Ecophysiological and morphological comparison of two populations of Chlainomonas sp. (Chlorophyta) causing red snow on ice-covered lakes in the High Tatras and Austrian Alps

Hypothesized life cycle of the snow algae Chlainomonas sp. (Chlamydomonadales, Chlorophyta) from the Cascade Mountains, USA

Chlainomonas (Chlamydomonadales, Chlorophyta) is one of the four genera of snow algae known to produce annual pink or red blooms in alpine snow. No Chlainomonas species have been successfully cultured in the laboratory, but diverse cell types have been observed from many field-collected samples, from multiple species. The diversity of morphologies suggests these algae have complex life cycles with changes in ploidy. Over 7 years (2017-2023), we observed seasonal blooms dominated by a Chlainomonas species from late spring through the summer months on a snow-on-lake habitat in an alpine basin in the North Cascade Mountains of Washington, USA. The Bagley Lake Chlainomonas is distinct from previously reported species based on morphology and sequence data. We observed a similar collection of cell types observed in other Chlainomonas species, with the addition of swarming biflagellate cells that emerged from sporangia. We present a life cycle hypothesis for this species that links cell morphologies observed in the field to seasonally available habitat. The progression of cell types suggests cells are undergoing both meiosis and fertilization in the life cycle. Since the life cycle is the most fundamental biological feature of an organism, with direct consequences for evolutionary processes, it is critical to understand how snow algal life cycles will influence their responses to changes in their habitat driven by climate warming. For microbial taxa that live in extreme environments and are difficult to culture, temporal field studies, such as we report here, may be key to creating testable hypotheses for life cycles.

Rosetta gen. nov. (Chlorophyta): Resolving the identity of red snow algal rosettes

Thick-walled rosette-like snow algae were long thought to be a life stage of various other species of snow algae. Rosette-like cells have not been cultured, but by manually isolating cells from 38 field samples in southern British Columbia, we assigned a variety of rosette morphologies to DNA sequence. Phylogenetic analysis of Rubisco large-subunit (rbcL) gene, ribosomal internal transcribed spacer 2 (ITS2) rRNA region, and 18S rRNA gene revealed that the rosette-like cells form a new clade within the phylogroup Chloromonadinia. Based on these data, we designate a new genus, Rosetta, which comprises five novel species: R. castellata, R. floranivea, R. stellaria, R. rubriterra, and R. papavera. In a survey of 762 snow samples from British Columbia, we observed R. floranivea exclusively on snow overlying high-elevation glaciers, whereas R. castellata was observed at lower elevations, near the tree line. The other three species were rarely observed. Spherical red cells enveloped in a thin translucent sac were conspecific with Rosetta, possibly a developmental stage. These results highlight the unexplored diversity among snow algae and emphasize the utility of single-cell isolation to advance the centuries-old problem of disentangling life stages and cryptic species.

Novel insights in cryptic diversity of snow and glacier ice algae communities combining 18S rRNA gene and ITS2 amplicon sequencing

Melting snow and glacier surfaces host microalgal blooms in polar and mountainous regions. The aim of this study was to determine the dominant taxa at the species level in the European Arctic and the Alps. A standardized protocol for amplicon metabarcoding using the 18S rRNA gene and ITS2 markers was developed. This is important because previous biodiversity studies have been hampered by the dominance of closely related algal taxa in snow and ice. Due to the limited resolution of partial 18S rRNA Illumina sequences, the hypervariable ITS2 region was used to further discriminate between the genotypes. Our results show that red snow was caused by the cosmopolitan Sanguina nivaloides (Chlamydomonadales, Chlorophyta) and two as of yet undescribed Sanguina species. Arctic orange snow was dominated by S. aurantia, which was not found in the Alps. On glaciers, at least three Ancylonema species (Zygnematales, Streptophyta) dominated. Golden-brown blooms consisted of Hydrurus spp. (Hydrurales, Stramenophiles) and these were mainly an Arctic phenomenon. For chrysophytes, only the 18S rRNA gene but not ITS2 sequences were amplified, showcasing how delicate the selection of eukaryotic 'universal' primers for community studies is and that primer specificity will affect diversity results dramatically. We propose our approach as a 'best practice'.

