Melting glacier surfaces are increasingly affected by blooms of psychrophilic microalgae, which darken the ice and lower its albedo, accelerating melting. These microalgae contain distinct vacuoles filled with brownish pigments that were earlier described as the unusual plant phenol purpurogallin. Recently, we discovered so far unreported, large amounts of iron dissolved in aqueous extracts of the glacier ice algae Ancylonema alaskanum. Since the vacuole content was very dark but the chromatographically isolated, aforementioned phenol was only yellowish, a putative complexation of iron with purpurogallin was assumed to be the reason. Application of several protocols, including Raman microscopy on both living cells and extracts, provided strong evidence that this microalga sequesters iron and forms organic metal complexes. Consequently, substantial amounts of so far uncharacterised Fe-complexes of purpurogallin are inferred to be present in Ancylonema, and that putative polymerisation of this compound impeded an earlier analytical discovery. This finding holds significant ecological implications for cold regions. The pigmentation not only enhances the tolerance of glacier ice algae to excessive UV and visible radiation but also influences our current understanding of the biochemical iron cycle in cryosphere-dominated polar and alpine regions. Further downstream consequences of this biological iron source remain to be elucidated.

• Keywords
• Raman microscopy, cryoflora, glaciers, polyphenols, secondary pigmentation,
• MeSH
• Phenols * metabolism   chemistry   MeSH
• Ice Cover * microbiology   MeSH
• Microalgae * radiation effects   chemistry   metabolism   MeSH
• Light MeSH
• Ultraviolet Rays * MeSH
• Iron * metabolism   chemistry   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Phenols * MeSH
• Iron * MeSH

Filamentous green algae of the genus Zygnema are an essential part of hydro-terrestrial ecosystems. Despite several studies on their resistance to natural stresses, little is known about the composition of their assemblages and the changes they undergo over time. Two sites at altitudes above 2200 m a.s.l. in the Austrian Alps were selected for a 2-year observation period and sampled five times. Molecular phylogenetic analysis of the 152 isolated strains of Zygnema sp. was performed based on the rbcL and trnG sequences. Seven genotypes were found at these sites during the samplings, but their proportion varied throughout the seasons. The site with a more stable water regime also had a more stable representation of genotypes, in contrast to the site with fluctuating water availability. The mats formed resistant pre-akinetes at the end of the season with reduced photosynthetic activity. Contrary to expectations, the mats were not exposed to extremely cold temperatures in winter due to snow cover. Some genotypes have been previously observed at this site, indicating that the population composition is stable. This work highlights the importance of resistant pre-akinetes in surviving winter conditions, the ability of algae to re-establish mats, and the need to address the hidden diversity of the genus Zygnema.

• Keywords
• Chlorophyll fluorescence, Cryptic diversity, Freezing, Hidden diversity, Overwintering,
• MeSH
• Ecosystem * MeSH
• Phylogeny MeSH
• Seasons MeSH
• Streptophyta * MeSH
• Water MeSH
• Publication type
• Journal Article MeSH
• Geographicals
• Austria MeSH
• Names of Substances
• Water MeSH

The streptophyte green algal class Zygnematophyceae is the immediate sister lineage to land plants. Their special form of sexual reproduction via conjugation might have played a key role during terrestrialization. Thus, studying Zygnematophyceae and conjugation is crucial for understanding the conquest of land. Moreover, sexual reproduction features are important for species determination. We present a phylogenetic analysis of a field-sampled Zygnema strain and analyze its conjugation process and zygospore morphology, both at the micro- and nanoscale, including 3D-reconstructions of the zygospore architecture. Vegetative filament size (26.18 ± 1.07 μm) and reproductive features allowed morphological determination of Zygnema vaginatum, which was combined with molecular analyses based on rbcL sequencing. Transmission electron microscopy (TEM) depicted a thin cell wall in young zygospores, while mature cells exhibited a tripartite wall, including a massive and sculptured mesospore. During development, cytological reorganizations were visualized by focused ion beam scanning electron microscopy (FIB-SEM). Pyrenoids were reorganized, and the gyroid cubic central thylakoid membranes, as well as the surrounding starch granules, degraded (starch granule volume: 3.58 ± 2.35 μm3 in young cells; 0.68 ± 0.74 μm3 at an intermediate stage of zygospore maturation). Additionally, lipid droplets (LDs) changed drastically in shape and abundance during zygospore maturation (LD/cell volume: 11.77% in young cells; 8.79% in intermediate cells, 19.45% in old cells). In summary, we provide the first TEM images and 3D-reconstructions of Zygnema zygospores, giving insights into the physiological processes involved in their maturation. These observations help to understand mechanisms that facilitated the transition from water to land in Zygnematophyceae.

• MeSH
• Cell Wall MeSH
• Ecosystem MeSH
• Phylogeny MeSH
• Charophyceae * MeSH
• Starch MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Starch MeSH

UNLABELLED: Red snow caused by spherical cysts can be found worldwide, while an orange snow phenomenon caused by spherical cells is restricted to (Sub-)Arctic climates. Both bloom types, occurring in the same localities at Svalbard, were compared ecophysiologically. Using a combination of molecular markers and light- and transmission electron microscopy, cells were identified as Sanguina nivaloides and Sanguina aurantia (Chlorophyceae). In search for reasons for a cosmopolitan vs. a more restricted distribution of these microbes, significant differences in fatty acid and pigment profiles of field samples were found. S. aurantia accumulated much lower levels of polyunsaturated fatty acids (21% vs. 48% of total fatty acids) and exhibited lower astaxanthin-to-chlorophyll-a ratio (2-8 vs. 12-18). These compounds play an important role in adaptation to extreme conditions at the snow surface and within snow drifts. Accordingly, the performance of photosystem II showed that one third to nearly half of the photosynthetic active irradiation was sufficient in S. aurantia, compared to S. nivaloides, to become light saturated. Furthermore, formation of plastoglobules observed in S. nivaloides but missing in S. aurantia may contribute to photoprotection. The rapid light curves of the two species show to a certain extent the shade-adapted photosynthesis under the light conditions at Svalbard (high α-value 0.16 vs. 0.11, low saturation point I k 59 vs. 86). These results indicate significant physiological and ultrastructural differences of the two genetically closely related cryoflora species, but the reasons why S. aurantia has not been found at conditions outside (Sub-)Arctic climate types remain unknown. SUPPLEMENTARY INFORMATION: The online version of this article (10.1007/s00300-020-02778-0) contains supplementary material, which is available to authorised users.

