial area and at poles, others often slit-like. On RL striae straight, slightly curved at poles, composed of 3-4, rarely 2 or 5, areolae. Areolae mostly round adjacent to axial area, elsewhere often transapically elongate, internally occluded by individual hymenes. A row of transapically elongated areolae on mantle separated from areolae on valve face by a hyaline area.

Achnanthidium tirolense sp. nov. (Figs 15; 16) HOLOTYPE. — Austria. Plansee, 18.IV.2019, I. Jüttner (holo-, NMW [NMW.C.2021.06.2019]; iso-, BM, BR, CANA). TYPE LOCALITY. — Austria. Tyrol, Kleiner Plansee, Plansee, 47°28’34.756”N, 10°46’50.861”E. HABITAT. — Epiphytic and epilithic in lake littoral. ETYMOLOGY. — The species was named after the region where it was found, Tyrol, Austria. REGISTRATION. — http://phycobank.org/103379 DESCRIPTION LM (Fig. 15 A-AG) Valves rhombic-lanceolate with slightly inflated valve centre. Margins tapering gradually towards long protracted, broadly rounded, rostrate poles. Valve dimensions (n = 25): length 14-23.5 µm, width at valve centre 3-4 µm. Central area on RV an acute-angled fascia. Raphe filiform, straight. Central pores slightly expanded, distantly placed at margin of central area. Central area on RL narrow lanceolate, sometimes a narrow acute-angled fascia on one or both sides formed by more widely spaced central striae. Axial area narrow linear on both valves. Striae radiate throughout on both valves, 28-30 in 10 µm (RV and RL). SEM (Fig. 16 A-F) Raphe located on slightly raised sternum, central raphe endings small, round, terminal raphe fissures straight with tear drop-shaped ends, terminating at valve/mantle junction. Internally, central raphe endings slightly deflected to opposite sides, terminal raphe endings form small helictoglossae. Striae straight or slightly curved, on RV composed of 3-5 areolae, on RL composed of 2-5 areolae. Areolae round, those adjacent to valve margin round or slit-like. One row of transapically elongated areolae on mantle separated from areolae on valve face by a hyaline area.

Achnanthidium neomicrocephalum Lange-Bert. & F.Staab (Figs 9-13) Süsswasserflora von Mitteleuropa, Bacillariophyceae. 4. Teil Achnanthaceae: 431, pl. 89, figs 1-13 (Krammer & Lange-Bertalot 2004). DESCRIPTION LM (Fig. 9 A-DZ) Frustules slightly arched, with convex RL, concave RV.Valves linear-lanceolate. Margins slightly tapering towards small protracted, capitate poles. Valve margins slightly inflated in valve centre, sometimes more strongly on one side. Valve dimensions (n = 31): length 12-30.5 µm, width at valve centre 2-2.5 µm. On RV, central area small rounded, bordered by 1-2 slightly shortened, more widely spaced striae on each side, or small transverse fascia. Raphe filiform, straight, central pores terminating close to margin of central area. Central area on rapheless valve indistinct. Axial area narrow, linear on both valves, slightly widening towards valve centre. Striae radiate throughout on both valves, 30-32 in 10 µm (RV and RL). SEM (Figs 10 A-H; 11A-F; 12A-H; 13A-D) Raphe located on slightly raised sternum, terminal raphe fissures straight terminating at valve/mantle junction. Internally, central raphe endings shortly deflected to opposite sides. Striae composed of 2-3 areolae (RV and RL). Areolae round or oval adjacent to axial area, sometimes slit-like adjacent to valve margin. Row of transapically elongated areolae on mantle separated from areolae on valve face by hyaline area., Published as part of Jüttner, Ingrid, Hamilton, Paul B., Wetzel, Carlos E., De, Bart Van, Vijver, King, Lydia, Kelly, Martyn G., Williams, David M. & Ector, Luc, 2022, A study of the morphology and distribution of four Achnanthidium Kütz. species (Bacillariophyta), implications for ecological status assessment, and description of two new European species, pp. 147-176 in Cryptogamie, Algologie 20 (10) on page 155, DOI: 10.5252/cryptogamie-algologie2022v43a10, http://zenodo.org/record/7819378

