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Planothidium marganaiensis G.G.Lai, Ector & C.E.Wetzel sp. nov. (Figs 2���65) Light microscopy (Figs 2���57):��� Frustules in girdle view rectangular, mostly single or forming short chains (up to two cells), clearly bent in the middle (Figs 29 ��� 32). Valves elliptical to weakly lanceolate with convex margins and subrostrate or slightly protracted apices (never capitate). Valve dimensions (n=59): length 5.0���12.3 ��m, width 2.8���4.0 ��m. Rapheless valve (Figs 2���28): Axial area linear, narrow and straight, sometimes lanceolate and slightly expanded in the middle of the valve. Central area asymmetrical with a unilateral small horseshoe-shaped hyaline area presenting a cavum. On the opposite side, absence of shortened striae or presence of one to two slightly shortened striae. Striae weakly radiate throughout the entire valve, 11���14 in 10 ��m, measured at the central part of the valve opposite to the unilateral expansion. Raphe valve (Figs 33���57): Axial area linear and narrow, slightly widened near the middle of the valve. Central area transapically rectangular to rounded, bordered by one to three shortened striae on each side, usually more distant than the others. Raphe branches straight with expanded, drop-like proximal raphe endings. Terminal raphe fissures not discernible in LM. Striae radiate throughout the entire valve, 11���14 in 10 ��m. Scanning electron microscopy (Figs 58���65):��� Rapheless valve (SV) (Figs 58���62): Valve surface with shallow, rounded to irregularly shaped depressions along the apical axis, mainly concentrated in the central area (Figs 58���60). Externally, striae multiseriate, composed of three to five rows of small same-sized round areolae (Figs 58���60). Striae continuing shortly onto the valve mantle, without interruption (Figs 58���59). Internally, cavum covered by a hood with a narrow aperture towards the mantle (Figs 61, 62). Raphe valve (RV) (Figs 63���65): Striae composed of three to five rows of rounded areolae, not extended over the valve mantle (Fig. 63). Virgae broader than striae, except near the apices. Externally, proximal raphe endings straight, terminating in expanded drop���like pores. Distal raphe fissures bent, continuing shortly onto the valve mantle (Fig. 63). Internally, striae sunken between raised virgae and composed of areolae covered by individual hymenes (Figs 64, 65). Central nodule raised (Fig. 65). Proximal raphe endings slightly deflected into opposite sides, terminating inconspicuously (Figs 64, 65). Distal raphe endings terminating on poorly developed helictoglossae, continuing shortly onto the valve mantle (Fig. 64). Type:��� ITALY. Carbonia-Iglesias: San Giovanni cave at Domusnovas (Monte Acqua, Marganai massif), 190 m a.s.l., 39��20���13.4������N; 008��37���37.1������E, sampling dates 21 September 2016 and 31 January 2017, collected by Giuseppina Grazia Lai (designated here, holotype: slide no. BR- 4636 Meise Botanic Garden, Belgium. Specimen here depicted in Fig. 9). Etymology:��� The name refers to the Marganai massif where the karst spring San Giovanni originates. Marganai (Margan��i) (Iglesias)���It is an elongated mountain or a mountain of a small mountain chain: it is likely that its name is to be compared���not derived���with Lat. margo, -inis ���margin��� (Indo-European). Cfr. Marghine (Source: Pittau, 2013)., Published as part of Lai, Giuseppina G., Ector, Luc, Padedda, Bachisio M. & Wetzel, Carlos E., 2021, Planothidium marganaiensis sp. nov. (Bacillariophyta), a new cavum-bearing species from a karst spring in south-western Sardinia (Italy), pp. 140-154 in Phytotaxa 489 (2) on pages 143-146, DOI: 10.11646/phytotaxa.489.2.3, http://zenodo.org/record/5756790, {"references":["Pittau, M. (2013) Toponimi della Sardegna Meridionale. Significato e origine. [http: // www. pittau. it / Sardo / top _ sard _ meridionale. html]"]}

Lai, Giuseppina G., Ector, Luc, Padedda, Bachisio M., Wetzel, Carlos E. (2021): Planothidium marganaiensis sp. nov. (Bacillariophyta), a new cavum-bearing species from a karst spring in south-western Sardinia (Italy). Phytotaxa 489 (2): 140-154, DOI: 10.11646/phytotaxa.489.2.3, URL: http://dx.doi.org/10.11646/phytotaxa.489.2.3

Karst ecosystems are considered as priority environments for the protection of biodiversity on a global scale. This study provides a first comparative analysis of epilithic diatom flora from karst springs in two Mediterranean geographic areas (Spain and Italy) with contrasting characteristics (islands vs mainland). We investigated twenty-three springs with different anthropogenic impact levels once in the winter season between 2007 and 2017 (N = 23). A total of 176 diatom taxa (56 genera) were found of which 101 (44 genera) were observed in single sites. A general good biotic integrity was revealed by structural indices (species richness, diversity and evenness). However, crenophilous species were generally present and abundant in less impacted springs. Comparing islands and mainland, significant differences were found in species composition and diversity (H&rsquo, ) based on multivariate analyses (global R = 0.610, p = 0.001) and t-test (t = 2.304, p = 0.031). Discharge and Cl&minus, were the most significant variables in determining diatom assemblages. Our results confirm the role of springs as multiple ecotones and refuges for rare species and suggest that the geographic insularity may be an important factor in maintaining diatom biodiversity.