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Terrestrial biota in the Antarctic are more globally distinct and highly structured biogeographically than previously believed, but information on biogeographic patterns and endemism in freshwater communities is largely lacking. We studied biogeographic patterns of Antarctic freshwater diatoms based on the analysis of species occurrences in a dataset of 439 lakes spread across the Antarctic realm. Highly distinct diatom floras, both in terms of composition and richness, characterize Continental Antarctica, Maritime Antarctica and the sub-Antarctic islands, with marked biogeographic provincialism in each region. A total of 44% of all species is estimated to be endemic to the Antarctic, and most of them are confined to a single biogeographic region. The level of endemism significantly increases with increasing latitude and geographic isolation. Our results have implications for conservation planning, and suggest that successful dispersal of freshwater diatoms to and within the Antarctic is limited, fostering the evolution of highly endemic diatom floras.

A recent detailed survey of the Maritime Antarctic diatom flora using a fine-grained taxonomy resulted in the description of many new species of Pinnularia in general and the section Distantes, including the P. borealis species complex, in particular. Moreover, DNA-based studies of P. borealis revealed that many more species need to be described within this complex. During a survey of the freshwater littoral diatom flora of James Ross Island (Ulu Peninsula) and Vega Island in Maritime Antarctica, a previously unknown chain-forming species in the P. borealis species complex of section Distantes was cultured from three different localities. Molecular phylogenies based on the nuclear-encoded D1-D3 large-subunit ribosomal DNA and plastid rbcL genes revealed that all cultures belong to a distinct highly supported lineage within the P. borealis species complex. Pinnularia catenaborealis sp. nov. is characterised by the presence of small spines located on a raised, thin silica ridge that almost entirely surrounds the valve face near the valve face/mantle junction, and the presence of small silica plates near the apices. In culture, P. catenaborealis formed chains of several tens of cells and in oxidised natural material, chains up to seven frustules were observed. Pinnularia catenaborealis is described from the littoral zone of freshwater Black Lake (Ulu Peninsula, James Ross Island) and has also been observed on nearby Vega Island. Although P. borealis is generally regarded as a (semi-) terrestrial diatom complex mainly occurring in (moist) soils and mosses, P. catenaborealis was found in freshwater habitats with an alkaline pH and low conductivity.

Despite the frequent application of diatoms in palaeoecological research in the High Arctic, our knowledge concerning the diversity, ecology, community associations, community dynamics and survival strategies of High Arctic freshwater littoral diatom floras is still rather limited. In the present study, the diversity, ecological characteristics and community structures of the freshwater littoral diatom flora from ponds and lakes of Petuniabukta (Spitsbergen, Svalbard Archipelago) were studied. In total, 87 samples from the littoral zone of 53 freshwater ponds and lakes were examined using light and scanning electron microscopy. A highly diverse diatom flora dominated by Achnanthidium and Nitzschia was observed. In total, 310 diatom taxa belonging to 59 genera were found, of which about one-third could not be identified beyond the genus level. Based on species composition, four main diatom communities were distinguished. Differences in environmental characteristics, such as habitat type, habitat diversity, habitat stability, streams, water pH and influence of the fjord, biota and human activities, appeared to be important factors in determining the observed diatom communities. The high number of unidentified taxa in this study indicates that a profound revision of the Arctic diatom flora is highly desired, not only to improve our fundamental knowledge concerning the diversity, ecology, community associations and biogeography of Arctic diatoms but also to aid applied (palaeo)environmental sciences.

Diatoms are one of the most abundant and arguably the most species-rich group of protists. Diatom species delimitation has often been based exclusively on the recognition of morphological discontinuities without investigation of other lines of evidence. Even though DNA sequences and reproductive experiments have revealed several examples of (pseudo)cryptic diversity, our understanding of diatom species boundaries and diversity remains limited. The cosmopolitan pennate raphid diatom genus Pinnularia represents one of the most taxon-rich diatom genera. In this study, we focused on the delimitation of species in one of the major clades of the genus, the Pinnularia subgibba group, based on 105 strains from a worldwide origin. We compared genetic distances between the sequences of seven molecular markers and selected the most variable pair, the mitochondrial cox1 and nuclear encoded LSU rDNA, to formulate a primary hypothesis on the species limits using three single-locus automated species delimitation methods. We compared the DNA-based primary hypotheses with morphology and with other available lines of evidence. The results indicate that our data set comprised 15 species of the P. subgibba group. The vast majority of these taxa have an uncertain taxonomic identity, suggesting that several may be unknown to science and/or members of (pseudo)cryptic species complexes within the P. subgibba group.

