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While conducting a systematic investigation of phytoplankton assemblages in the northeastern Gulf of Mexico (GOM) in the aftermath of the Deepwater Horizon blowout we took the opportunity to collect sediment samples from off the coast of Pensacola Bay, Florida. These samples contained sufficient numbers of a monoraphid diatom initially identified as Achnanthes controversa Hustedt (Navicula marginulata Cleve) to examine its fine structure in detail. The results of these investigations clearly show that this taxon could not be accommodated in Achnanthes as currently understood, or in any currently described genus in the Achnanthidiaceae. We, therefore, propose a new genus, Vallithidium, with Vallithidium marginulatum (Cleve), J.A. Nienow & A.K.S.K Prasad as the generitype. Our investigations also show that the taxon Hustedt illustrated from Campeche Bay is not the same as the taxon described from the northeastern Gulf. We therefore describe it as a separate species in the new genus, Vallithidium campechianum sp. nov. [ABSTRACT FROM AUTHOR]

Six species of the genus Achnanthidium Kütz. with straight terminal raphe fissures including Achnanthidium caledonicum (Lange-Bert.) Lange-Bert., A. sieminskae Witkowski, Kulikowskiy & Riaux-Gob., Achnanthidium neomicrocephalum Lange-Bert. & F.Staab and Achnanthidium minutissimum (Kütz.) Czarn. were studied using light and scanning electron microscopy, and shape analysis. The type of Achnanthes microcephala f. scotica J.R.Carter (synonym: A. caledonica) and a population of A. neomicrocephalum from its type locality were investigated. Two new species, Achnanthidium tirolense sp. nov. and Achnanthidium lacuslustense sp. nov., found in two oligotrophic lakes in Germany and Austria, were described. Achnanthidium tirolense sp. nov. is distinguished by its rhombic-lanceolate valves with a slightly inflated valve centre and an acute-angled fascia on the raphe valve, and Achnanthidium lacuslustense sp. nov. by its large capitate poles, broad fascia on the raphe valve and strongly curved frustules. The distribution of A. caledonicum, A. sieminskae, A. neomicrocephalum and A. minutissimum in relation to land use and their species associations were investigated in 52 rivers, streams, and lakes of Scotland. Achnanthidium sieminskae, A. caledonicum, and A. neomicrocephalum were found in areas with low human impact where seminatural vegetation was dominant. The latter two species are typical in mountainous areas. In contrast, A. minutissimum sensu stricto was found in locations where human impact was greater. Although further data are required to determine which environmental variables underlie these distributions, our results suggest that the treatment of these species should be revised in ecological status assessments, and taxa aggregated under A. minutissimum sensu lato should be distinguished in ecological and biogeographical studies. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]