Jüttner, Ingrid, Hamilton, Paul B., Wetzel, Carlos E., De, Bart Van, Vijver, King, Lydia, Kelly, Martyn G., Williams, David M., Ector, Luc (2022): A study of the morphology and distribution of four Achnanthidium Kütz. species (Bacillariophyta), implications for ecological status assessment, and description of two new European species. Cryptogamie, Algologie 20 (10): 147-176, DOI: 10.5252/cryptogamie-algologie2022v43a10

Achnanthidium caledonicum (Lange-Bert.) Lange-Bert. (Figs 2-4) Iconographia Diatomologica 6: 270-283 (Lange-Bertalot 1999). — Achnanthes microcephala f. scotica J.R.Carter in Carter & Bailey- Watts, Nova Hedwigia 33: 534, pl. 1, fig. 31 (Carter & Bailey-Watts 1981). — Achnanthes minutissima var. scotica (J.R.Carter) Lange- Bert.in Lange-Bertalot & Krammer, Bibliotheca Diatomologica 18: 106, pl. 55, figs 4-12, pl. 57, fig. 11, pl. 67, figs 35-56 (Lange-Bertalot & Krammer 1989). — Achnanthes scotica (J.R.Carter) Lange-Bert., Bibliotheca Diatomologica 27: 8 (Lange-Bertalot 1993), nom. illeg., non Achnanthes scotica Flower & V.J.Jones, Diatom Research 4: 228, figs 1-7, 42-53 (Flower & Jones 1989). — Achnanthidium minutissimum var. scoticum (J.R.Carter) H.Cremer in Cremer & Wagner, Polar Biology 26: 108 (Cremer & Wagner 2003). — Basionym: Achnanthes caledonica Lange-Bert. in Lange-Bertalot & Moser, Bibliotheca Diatomologica 29: 95 (Lange-Bertalot & Moser 1994). DESCRIPTION LM (Fig. 2 A-AN) Frustules moderately arched, with convex rapheless valves, concave raphe valves, and recurved poles. Valves linear with large, subcapitate to capitate poles, the latter sometimes wider than the maximum valve width at valve centre. Valve margins tapering slightly towards a small constriction before poles. At valve centre, margins slightly inflated, sometimes unilaterally, or straight. Valve dimensions (n = 20): length 17.5-37.5 µm, width at valve centre: 3.0-3.5 µm. On raphe valve (RV), shape of central area variable, either a small transverse fascia, or oval to round, bordered by one or several slightly shortened striae. Raphe filiform, straight, central pores hardly widened, distantly placed at margin of central area. Central area on rapheless valve (RL) narrow lanceolate. Axial area narrow linear on both valves. Striae radiate throughout on both valves, more distantly placed in valve centre, 30-32 in 10 µm. SEM (Figs 3 A-F; 4A-D) Raphe located on slightly raised sternum, terminal fissures straight terminating at valve/mantle junction. Internally, central raphe endings shortly deflected to opposite sides, terminal raphe endings form small helictoglossae. Striae composed of 2-4 (RV) and 2-5 (RL) areolae. Areolae round or oval, areolae adjacent to valve margin slit-like. Row of transapically elongated areolae on mantle separated from areolae on valve face by hyaline area. On the type slide (lecto-, BM[BM 94890], ex J.R. Carter 3665), the species is common, but many specimens are positioned in girdle view. The central area is often a fascia, as mentioned by Carter (Carter & Bailey-Watts 1981), but sometimes less conspicuous, irregular, and formed by more widely spaced central striae., Published as part of Jüttner, Ingrid, Hamilton, Paul B., Wetzel, Carlos E., De, Bart Van, Vijver, King, Lydia, Kelly, Martyn G., Williams, David M. & Ector, Luc, 2022, A study of the morphology and distribution of four Achnanthidium Kütz. species (Bacillariophyta), implications for ecological status assessment, and description of two new European species, pp. 147-176 in Cryptogamie, Algologie 20 (10) on pages 152-153, DOI: 10.5252/cryptogamie-algologie2022v43a10, http://zenodo.org/record/7819378, {"references":["FLOWER R. J. & JONES V. J. 1989. - Taxonomic descriptions and occurrences of new Achnanthes taxa in acid lakes in the U. K. Diatom Research 4 (2): 227 - 239 https: // doi. org / 10.1080 / 0269 249 X. 1989.9705072","CREMER H. & WAGNER B. 2003. - The diatom flora in the ultraoligotrophic Lake El'gygytgyn, Chukotka. Polar Biology 26: 105 - 114. https: // doi. org / 10.1007 / s 00300 - 002 - 0445 - 0"]}