Patchy and Pink: Dynamics of a Chlainomonas sp. (Chlamydomonadales, chlorophyta) algal bloom on Bagley Lake, North Cascades, WA

Snow algal blooms frequently occur throughout alpine and polar environments during spring and summer months; however, our understanding of bloom dynamics is limited. We tracked a recurrent bloom of Chlainomonas sp. on Upper Bagley Lake in the North Cascade Mountains, USA, to assess the spatiotemporal dynamics in bloom color intensity, community photophysiology, and community composition over eight weeks. We found that the algae biomass had a dynamic patchy distribution over space and time, which was decoupled from changes in community composition and life-cycle progress averaged across the bloom. The proportional representation of Chlainomonas sp. remained consistent throughout the study while the overall community composition shows a progression through the bloom. We found that community photophysiology, measured by the maximum quantum yield of PSII (Fv/Fm), decreased on average throughout the bloom. These findings suggest that the Chlainomonas sp. community on Bagley Lake is not simply an algal bloom with rapid increase in biomass followed by a population crash, as is often seen in aquatic systems, though there is a physiological trajectory and sensitivity to environmental stress. These results contribute to our understanding of the biology of Chlainomonas sp. and its response to environmental stress, specifically an extreme warming event.

Antarctic snow algae: unraveling the processes underlying microbial community assembly during blooms formation

BACKGROUND AND AIMS

At the West Antarctic Peninsula, snow algae blooms are composed of complex microbial communities dominated by green microalgae and bacteria. During their progression, the assembly of these microbial communities occurs under harsh environmental conditions and variable nutrient content due to fast snow melting. To date, it is still unclear what are the ecological mechanisms governing the composition and abundance of microorganisms during the formation of snow algae blooms. In this study, we aim to examine the main ecological mechanisms governing the assembly of snow algae blooms from early stages to colorful stages blooms.

METHODS

The composition of the microbial communities within snow algae blooms was recorded in the West Antarctic Peninsula (Isabel Riquelme Islet) during a 35-day period using 16S rRNA and 18S rRNA metabarcoding. In addition, the contribution of different ecological processes to the assembly of the microbial community was quantified using phylogenetic bin-based null model analysis.

RESULTS

Our results showed that alpha diversity indices of the eukaryotic communities displayed a higher variation during the formation of the algae bloom compared with the bacterial community. Additionally, in a macronutrients rich environment, the content of nitrate, ammonium, phosphate, and organic carbon did not play a major role in structuring the community. The quantification of ecological processes showed that the bacterial community assembly was governed by selective processes such as homogenous selection. In contrast, stochastic processes such as dispersal limitation and drift, and to a lesser extent, homogenous selection, regulate the eukaryotic community.

CONCLUSIONS

Overall, our study highlights the differences in the microbial assembly between bacteria and eukaryotes in snow algae blooms and proposes a model to integrate both assembly processes. Video Abstract.

The austral biflagellate Chloromonas rubroleosa (Chlorophyceae) is the closest relative of the unusual quadriflagellate genus Chlainomonas, both found in snow

The quadriflagellate genus Chlainomonas frequently dominates red snow globally. It is unusual in several respects, with two separated pairs of flagella, apparent cell division via extrusion of cytoplasmic threads, and being nested phylogenetically within the biflagellate genus Chloromonas. Here, we showed that the austral species Chloromonas (Cr.) rubroleosa, originally described from Antarctic red snow, is a close biflagellate relative of Chlainomonas, challenging the monophyly of Chlainomonas as currently conceived. Sequences of the 18S rRNA gene robustly linked Cr. rubroleosa with near-identical environmental sequences from Antarctic red snow and Chlainomonas from North America, Japan, and Europe. Furthermore, the 18S rRNA and rbcL gene sequences of Cr. rubroleosa were almost identical to New Zealand and North American collections of Chlainomonas. Cr. rubroleosa and New Zealand Chlainomonas are separated by only a single-base substitution across the ITS1-5.8S-ITS2 rRNA loci (and according to ITS2, the North American collection is the next closest relative). This again raises the possibility that Chlainomonas is a life-cycle stage of vegetatively biflagellate organisms, although this remains confounded by the scarcity of biflagellates in field populations, the apparent cell division by quadriflagellates, and the absence of Chlainomonas-type cells in cultures of Cr. rubroleosa. The latter species is broadly similar to Chlainomonas, being poor at swimming, with similar pigment, chloroplast arrangement and ultrastructure, and is relatively large. Increased size is a feature of the wider clade of "Group D" snow algae. A synthesis of field and laboratory investigations may be needed to unravel the life cycle and correct the systematics of this group.