Cryoconite holes are small, extreme habitats, widespread in the ablation zones of gla-ciers worldwide. They can provide a suitable environment for microorganisms including bacteria, cyanobacteria, algae, fungi, and invertebrates. Diatoms have been previously recovered from cryoconite holes of Greenland and of Svalbard, and recent findings from Antarctica suggest that cryoconite holes may harbor a unique diatom flora distinct from other aquatic habitats nearby. In the present study, we characterize the diatom communi-ties of Nordenskiöld glacier cryoconite holes in Billefjorden (Svalbard, Spitsbergen), and multivariate approaches were used to compare them with three freshwater localities in the immediate vicinity to investigate possible sources of the species pool. We found cryoconite holes to have similar or greater average genus-richness than adjacent lake/ ponds habitats, even though lower numbers of valves were recovered. Overall, cryoconite hole diatom communities differed significantly from those observed in lakes, suggesting that other sources actively contribute to these communities than nearby lakes alone. This further suggests that (i) diatoms present in cryoconite might not exclusively originate from aquatic habitats, but also from (semi-) terrestrial ones; and (ii) that a much wider area than the immediate surroundings should be considered as a possible source for cryoconite diatom flora.

Diatoms constitute the most diverse group of microalgae and have long been recognised for their large biotechnological potential. In the wake of growing research interest in new model species and development of commercial applications, there is a pressing need for long-term preservation of diatom strains. While cryopreservation using dimethylsulfoxide (DMSO) as a cryoprotective agent is the preferred method for long-term strain preservation, many diatom species cannot be successfully cryopreserved using DMSO. Therefore, in this study, we studied cryopreservation success in six different diatom species, representing the major morphological and ecological diatom groups, using a range of DMSO concentrations and Plant Vitrification Solution 2 (PVS2) as an alternative cryoprotectant to DMSO. In addition, we tested whether suppressing bacterial growth by antibiotics accelerates the post-thaw recovery process. Our results show that the effects of cryoprotectant choice, its concentration and the addition of antibiotics are highly species specific. In addition, we showed that PVS2 and antibiotics are useful agents to optimize cryopreservation of algae that cannot survive the traditional cryopreservation protocol using DMSO. We conclude that a species-specific approach will remain necessary to develop protocols for diatom cryopreservation and to increase their representation in public culture collections.

During a survey conducted on the freshwater diatom flora of small shallow pools on the Ecology Glacier forefield (King George Island, Maritime Antarctic Region), an unknown spine-bearing chain-forming Pinnularia species, belonging to the Pinnularia borealis species complex, was found. Although it closely resembles the recently described Pinnularia catenaborealis from James Ross Island and Vega Island (Antarctic Peninsula), a unique set of morphological characteristics revealed in both light and scanning electron microscopy clearly discriminates the specimens of King George Island as a new species. Pinnularia subcatenaborealis Kochman-Kędziora, Pinseel & Van de Vijver sp. nov. can be distinguished from P. catenaborealis by an overall smaller valve size, the presence of irregularly formed silica outgrowths on the mantle and small, irregular plates located near the apices. The new taxon is so far only recorded from a small pool with circumneutral pH and very low conductivity.

Soil micro-organisms drive the global carbon and nutrient cycles that underlie essential ecosystem functions. Yet, we are only beginning to grasp the drivers of terrestrial microbial diversity and biogeography, which presents a substantial barrier to understanding community dynamics and ecosystem functioning. This is especially true for soil protists, which despite their functional significance have received comparatively less interest than their bacterial counterparts. Here, we investigate the diversification of Pinnularia borealis, a rare biosphere soil diatom species complex, using a global sampling of >800 strains. We document unprecedented high levels of species-diversity, reflecting a global radiation since the Eocene/Oligocene global cooling. Our analyses suggest diversification was largely driven by colonization of novel geographic areas and subsequent evolution in isolation. These results illuminate our understanding of how protist diversity, biogeographical patterns, and members of the rare biosphere are generated, and suggest allopatric speciation to be a powerful mechanism for diversification of micro-organisms., It is generally thought many microbes, owing to their ubiquity and dispersal capability, lack biogeographic structuring and clear speciation patterns compared to macroorganisms. However, Pinseel et al. demonstrate multiple cycles of colonization and diversification in Pinnularia borealis, a rare biosphere soil diatom.

During a survey of freshwater diatoms from lakes in the region of Petuniabukta on Spitsbergen (Svalbard Archipelago) a new Gomphonema species, G. svalbardense sp. nov., has been recorded. The new taxon was previously cited in the literature as G. angustatum var. undulatum but this identification proved to be erroneous. Detailed morphology description of G. svalbardense based on light and scanning electron microscopy is presented in this paper and the morphological features of the taxon have been compared with similar species. Gomphonema svalbardense is characterized by its typical linear, almost naviculoid outline with undulating margins, with clearly inflated central part, asymmetric central area, lateral raphe with simple straight proximal endings and the weakly radiate striae. History, ecology and biogeography of the species, mainly based on literature data, have been included.