Achnanthidium lacuslustense sp. nov. (Figs 17; 18) HOLOTYPE. — Germany. Lustsee, 14.IV.2013, I. Jüttner (holo-, NMW [NMW.C.2013.010.2013]; iso-, BM, BR, CANA). TYPE LOCALITY. — Germany. Upper Bavaria, Osterseen area, Lustsee, 47°48’33.509”N, 11°17’36.959”E. HABITAT. — Epiphytic and epipsammic in lake littoral. ETYMOLOGY. — The species was named after the location where it was found, the Lustsee in Upper Bavaria, Germany. REGISTRATION. — http://phycobank.org/103380 DESCRIPTION LM (Fig. 17 A-M) Frustules with strongly concave RV and convex RL with recurved poles.Valves lanceolate,margins tapering gradually towards large capitate poles.Width of poles slightly less or as wide as the maximum width at valve centre. Valve dimensions (n = 14): length 15.5-29.0 µm, width at valve centre 3-4 µm.Central area on RV a moderately broad fascia.Raphe filiform, straight.Central pores slightly expanded, terminating at margin of central area.Central area on RL rectangular, bordered by 2 short striae on each side. Axial area narrow linear on both valves. Striae radiate throughout on both valves, 28-30 in 10 µm (RV), 30 in 10 µm (RL). SEM (Fig. 18 A-H) Raphe located on raised sternum, central raphe endings slightly expanded, tear drop-shaped, terminal raphe fissures straight with tear drop-shaped ends, terminating at valve/ mantle junction. Internally, central raphe endings very slightly deflected to opposite sides, terminating on side of central nod - ule, terminal raphe endings form small helictoglossae. Striae straight on RV, composed of 3-4, rarely 2, areolae. Areolae round adjacent to axial area and at poles, others often slit-like. On RL striae straight, slightly curved at poles, composed of 3-4, rarely 2 or 5, areolae. Areolae mostly round adjacent to axial area, elsewhere often transapically elongate, internally occluded by individual hymenes. A row of transapically elongated areolae on mantle separated from areolae on valve face by a hyaline area., Published as part of Jüttner, Ingrid, Hamilton, Paul B., Wetzel, Carlos E., De, Bart Van, Vijver, King, Lydia, Kelly, Martyn G., Williams, David M. & Ector, Luc, 2022, A study of the morphology and distribution of four Achnanthidium Kütz. species (Bacillariophyta), implications for ecological status assessment, and description of two new European species, pp. 147-176 in Cryptogamie, Algologie 20 (10) on page 158, DOI: 10.5252/cryptogamie-algologie2022v43a10, http://zenodo.org/record/7819378

We describe a new genus of monoraphid diatom based on Achnanthidium ovatum T.Watanabe et Tuji in Watanabe et al., a species described originally from Japan. We encountered this species from Yunnan Province in the southwestern part to eastern China, as well as in Luding, Sichuan Province and Xianju, Zhejiang Province, in pristine lotic environments. The species differs from the genus Achnanthidium by being asymmetrical to the transapical axis and has a pseudoseptum at the poles of the interior of the raphe valve which is fused with each helictoglossa. Moreover, a distinct group of condensed striae can be found at the narrower pole ('footpole') of the raphe valve. This appears to be the only known genus of freshwater monoraphid diatoms that is restricted to a single continent.