The snow alga Chloromonas kaweckae sp. nov. (Volvocales, Chlorophyta) causes green surface blooms in the high tatras (Slovakia) and tolerates high irradiance

Seasonally slowly melting mountain snowfields are populated by extremophilic microalgae. In alpine habitats, high-light sensitive, green phytoflagellates are usually observed in subsurface layers deeper in the snowpack under dim conditions, while robust orange to reddish cyst stages can be seen exposed on the surface. In this study, uncommon surface green snow was investigated in the High Tatra Mountains (Slovakia). The monospecific community found in the green surface bloom consisted of vegetative Chloromonas cells (Volvocales, Chlorophyta). Molecular data demonstrated that the field sample and the strain isolated and established from the bloom were conspecific, and they represent a new species, Chloromonas kaweckae sp. nov., which is described based on the morphology of the vegetative cells and asexual reproduction and on molecular analyses of the strain. Cells of C. kaweckae accumulated approximately 50% polyunsaturated fatty acids, which is advantageous at low temperatures. In addition, this new species performed active photosynthesis at temperatures close to the freezing point showed a light compensation point of 126 ± 22 μmol photons · m-2  · s-1 and some signs of photoinhibition at irradiances greater than 600 μmol photons · m-2  · s-1 . These data indicate that the photosynthetic apparatus of C. kaweckae could be regarded as adapted to relatively high light intensities, otherwise unusual for most flagellate stages of snow algae.

Diversity and Distribution of Carotenogenic Algae in Europe: A Review

Microalgae are the richest source of natural carotenoids, which are valuable pigments with a high share of benefits. Often, carotenoid-producing algae inhabit specific biotopes with unfavorable or even extremal conditions. Such biotopes, including alpine snow fields and hypersaline ponds, are widely distributed in Europe. They can serve as a source of new strains for biotechnology. The number of algal species used for obtaining these compounds on an industrial scale is limited. The data on them are poor. Moreover, some of them have been reported in non-English local scientific articles and theses. This review aims to summarize existing data on microalgal species, which are known as potential carotenoid producers in biotechnology. These include Haematococcus and Dunaliella, both well-known to the scientific community, as well as less-elucidated representatives. Their distribution will be covered throughout Europe: from the Greek Mediterranean coast in the south to the snow valleys in Norway in the north, and from the ponds in Amieiro (Portugal) in the west to the saline lakes and mountains in Crimea (Ukraine) in the east. A wide spectrum of algal secondary carotenoids is reviewed: β-carotene, astaxanthin, canthaxanthin, echinenone, adonixanthin, and adonirubin. For convenience, the main concepts of biology of carotenoid-producing algae are briefly explained.

The expression pattern of β-carotene ketolase gene restricts the accumulation of astaxanthin in Dunaliella under salt stress

Dunaliella salina can accumulate a large amount of β-carotene which is generally considered to be its terminal product of carotenoid metabolism. In this study, it was proved that D. salina has the ketolase (DsBKT) of catalyzing the synthesis of astaxanthin, the downstream products of β-carotene. Therefore, the reason why D. salina does not synthesize astaxanthin is the purpose of this study. The enzymatic activity of DsBKT was detected by functional complementation assays in Escherichia coli, results showed that DsBKT had efficient ketolase activity toward β-carotene and zeaxanthin to produce astaxanthin, indicating that there were complete astaxanthin-producing genes in Dunaliella. Unlike the induced expression of Lycopene cyclase (catalyzing β-carotene synthesis) under salt stress, the expression of DsBKT was very low under both normal and stress conditions, which may be the main reason why D. salina cannot accumulate astaxanthin. On the contrary, with the astaxanthin-rich Haematococcus pluvialis as a control, its BKT gene was significantly upregulated under salt stress. Further study showed that DsBKT promoter had strong promoter ability and could stably drive the expression of ble-egfp in D. salina. Obviously, DsBKT promoter is not the reason of DsBKT not being expressed which may be caused by Noncoding RNA.

