Mesotaenium berggrenii is one of few autotrophs that thrive on bare glacier surfaces in alpine and polar regions. This extremophilic alga produces high amounts of a brownish vacuolar pigment, whose chemical constitution and ecological function is largely unknown until now. Field material was harvested to isolate and characterize this pigment. Its tannin nature was determined by photometric methods, and the structure determination was carried out by means of HPLC-MS and 1D- and 2D-NMR spectroscopy. The main constituent turned out to be purpurogallin carboxylic acid-6-O-β-d-glucopyranoside. This is the first report of such a phenolic compound in this group of algae. Because of its broad absorption capacities of harmful UV and excessive VIS radiation, this secondary metabolite seems to play an important role for the survival of this alga at exposed sites. Attributes and abundances of the purpurogallins found in M. berggrenii strongly suggest that they are of principal ecophysiological relevance like analogous protective pigments of other extremophilic microorganisms. To prove that M. berggrenii is a true psychrophile, photosynthesis measurements at ambient conditions were carried out. Sequencing of the 18S rRNA gene of this alpine species and of its arctic relative, the filamentous Ancylonema nordenskiöldii, underlined their distinct taxonomic position within the Zygnematophyceae.

At the arctic archipelago of Svalbard, bare glacier surfaces are populated by microalgae like Ancylonema nordenskio¨ldii (Zygnematales, Streptophyta). The resulting blooms cause, due to a vacuolar pigmentation, brownish colourations of the glacier surface. This freshwater ice alga has been described from several polar and alpine glaciers; however, these reports lacked data about the ecophysiology or ultrastructure. Considering the harsh environmental conditions of the exceptional habitat, such as permanently low temperatures, exposure to high irradiation or a short vegetation period, the aim of this study was to elucidate cellular adaptations of A. nordenskio¨ldii. Thus, samples were collected at two glaciers in Spitsbergen. The cytoarchitecture of the cylindrical cells, which are arranged in unbranched filaments, demonstrates active cells with Golgi bodies, mitochondria and rough endoplasmic reticulum close to the nucleus when investigated by transmission electron microscopy (TEM). The cell walls are pore less and only 90 nm thin. A. nordenskio¨ldii only sporadically produces oblong zygotes when two filaments conjugate. The most remarkable cytological feature is peripheral brownish vacuoles, appearing osmiophil and electron dense by TEM. Aqueous extracts of this pigmentation show a broad absorption in the visible light and in the UV. Consequently, a protection against excessive irradiation is provided. Photosynthesis measurements performed at different temperatures and light levels indicate that the metabolism is adapted to temperatures close to the freezing point as well as to high light conditions. Therefore, A. nordenskio¨ldii can be regarded as metabolically and cytological well adapted to live on glaciers.

The filamentous green alga Zygogonium ericetorum (Zygnematophyceae, Streptophyta) was collected in a high-alpine rivulet in Tyrol, Austria. Two different morphotypes of this alga were found: a purple morph with a visible purple vacuolar content and a green morph lacking this coloration. These morphotypes were compared with respect to their secondary metabolites, ultrastructure, and ecophysiological properties. Colorimetric tests with aqueous extracts of the purple morph indicated the presence of soluble compounds such as phenolics and hydrolyzable tannins. High-performance liquid chromatography-screening showed that Z. ericetorum contained several large phenolic peaks with absorption maxima at ~280 nm and sometimes with minor maxima at ~380 nm. Such compounds are uncommon for freshwater green microalgae, and could contribute to protect the organism against increased UV and visible (VIS) irradiation. The purple Z. ericetorum contained larger amounts (per dry weight) of the putative phenolic substances than the green morph; exposure to irradiation may be a key factor for accumulation of these phenolic compounds. Transmission electron microscopy of the purple morph showed massive vacuolization with homogenous medium electron-dense content in the cell periphery, which possibly contains the secondary compounds. In contrast, the green morph had smaller, electron-translucent vacuoles. The ecophysiological data on photosynthesis and desiccation tolerance indicated that increasing photon fluence densities led to much higher relative electron transport rates (rETR) in the purple than in the green morph. These data suggest that the secondary metabolites in the purple morph are important for light acclimation in high-alpine habitats. However, the green morph recovered better after 4 d of rehydration following desiccation stress.

Species of Zygnema form macroscopically visible mats in polar and temperate terrestrial habitats, where they are exposed to environmental stresses. Three previously characterized isolates (Arctic Zygnema sp. B, Antarctic Zygnema sp. C, and temperate Zygnema sp. S) were tested for their tolerance to experimental UV radiation. Samples of young vegetative cells (1 month old) and pre-akinetes (6 months old) were exposed to photosynthetically active radiation (PAR, 400–700 nm, 400 μmol photons m−2 s−1) in combination with experimental UV-A (315–400 nm, 5.7 W m−2, no UV-B), designated as PA, or UV-A (10.1 W m−2) + UV-B (280–315 nm, 1.0 W m−2), designated as PAB. The experimental period lasted for 74 h; the radiation period was 16 h PAR/UV-A per day, or with additional UV-B for 14 h per day. The effective quantum yield, generally lower in pre-akinetes, was mostly reduced during the UV treatment, and recovery was significantly higher in young vegetative cells vs. pre-akinetes during the experiment. Analysis of the deepoxidation state of the xanthophyll-cycle pigments revealed a statistically significant (p < 0.05) increase in Zygnema spp. C and S. The content of UV-absorbing phenolic compounds was significantly higher (p < 0.05) in young vegetative cells compared to pre-akinetes. In young vegetative Zygnema sp. S, these phenolic compounds significantly increased (p < 0.05) upon PA and PAB. Transmission electron microscopy showed an intact ultrastructure with massive starch accumulations at the pyrenoids under PA and PAB. A possible increase in electron-dense bodies in PAB-treated cells and the occurrence of cubic membranes in the chloroplasts are likely protection strategies. Metabolite profiling by non-targeted RP-UHPLC-qToF-MS allowed a clear separation of the strains, but could not detect changes due to the PA and PAB treatments. Six hundred seventeen distinct molecular masses were detected, of which around 200 could be annotated from databases. These results indicate that young vegetative cells can adapt better to the experimental UV-B stress than pre-akinetes.Electronic supplementary materialThe online version of this article (10.1007/s00709-018-1225-1) contains supplementary material, which is available to authorized users.

Adaptation to Aquatic and Terrestrial Environments in Chlorella vulgaris (Chlorophyta).