A new monoraphid diatom species Achnanthidium tinea Tseplik, Kulikovskiy, Kociolek & Maltsev, sp. nov. is described from Indonesia. The species is described on the basis of molecular and morphological analyses. According to molecular data the new species belongs to the clade that includes strains of Achnanthidium minutissimum, Achnanthidium saprophilum and Achnanthidium digitatum. Morphologically, the new species differs quite significantly from other species of the same genus because of linear-elliptic valves with almost parallel sides and strongly radiate striae and a butterfly-shaped fascia on the raphe valve. The morphology and phylogeny of the new species are discussed, and thoughts on the current state of the taxonomy of the genus Achnanthidium are expressed. Our work shows the importance of using molecular data in diatom systematics and also demonstrates the need to investigate rarely studied regions of our planet.

Achnanthidium tibeticum Y. Liu & J.P. Kociolek sp. nov. Figures 1���32 Type:����� CHINA. Tibet: Medog County, Yarang Village, Streams, 95��28���22.8 E, 29��29���97.0 N,19 June, 2019. (holotype HANU! Slide no. THTYJZL12, College of Life Science and Technology, Harbin Normal University, Harbin, China. Figs 1 & 2 are of the holotype specimen). Isotype: Material no. YJZL12-12, slide no. YJZL12-12 #. Cleaned material stored in Key laboratory of Algal Biology, Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan, China. Description:����� LM (Figs 1���14): Raphe valve slightly convex, rapheless valve slightly concave. Valves linear to elliptic, with apices rounded. Length 7.9���3.6 ��m, width 2.9���3.9 ��m (n=25). Axial area narrow and linear. On the raphe valve, striae become coarser in the middle of valve, central area is inconspicuous. On the rapheless valves, central area is absent. On the raphe valves, striae slightly radiate in the center, becoming radiate towards the ends; 9���12 per 10 ��m in center of raphe valve, denser near the ends at 18���20 per 10 ��m. On the rapheless valve, striae nearly parallel in the center and become slightly radiate towards the ends; 12���15 per 10 ��m. SEM: Raphe valve (Figs 15���26): Externally, raphe on the valve face never extends onto the mantle, straight, distal raphe ends?-shaped, curved to the same direction (Figs 15���17, 20), proximal raphe ends slightly expanded, teardrop-shaped. Striae uniseriate, areolae almost round, with varying sizes. One row of slit-like areolae present in the mantle. Internally, raphe straight, distal raphe ends terminate in small helictoglossae, proximal raphe ends curved to the opposite directions. Areolae are occluded by fine hymenate structures. Rapheless valve (Figs 27���32): Externally, axial area narrow and linear; central area absent, areolae almost round. One row of slit-like areolae present on the mantle. Internally, sternum slightly raised on the apices, areolae structure similar to the raphe valve, with finely structured occlusions. Etymology:����� Named for the province of the type locality., Published as part of Ge, Danyang, Zhang, Simeng, Liu, Hongyan, Liu, Yan, Kociolek, Patrick John, Zhu, Huan, Liu, Guoxiang & Fan, Yawen, 2022, One new species of Achnanthidium K��tzing (Bacillariophyta, Achnanthidiaceae) from Tibet, China, pp. 268-280 in Phytotaxa 538 (4) on pages 271-275, DOI: 10.11646/phytotaxa.538.4.1, http://zenodo.org/record/6340368

Tibet is a very important biodiversity hot spot in China and the diatom flora of this region has drawn a lot of attention. Diatom samples were collected from several places across Tibet. During research on the biodiversity of diatoms from Tibet, one Achnanthidium species showed unique features compared with other known taxa. It has linear-lanceolate valves with rounded apices, tear-drop shaped proximal raphe ends and ?-shaped distal raphe ends. The most distinctive character of this species, however, is the shape of areolae; since this species has nearly round external areolar openings on the valve face but more typical slit-like areolae present on the mantle. Here we describe it as new to science as Achnanthidium tibeticum Y. Liu & Kociolek. We compare this new species with other, similar taxa within the genus.