Extremophilic Microorganisms in Central Europe

Extremophiles inhabit a wide variety of environments. Here we focus on extremophiles in moderate climates in central Europe, and particularly in Slovenia. Although multiple types of stress often occur in the same habitat, extremophiles are generally combined into groups according to the main stressor to which they are adapted. Several types of extremophiles, e.g., oligotrophs, are well represented and diverse in subsurface environments and karst regions. Psychrophiles thrive in ice caves and depressions with eternal snow and ice, with several globally distributed snow algae and psychrophilic bacteria that have been discovered in alpine glaciers. However, this area requires further research. Halophiles thrive in salterns while thermophiles inhabit thermal springs, although there is little data on such microorganisms in central Europe, despite many taxa being found globally. This review also includes the potential use of extremophiles in biotechnology and bioremediation applications.

Spatial and Temporal Variations in Pigment and Species Compositions of Snow Algae on Mt. Tateyama in Toyama Prefecture, Japan

Snow algae are photosynthetic microbes that inhabit the melting snow surface in alpine and polar regions. We analyzed the pigment and species composition of colored snow collected on Mt. Tateyama in Japan during the melting seasons of 2015 and 2016. High-performance liquid chromatographic analyses of the pigments extracted from the colored snow showed that their composition varied within the study area and were classified into four types: Type A (astaxanthin-monoester dominant), Type B (medium astaxanthin-monoester content), Type C (abundant primary carotenoids and free-astaxanthin), and Type D (abundant primary carotenoids and astaxanthin diesters). Types A and B were most commonly observed in the study area, whereas Types C and D appeared only at specific sites. Analysis of the 18S ribosomal RNA (18S rRNA) gene revealed six major amplicon sequence variants (ASVs) of snow algae, belonging to the Sanguina, Chloromonas, and Chlainomonas groups. The relative abundance of the algal ASVs showed that Sanguina was dominant (>48%) in both Types A and B, suggesting that the difference in astaxanthin abundance between the two types was caused by the production of pigments in the algal cells. The algal community structures of Types C and D differed from those of Types A and B, indicating that the primary carotenoids and astaxanthin diesters were derived from certain algal species in these types. Therefore, astaxanthin-rich Sanguina algae mostly induced the red snow that appeared widely in this alpine area; however, they were partially dominated by Chloromonas or Chlainomonas algae, causing different pigment compositions.

Cysts of the Snow Alga Chloromonas krienitzii (Chlorophyceae) Show Increased Tolerance to Ultraviolet Radiation and Elevated Visible Light

Melting mountainous snowfields are populated by extremophilic microorganisms. An alga causing orange snow above timberline in the High Tatra Mountains (Poland) was characterised using multiple methods examining its ultrastructure, genetics, life cycle, photosynthesis and ecophysiology. Based on light and electron microscopy and ITS2 rDNA, the species was identified as Chloromonas krienitzii (Chlorophyceae). Recently, the taxon was described from Japan. However, cellular adaptations to its harsh environment and details about the life cycle were so far unknown. In this study, the snow surface population consisted of egg-shaped cysts containing large numbers of lipid bodies filled presumably with the secondary carotenoid astaxanthin. The outer, spiked cell wall was shed during cell maturation. Before this developmental step, the cysts resembled a different snow alga, Chloromonas brevispina. The remaining, long-lasting smooth cell wall showed a striking UV-induced blue autofluorescence, indicating the presence of short wavelengths absorbing, protective compounds, potentially sporopollenin containing polyphenolic components. Applying a chlorophyll fluorescence assay on intact cells, a significant UV-A and UV-B screening capability of about 30 and 50%, respectively, was measured. Moreover, intracellular secondary carotenoids were responsible for a reduction of blue-green light absorbed by chloroplasts by about 50%. These results revealed the high capacity of cysts to reduce the impact of harmful UV and high visible irradiation to the chloroplast and nucleus when exposed at alpine snow surfaces during melting. Consistently, the observed photosynthetic performance of photosystem II (evaluated by fluorometry) showed no decline up to 2100 μmol photons m-2 s-1. Cysts accumulated high contents of polyunsaturated fatty acids (about 60% of fatty acids), which are advantageous at low temperatures. In the course of this study, C. krienitzii was found also in Slovakia, Italy, Greece and the United States, indicating a widespread distribution in the Northern Hemisphere.

Ecophysiological and ultrastructural characterisation of the circumpolar orange snow alga Sanguina aurantia compared to the cosmopolitan red snow alga Sanguina nivaloides (Chlorophyta)

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.

