Filamentous Zygnematophyceae are typical components of algal mats in the polar hydro-terrestrial environment. Under field conditions, they form senescent vegetative cells, designated as pre-akinetes, which are tolerant to desiccation and osmotic stress. Pre-akinete formation and desiccation tolerance was investigated experimentally under monitored laboratory conditions in four strains of Arctic and Antarctic isolates with vegetative Zygnema sp. morphology. Phylogenetic analyses of rbcL sequences revealed one Arctic strain as genus Zygnemopsis, phylogenetically distant from the closely related Zygnema strains. Algae were cultivated in liquid or on solidified medium (9 weeks), supplemented with or lacking nitrogen. Nitrogen-free cultures (liquid as well as solidified) consisted of well-developed pre-akinetes after this period. Desiccation experiments were performed at three different drying rates (rapid: 10% relative humidity, slow: 86% rh and very slow); viability, effective quantum yield of PS II, visual and ultrastructural changes were monitored. Recovery and viability of pre-akinetes were clearly dependent on the drying rate: slower desiccation led to higher levels of survival. Pre-akinetes survived rapid drying after acclimation by very slow desiccation. The formation of pre-akinetes in polar Zygnema spp. and Zygnemopsis sp. is induced by nitrogen limitation. Pre-akinetes, modified vegetative cells, rather than specialized stages of the life cycle, can be hardened by mild desiccation stress to survive rapid drying. Naturally hardened pre-akinetes play a key role in stress tolerance and dispersal under the extreme conditions of polar regions, where sexual reproduction and production of dormant stages is largely suppressed.

Filamentous conjugating green microalgae (Zygnematophyceae, Streptophyta) belong to the most common primary producers in polar hydro-terrestrial environments such as meltwater streamlets and shallow pools. The mats formed by these organisms are mostly composed of sterile filaments with Zygnema morphology, but the extent of their diversity remains unknown. Traditional taxonomy of this group is based on reproductive morphology, but sexual reproduction (conjugation and formation of resistant zygospores) is very rare in extreme conditions. In the present study we gave the first record of zygospore formation in Svalbard field samples, and identified conjugating filaments as Zygnemopsis lamellata and Zygnema cf. calosporum. We applied molecular phylogeny to study genetic diversity of sterile Zygnema filaments from Svalbard in the High Arctic. Based on analysis of 143 rbcL sequences, we revealed a surprisingly high molecular diversity: 12 Arctic Zygnema genotypes and one Zygnemopsis genotype were found. In addition, we characterized individual Arctic genotypes based on cell width and chloroplast morphology using light and confocal laser scanning microscopy. Our findings highlight the importance of a molecular approach when working with sterile filamentous Zygnematophyceae, as hidden diversity might be very beneficial for adaptation to harsh environmental conditions, and experimental results could be misinterpreted when hidden diversity is neglected.

The frustule architecture of diatoms and the nature of the vegetative cell division phase of their life cycle constrain cell size and shape. For decades, diatomists have observed that size diminution is accompanied by valve shape changes. However, allometric shape changes have rarely been assessed using quantitative statistical tools. In the present study, we employed geometric morphometrics to examine the shape dynamics of raphid diatom frustules. An investigation was carried out to explore whether shape characteristics, such as circularity or asymmetry, and variation of valve outline, increase with decreasing cell size. Four monoclonal strains (Luticola dismutica strain 1, L. dismutica strain 2, Navicula cryptocephala, and Sellaphora pupula) were cultivated under stable conditions for two years in order to capture the complete range of cell sizes from initial to sexually competent cells. Shape changes and the pattern of shape change relative to size were quantified using geometric morphometrics. A quantitative shape analysis revealed similar allometric trends among the different strains and genera. With decreasing cell size, circularity of the valve outlines increased, that is, the complexity of the valves decreased. However, shape variation of valves within the populations increased with decreasing cell size. The levels of asymmetry did not change consistently throughout the size diminution phase. In two out of four strains, horizontal (dorsiventral) asymmetry was significantly lower than vertical (heteropolar) and transversal (sigmoid) asymmetries. The increasing morphological variation in clonal strains was likely caused by an accumulation of structural deviations during morphogenesis. In this respect, this is a specific example of the structural inheritance of morphological characteristics, which is naturally related to the peculiar vegetative life cycle of the diatoms.

Green algae in corticolous biofilms are simple coccoid cells or filamentous thalli with strikingly low morphological diversity. Consequently, microscopic identification of these organisms is difficult, and often possible only to higher taxonomic units. We investigated the taxonomic and phylogenetic composition of green microalgae isolated from biofilms growing on the bark of Quercus pubescens and Pinus nigra. The study was based on 122 partial sequences of the plastid-encoded rbcL gene. In total, 29 operational taxonomic units (OTUs), differing in their rbcL sequences, were encountered. Members of the Trebouxiophyceae formed 97.5% of the isolates; Streptophyta made up 2.5%. The most frequently occurring OTUs were in the genera Coccomyxa, Parachloroidium and Stichococcus. Within the Watanabea clade, we have probably discovered an as-yet undescribed generic lineage with chlorelloid morphology. OTUs belonging to the recently described trebouxiophycean genera Kalinella, Leptochlorella and Xylochloris were also encountered, which indicates that these genera are probably widely distributed in subaerial microhabitats, such as tree bark. The samples taken from oak trees were more diverse in their OTU composition than those taken from pine trees, but the average phylogenetic distances of OTUs in samples did not differ between the host tree taxa. Host tree species had a stronger effect on the community structure of algae than the sampling locality. This indicates that habitat filtering is important for the distribution of individual microalgal phylogenetic taxa.

Molecular-assisted alpha taxonomy, which combines molecular species delimitation with post-hoc morphological examinations, has proved to be an effective tool for the classification of morphologically similar species. We employed this approach to examine the diversity of the genus Frustulia in northern Europe. First, we used two molecular markers to delimit species and then characterized their morphology using conventional and geometric morphometrics. Next, we employed machine-learning methods to identify valves in benthic diatom communities in order to infer the distribution and pH preference of individual species. Unlike previous studies using automated identification of diatom species, we examined the performance of the semi-supervised classifier. Supervised methods, which have been used before, only employ labelled valves to train the classification algorithm. The semi-supervised approach is, in addition, able to benefit from unlabelled valves in the natural populations. It is usually superior in cases in which there are few labelled data available. Finally, we compared the classification accuracy of the algorithms and five volunteer specialists. We found five molecular lineages, the F. crassinervia-saxonica species complex, F. gaertnerae, F. septentrionalis, F. krammeri, and F. cf. maoriana. The most valuable characteristics for species identification were length, width, striation pattern, and allometric shape changes described by the first axis in the geometric morphometric analysis. We found that a semi-supervised approach that does not rely solely on the morphology of isolated cells, but also accounts for variation among valves from natural populations, has superior performance. Based on valves from natural populations, we observed marked differences in species abundances and pH tolerances that have a bearing on their geographical distributions.