A new diatom genus, Gogorevia, is described on the basis of an integrated molecular and morphological investigation. Two species are described from Vietnam: the generitype, G. rinatii sp. nov., and G. ovalis sp. nov. These species share many morphological features with Achnanthes exigua and allied taxa that were previously transferred to the genera Achnanthidium or Lemnicola. Our current molecular investigation shows that Gogorevia is distinct and phylogenetically removed from the above-mentioned genera. Morphologically, Gogorevia differs from Achnanthidium and Lemnicola by the shape of the valves, possessing uniseriate striae, raphe morphology, and the presence of an evident sternum. The recently described species Lemnicola uniseriata is transferred to Gogorevia on the basis that it possesses the same morphological features and phylogenetic position as other members of the genus.

During a survey of benthic diatoms from the coral reefs of the Indian Ocean (Scattered Islands) and Pacific Ocean (Tuamotu Archipelago), an interesting monoraphid diatom was observed and examined by light microscopy and various electron microscopy methods including Focus Ion Beam milling. Our thorough analysis revealed the similarity of this diatom to Bennettella R.W.Holmes, which we reference in the name: Xenobennettella Witkowski & Riaux-Gobin gen. nov., with Xenobennettella coralliensis Witkowski & Riaux-Gobin sp. nov. as the generitype. The type habitat for this new species is the sublittoral coral reef of Juan de Nova in the Mozambique Channel. The sternum valve of the new genus is characterized by an alveolate ultrastructure with the rim of the alveola opening along the valve margin, resembling the sternum valve of Bennettella. Internally, Xenobennettella differs from the latter by possessing a cavum (horseshoe-shaped chamber) on one side of the valve, in a central axial position. The raphe valve of Xenobennettella has small, marginal, apically elongate chambers, which are internally delineated by transapical ribs that are very similar to Bennettella. However, the raphe in the new genus is different from the latter, resembling some Cocconeis and Planothidium with internal raphe endings bent in the opposite direction, while resembling some Planothidium taxa externally by ending on the apical part of the mantle. This contrasts to Bennettella, which has a unique raphe system, with external raphe endings below the apices, a prominent axial structure and a transapically expanded central area. Likewise, the external surface of Bennettella is different from that of the new genus with a complex mantle structure and biseriate striae. In Xenobennettella, the valve mantle of the raphe valve is simple and perforated by areola. The transapical striae occur in the valve margin and the axial area is ornamented along its course with a single row of densely packed areola on both sides. The characteristics of the raphe valve and alveolate sternum valve place the new genus among the Achnanthidiaceae.

The upper Han River plays an important role in the Middle Route of the North to South Water Transfer Project in China. During an investigation of diatom biodiversity in this area, a new benthic monoraphid diatom, Achnanthidium ampliatum sp. nov. was observed from an alkaline tributary. This new species has relatively large valves, with a lanceolate axial area and distal raphe ends curved to the same side on the raphe valve, linear axial area in rapheless valves, with uniseriate striae and round to short slit-like areolae on both valves. We present light and scanning electron microscopic observations to document the new species. A. ampliatum sp. nov. is easily separated from other, similar Achnanthidium and Psammothidum taxa, and we provide comparisons with species that have similar outlines and valve features.

Temporary streams represent ca. half of the river network at the worldwide level, and in regions such as southern Europe their dominance of the landscape is evident (Larned et al. 2010, McDonough et al. 2011, Barbosa et al. 2020). Currently, these streams are receiving increased attention due to their importance as high diversity spots for algae, arthropods, plants, etc., and due to the ecosystem services they provide (Acuña et al. 2014, Datry et al. 2018b). Because of their geographical position and the land use changes in their watersheds (increased urbanization, agriculture, etc.), climate change, contamination and habitat fragmentation constitute forces threatening their integrity and ecological characteristics (Datry et al. 2018a, Bonada et al. 2020).