Sanguina nivaloides and Sanguina aurantia gen. et spp. nov. (Chlorophyta): the taxonomy, phylogeny, biogeography and ecology of two newly recognised algae causing red and orange snow

Melting snowfields in polar and alpine regions often exhibit a red and orange colouration caused by microalgae. The diversity of these organisms is still poorly understood. We applied a polyphasic approach using three molecular markers and light and electron microscopy to investigate spherical cysts sampled from alpine mountains in Europe, North America and South America as well as from both polar regions. Molecular analyses revealed the presence of a single independent lineage within the Chlamydomonadales. The genus Sanguina is described, with Sanguina nivaloides as its type. It is distinguishable from other red cysts forming alga by the number of cell wall layers, cell size, cell surface morphology and habitat preference. Sanguina nivaloides is a diverse species containing a total of 18 haplotypes according to nuclear ribosomal DNA internal transcribed spacer 2, with low nucleotide divergence (≤3.5%). Based on molecular data we demonstrate that it has a cosmopolitan distribution with an absence of geographical structuring, indicating an effective dispersal strategy with the cysts being transported all around the globe, including trans-equatorially. Additionally, Sanguina aurantia is described, with small spherical orange cysts often clustered by means of mucilaginous sheaths, and causing orange blooms in snow in subarctic and Arctic regions.

Variation in Snow Algae Blooms in the Coast Range of British Columbia

Snow algae blooms cover vast areas of summer snowfields worldwide, reducing albedo and increasing snow melt. Despite their global prevalence, little is known about the algae species that comprise these blooms. We used 18S and rbcL metabarcoding and light microscopy to characterize algae species composition in 31 snow algae blooms in the Coast Range of British Columbia, Canada. This study is the first to thoroughly document regional variation between blooms. We found all blooms were dominated by the genera Sanguina, Chloromonas, and Chlainomonas. There was considerable variation between blooms, most notably species assemblages above treeline were distinct from forested sites. In contrast to previous studies, the snow algae genus Chlainomonas was abundant and widespread in snow algae blooms. We found few taxa using traditional 18S metabarcoding, but the high taxonomic resolution of rbcL revealed substantial diversity, including OTUs that likely represent unnamed species of snow algae. These three cross-referenced datasets (rbcL, 18S, and microscopy) reveal that alpine snow algae blooms are more diverse than previously thought, with different species of algae dominating different elevations.

Snow and Glacial Algae: A Review1

Snow or glacial algae are found on all continents, and most species are in the Chlamydomonadales (Chlorophyta) and Zygnematales (Streptophyta). Other algal groups include euglenoids, cryptomonads, chrysophytes, dinoflagellates, and cyanobacteria. They may live under extreme conditions of temperatures near 0°C, high irradiance levels in open exposures, low irradiance levels under tree canopies or deep in snow, acidic pH, low conductivity, and desiccation after snow melt. These primary producers may color snow green, golden-brown, red, pink, orange, or purple-grey, and they are part of communities that include other eukaryotes, bacteria, archaea, viruses, and fungi. They are an important component of the global biosphere and carbon and water cycles. Life cycles in the Chlamydomonas-Chloromonas-Chlainomonas complex include migration of flagellates in liquid water and formation of resistant cysts, many of which were identified previously as other algae. Species differentiation has been updated through the use of metagenomics, lipidomics, high-throughput sequencing (HTS), multi-gene analysis, and ITS. Secondary metabolites (astaxanthin in snow algae and purpurogallin in glacial algae) protect chloroplasts and nuclei from damaging PAR and UV, and ice binding proteins (IBPs) and polyunsaturated fatty acids (PUFAs) reduce cell damage in subfreezing temperatures. Molecular phylogenies reveal that snow algae in the Chlamydomonas-Chloromonas complex have invaded the snow habitat at least twice, and some species are polyphyletic. Snow and glacial algae reduce albedo, accelerate the melt of snowpacks and glaciers, and are used to monitor climate change. Selected strains of these algae have potential for producing food or fuel products.