• Keywords
• Arctic, Astaxanthin, Chlamydomonas nivalis, Cryoflora, Green algae, Polyunsaturated fatty acid,
• Publication type
• Journal Article MeSH

In streptophytic green algae in the genus Zygnema, pre-akinete formation is considered a key survival strategy under extreme environmental conditions in alpine and polar regions. The transition from young, dividing cells to pre-akinetes is associated with morphological changes and the accumulation of storage products. Understanding the underlying metabolic changes could provide insights into survival strategies in polar habitats. Here, GC-MS-based metabolite profiling was used to study the metabolic signature associated with pre-akinete formation in Zygnema sp. from polar regions under laboratory conditions, induced by water and nutrient depletion, or collected in the field. Light microscopy and TEM revealed drastic changes in chloroplast morphology and ultrastructure, degradation of starch grains, and accumulation of lipid bodies in pre-akinetes. Accordingly, the metabolite profiles upon pre-akinete formation reflected a gradual shift in metabolic activity. Compared with young cells, pre-akinetes showed an overall reduction in primary metabolites such as amino acids and intermediates of the tricarboxylic acid (TCA) cycle, consistent with a lower metabolic turnover, while they accumulated lipids and oligosaccharides. Overall, the transition to the pre-akinete stage involves re-allocation of photosynthetically fixed energy into storage instead of growth, supporting survival of extreme environmental conditions.

• Keywords
• Zygnema, Abiotic stress, green algae, metabolomics, pre-akinete, streptophyte,
• MeSH
• Chlorophyta * MeSH
• Ecosystem MeSH
• Lipid Droplets MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH

Green algae of the genus Zygnema form extensive mats and produce large amounts of biomass in shallow freshwater habitats. Environmental stresses including freezing may perturb these mats, which usually have only annual character. To estimate the limits of survival at subzero temperatures, freezing resistance of young Zygnema sp. (strain MP2011Skan) cells and pre-akinetes was investigated. Young, 2-week-old cultures were exposed to temperatures of 0 to - 14 °C at 2-K steps, whereas 8-month-old cultures were frozen from - 10 to - 70 °C at 10-K intervals. Cell viability after freezing was determined by 0.1% auramine O vital fluorescence staining and measurements of the effective quantum yield of photosystem II (ФPSII). At - 8 °C, the young vegetative cells were unable to recover from severe frost damage. But temperatures even slightly below zero (- 2 °C) negatively affected the cells' physiology. Single pre-akinetes could survive even at - 70 °C, but their LT50 value was - 26.2 °C. Severe freezing cytorrhysis was observed via cryo-microscopy at - 10 °C, a temperature found to be lethal for young cells. The ultrastructure of young cells appeared unchanged at - 2 °C, but severe damage to biomembranes and formation of small foamy vacuoles was observed at - 10 °C. Pre-akinetes did not show ultrastructural changes at - 20 °C; however, vacuolization increased, and gas bubbles appeared at - 70 °C. Our results demonstrate that the formation of pre-akinetes increases freezing resistance. This adaptation is crucial for surviving the harsh temperature conditions prevailing in the High Arctic in winter and a key feature in seasonal dynamics of Zygnema sp.

• Keywords
• Auramine O, Chlorophyll fluorescence, Freezing, Ice, Live cell staining, Ultrastructure,
• MeSH
• Chlorophyll chemistry   MeSH
• Freezing MeSH
• Publication type
• Journal Article MeSH
• Geographicals
• Arctic Regions MeSH
• Names of Substances
• Chlorophyll MeSH

Within streptophyte green algae Zygnematophyceae are the sister group to the land plants that inherited several traits conferring stress protection. Zygnema sp., a mat-forming alga thriving in extreme habitats, was collected from a field site in Svalbard, where the bottom layers are protected by the top layers. The two layers were investigated by a metatranscriptomic approach and GC-MS-based metabolite profiling. In the top layer, 6569 genes were significantly upregulated and 149 were downregulated. Upregulated genes coded for components of the photosynthetic apparatus, chlorophyll synthesis, early light-inducible proteins, cell wall and carbohydrate metabolism, including starch-degrading enzymes. An increase in maltose in the top layer and degraded starch grains at the ultrastructural levels corroborated these findings. Genes involved in amino acid, redox metabolism and DNA repair were upregulated. A total of 29 differentially accumulated metabolites (out of 173 identified ones) confirmed higher metabolic turnover in the top layer. For several of these metabolites, differential accumulation matched the transcriptional changes of enzymes involved in associated pathways. In summary, the findings support the hypothesis that in a Zygnema mat the top layer shields the bottom layers from abiotic stress factors such as excessive irradiation.

• MeSH
• Chlorophyta genetics   metabolism   MeSH
• Ecosystem MeSH
• Photosynthesis genetics   MeSH
• Stress, Physiological MeSH
• Metabolome MeSH
• Streptophyta genetics   metabolism   MeSH
• Transcriptome MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Geographicals
• Arctic Regions MeSH
• Svalbard MeSH