Приводятся результаты анализа литературных и оригинальных данных по экологии и географическому распространению 5 видов рода Eucocconeis P.T. Cleve во флоре водорослей Западной Сибири. Для каждого вида приводятся сведения по отношению к галобности, активной реакции воды, сапробности и географическому распространению.

We describe three new Achnanthidium species, A. lacustre sp. nov., A. sublanceolatum sp. nov., and A. taipingensis sp. nov., from Taiping Lake, Anhui Province (China) based on light and scanning electron microscopy. A. lacustre sp. nov. belongs to the "A. minutissimum complex" of the genus, based on it having straight external distal raphe fissures and round to elliptical areolae. Both A. sublanceolatum sp. nov., and A. taipingensis sp. nov. belong to the "A. pyrenaicum complex" of the genus, based on them having transpically-elongated areolae and deflected external distal raphe fissures. All three species are sufficiently different from other similar species based on valve outline, shape of the axial and center areas, and striae density. These three new species are all observed in benthic collections from Taiping Lake.

A new freshwater species, Achnanthidium longissimum sp. nov., is described from Jiuzhai Valley, Sichuan Province (Southwest China), based on light and scanning electron microscopy. This species belongs to the ‘Achnanthidium pyrenaicum complex’, based on having transpically elongated areolae and deflected distal raphe fissures. Achnanthidium longissimum sp. nov. possesses a unique combination of features that includes having the longest valves in the genus (nearly 50 µm long), capitate ends, and raphe vestiges near the apices of the rapheless valve. Furthermore, the mantle has a single row of linear areolae, and a narrow hyaline area at the valve face – mantle junction on both valves. Achnanthidium longissimum is compared with morphologically similar species. The new species is known from epilithic and epiphytic collections from three alkaline lakes in Jiuzhai Valley.

A population of an unknown Psammothidium species (Bacillariophyta, Achnanthidiaceae) was found in core sediments collected from Cimera Lake, an oligotrophic, undisturbed mountain lake in Central Spain (Gredos mountain range). The morphology and ultrastructure of this taxon is hereby documented in detail by means of light (LM) and scanning electron (SEM) micrographs. Morphologically, the closest taxon is P. levanderi, and the type of this species is analyzed here to provide a differential diagnosis. A comparison with other similar small Psammothidium species with an elliptic outline show that the combination of features exhibited by this taxon is unique and it is thus described here as Psammothidium toroi sp. nov.

Planothidium rostrolanceolatum Van de Vijver, Kopalová & Zidarova Figures 3 (55, 56), 5(6) Morphometric information: length: 15.2–23 μm; width: 6–8 μm; striae in 10 µm: 13–15, Published as part of Ferreira da Silva, Juliana, Oliveira, Maria Angélica, Paidano Alves, Rodrigo, Vestena Cassol, Ana Paula, Ribeiro da Anunciação, Raylane, Pereira da Silva, Eduardo, Schünemann, Adriano Luis & Batista Pereira, Antônio, 2019, Geographic distribution of epilithic diatoms (Bacillariophyceae) in Antarctic lakes, South Shetland Islands, Maritime Antarctica Region, pp. 797-809 in Check List 15 (5) on page 806, DOI: 10.15560/15.5.797

Planothidium capitatum (O. Muller) Van de Vijver, Kopalová, Wetzel & Ector Figures 3 (52), 5(5) Morphometric information: length: 15–22 μm; width: 4.3–5 μm; striae in 10 µm: 14–15, Published as part of Ferreira da Silva, Juliana, Oliveira, Maria Angélica, Paidano Alves, Rodrigo, Vestena Cassol, Ana Paula, Ribeiro da Anunciação, Raylane, Pereira da Silva, Eduardo, Schünemann, Adriano Luis & Batista Pereira, Antônio, 2019, Geographic distribution of epilithic diatoms (Bacillariophyceae) in Antarctic lakes, South Shetland Islands, Maritime Antarctica Region, pp. 797-809 in Check List 15 (5) on page 806, DOI: 10.15560/15.5.797