Resilience to Freezing in the Vegetative Cells of the Microalga Lobosphaera incisa (Trebouxiophyceae, Chlorophyta)

The chlorophyte microalga Lobosphaera incisa was isolated from the snowy slopes of Mt. Tateyama in Japan. This microalga stores exceptionally high amounts of the omega-6 LC-PUFA arachidonic acid in triacylglycerols, and therefore represents a potent photosynthetic source for this essential LC-PUFA. Assuming that freezing tolerance may play a role in adaptation of L. incisa to specific ecological niches, we examined the capability of L. incisa to tolerate extreme sub-zero temperatures. We report here, that the vegetative cells of L. incisa survived freezing at -20°C and -80°C (over 1 month), without cryoprotective agents or prior treatments. Cells successfully recovered upon thawing and proliferated under optimal growth conditions (25°C). However, cells frozen at -80°C showed better recovery and lower cellular ROS generation upon thawing, compared to those preserved at -20°C. Photosynthetic yield of PSII, estimated by F_v /F_m , temporarily decreased at day 1 post freezing and resumed to the original level at day 3. Interestingly, the thawed algal cultures produced a higher level of chlorophylls, exceeding the control culture. The polar metabolome of the vegetative cells comprised a range of compatible solutes, dominated by glutamate, sucrose, and proline. We posit that the presence of endogenous cryoprotectants, a rigid multilayer cell wall, the high LC-PUFA content in membrane lipids, and putative cold-responsive proteins may contribute to the retention of functionality upon recovery from the frozen state, and therefore for the survival under cryospheric conditions. From the applied perspective, this beneficial property holds promise for the cryopreservation of starter cultures for research and commercial purposes.

Fatty acids as chemotaxonomic and ecophysiological traits in green microalgae (desmids, Zygnematophyceae, Streptophyta): A discriminant analysis approach

Desmids (Zygnematophyceae) are a group of poorly studied green microalgae. The aim of the present study was to identify fatty acids (FAs) that could be used as biomarkers in desmids in general, and to determine FAs as traits within different ecophysiological desmid groups. FA profiles of 29 desmid strains were determined and analysed with respect to their geographic origin, trophic preference and age of cultivation. It appeared that merely FAs present in relatively large proportions such as palmitic, linoleic, α-linolenic and hexadecatrienoic acids could be used as biomarkers for reliable categorization of this microalgal group. Linear discriminant analysis applied to three a priori defined groups of desmids, revealed clear strain-specific characteristics regarding FA distribution, influenced by climate and trophic conditions at the source sites as well as by the age of culture and growth phase. Accordingly, when considering FAs for the determination of lower taxonomic ranks we recommend using the term "trait" instead of "biomarker", as the latter designates unchangeable "fingerprint" of a specific taxon. Furthermore, despite that desmids were regarded as microalgae having stable genomes, long-term cultivation appeared to cause modifications in FA metabolic pathways, evident as a larger proportion of stearidonic acid in desmid strains cultivated over extensive time periods (>35 years).

Evaluating High-Throughput Sequencing Data of Microalgae Living in Melting Snow: Improvements and Limitations1

Melting snow fields are an extremophilic habitat dominated by closely related Chlamydomonadaceae (Chlorophyta). Microscopy-based classification of these cryophilic microalgae is challenging and may not reveal the true diversity. High-throughput sequencing (HTS) allows for a more comprehensive evaluation of the community. However, HTS approaches have been rarely used in such ecosystems and the output of their application has not been evaluated. Furthermore, there is no consensus on the choice for a suitable DNA marker or data processing workflow. We found that the correct placement of taxonomic strings onto OTUs strongly depends on the quality of the reference databases. We improved the assignments of the HST data by generating additional reference sequences of the locally abundant taxa, guided by light microscopy. Furthermore, a manual inspection of all automated OTU assignments, oligotyping of the most abundant 18S OTUs, as well as ITS2 secondary structure analyses were necessary for accurate species assignments. Moreover, the sole use of one marker can cause misleading results, either because of insufficient variability within the locus (18S) or the scarcity of reference sequences (ITS2). Our evaluation reveals that HTS output needs to be thoroughly checked when the studied habitats or organisms are poorly represented in publicly available databases. We recommend an optimized workflow for an improved biodiversity evaluation of not only snow algal communities, but generally 'exotic' ecosystems where similar problems arise. A consistent sampling strategy, two- molecular marker approach, light microscopy-based guidance, generation of appropriate reference sequences and final manual verification of all taxonomic assignments are highly recommended.