Planothidium australe (Manguin) Le Cohu Figure 3 (50, 51) Morphometric information: length: 21–32 μm; width: 6–8; striae in 10 µm: 15–20, Published as part of Ferreira da Silva, Juliana, Oliveira, Maria Angélica, Paidano Alves, Rodrigo, Vestena Cassol, Ana Paula, Ribeiro da Anunciação, Raylane, Pereira da Silva, Eduardo, Schünemann, Adriano Luis & Batista Pereira, Antônio, 2019, Geographic distribution of epilithic diatoms (Bacillariophyceae) in Antarctic lakes, South Shetland Islands, Maritime Antarctica Region, pp. 797-809 in Check List 15 (5) on page 806, DOI: 10.15560/15.5.797

The results of the analysis of literature and original data on ecology and geographical distribution of 5 Eucocconeis P.T. Cleve species in the algal flora of West Siberia are given. Information relevant to halobility, active water reaction, saprogenity and geographical distribution is presented for each of species., Приводятся результаты анализа литературных и оригинальных данных по экологии и географическому распространению 5 видов рода Eucocconeis P.T. Cleve во флоре водорослей Западной Сибири. Для каждого вида приводятся сведения по отношению к галобности, активной реакции воды, сапробности и географическому распространению.

Thirteen Planothidium species are illustrated and discussed in a detailed morphological account based on light and scanning electron microscopy analysis of modern and historic materials related to the names Planothidium rostratum (Ostrup) Lange-Bertalot and Planothidium frequentissimum (Lange-Bertalot) Lange-Bertalot. Eight species from freshwater enviromnents in Europe, Asia and South America are proposed as new. All taxa here discussed and illustrated belong to the group of species characterized by the presence of a cavum in the rapheless valve. Additional information concerning their distribution and ecology is briefly commented based on a thorough literature revision.

International audience; A small marine benthic diatom pertaining to the order Achnanthales (Bacillariophyta) was observed from the Scattered Islands (“Eparses”, Mozambique Channel, Indian Ocean) and from Napuka Atoll (North of the Tuamotu Archipelago, South Pacific). The latter taxon has unique characteristics (e.g., raphe valve striae composed of small areolae versus macroareolae on the sternum valve, low and plain mantle, simply hooked terminal raphe endings, high and complex cingulum composed of plain and open cingular bands, presence of a vestigial raphe) matching none of the genera split from Achnanthidium Kützing sensu lato (e.g., Planothidium Round & Bukhtiyarova, Karayevia Round & Bukhtiyarova ex Round, Psammothidium Bukhtiyarova & Round, Rossithidium Round & Bukhtiyarova), neither the new genus Madinithidium Witkowski, Desrosiers & Riaux-Gobin from which the striae of both valves are composed of macroareolae and the terminal raphe endings are doubly hooked. Pseudachnanthidium megapteropsis sp. nov and gen. nov is proposed. Due to its small size and to the fact that its striation is not discernible using light microscopy, the new taxon probably escaped the attention of taxonomists in the past. This taxon is found along with Madinithidium flexuistriatum that has close but distinctive morphology, clearly marked off under scanning electron microscope (SEM). Observation with a SEM is essential to differentiate the two latter small-celled and elusive taxa that have overlapping ecological niches. It remains unknown whether Pseudachnanthidium megapteropsis is a cosmopolitan-tropical taxon or if its environmental tolerance is wider. Two other small taxa possibly pertain to the same new genus: one from Reunion Island (Mascarene Archipelago, Indian Ocean) and the second from the Scattered Islands (Mozambique Channel).

Results of light and scanning electron microscopy investigations of valves of two freshwater achnanthoid diatoms are provided. Based on the structure of external terminal raphe endings, represented by simple pores, and cocconeoid habitus Achnanthes joursacensis Hérib. is transferred to the genus Platessa. Achnanthes rostellata A. Cleve is transferred to the genus Lemnicola due to presence of stauros on raphid valve and external terminal raphe fissures curved in opposite directions.