When Unity Is Strength: The Strategies Used by Chlamydomonas to Survive Environmental Stresses

The unicellular green alga Chlamydomonas reinhardtii is a valuable model system to study a wide spectrum of scientific fields, including responses to environmental conditions. Most studies are performed under optimal growth conditions or under mild stress. However, when environmental conditions become harsher, the behavior of this unicellular alga is less well known. In this review we will show that despite being a unicellular organism, Chlamydomonas can survive very severe environmental conditions. To do so, and depending on the intensity of the stress, the strategies used by Chlamydomonas can range from acclimation to the formation of multicellular structures, or involve programmed cell death.

Ecophysiology of Chloromonas hindakii sp. nov. (Chlorophyceae), Causing Orange Snow Blooms at Different Light Conditions

Slowly melting snowfields in mountain and polar regions are habitats of snow algae. Orange blooms were sampled in three European mountain ranges. The cysts within the blooms morphologically resembled those of Chloromonas nivalis (Chlorophyceae). Molecular and morphological traits of field and cultured material showed that they represent a new species, Chloromonas hindakii sp. nov. The performance of photosystem II was evaluated by fluorometry. For the first time for a snow alga, cyst stages collected in a wide altitudinal gradient and the laboratory strain were compared. The results showed that cysts were well adapted to medium and high irradiance. Cysts from high light conditions became photoinhibited at three times higher irradiances (600 µmol photons m⁻² s⁻¹) than those from low light conditions, or likewise compared to cultured flagellates. Therefore, the physiologic light preferences reflected the conditions in the original habitat. A high content of polyunsaturated fatty acids (about 60% of total lipids) and the accumulation of the carotenoid astaxanthin was observed. They are regarded as adaptations to cope with extreme environmental conditions of snow that include low temperatures, freeze-thaw cycles, and variable light intensity. The intraspecific ability of adaptation of the photosynthetic apparatus to different irradiance regimes seems to be advantageous for thriving in different snow habitats.

Analyzing carotenoids of snow algae by Raman microspectroscopy and high-performance liquid chromatography

We tested the potential of Raman microspectroscopy to determine carotenoid pigments - both primary (lutein, beta-carotene) and secondary (astaxanthin) carotenoids - in the different species and life-cycle stages of snow algae from the order Chlamydomonadales (Chlorophyta). We compared the performance of Raman spectrometry to a reference method of biological pigment analysis, high-performance liquid chromatography (HPLC). The three main carotenoid Raman bands of the astaxanthin-rich red cysts were located at 1520, 1156 and 1006 cm^-1. The shifts (orange aplanozygotes and green motile cells with flagella) in the position of the ν₁(CC) Raman band of the polyenic chain is consistent with the expected changes in the ratios of the various carotenoid pigments. Flagellated green cells commonly contain lutein as a major carotenoid, together with minor amounts of β‑carotene and varying amounts of antheraxanthin, violaxanthin and neoxanthin. Aplanozygotes contain mixtures of both primary and secondary carotenoids. In most cases, the ν₁(CC) band is an overlapping set of bands, which is due to the signal of all carotenoid pigments in the sample, and a deconvolution along with the band position shifts (mainly ν₁) could be used to characterize the mixture of carotenoids. However, the ability of Raman spectroscopy to discriminate between structurally slightly differing carotenoid pigments or several carotenoids in an admixture in an unknown biological system remains limited.

Ecology, cytology and phylogeny of the snow alga Scotiella cryophila K-1 (Chlamydomonadales, Chlorophyta) from the Austrian Alps

Long-lasting, slowly melting snowfields in mountainous regions are frequently populated by specialised microalgae whose diversity is still vastly underestimated. Cysts causing sub-surficial green snow were collected in the Austrian Alps, Tyrol, and morphologically accorded to the snow alga Scotiella cryophila sensu Chodat, initially described from Switzerland. The cytology and photobiology of this population were investigated to understand mechanisms of adaptation to the harsh habitat. Cysts of S. cryophila K-1 had secondary cell walls with pronounced rib-like surface structures and contained several small spherical plastids. The cytoplasm was dominated by lipid bodies, which developed reddish secondary pigmentation. Partial life cycle observations showed that daughter cells lacked structured cell walls. Cysts performed active photosynthesis at temperature conditions close to the freezing point and were photoinhibited at irradiances greater than 70 μmol m^-2 s^-1. This corresponded exactly to habitat conditions 20 to 40 cm below the snow surface. Phylogenetic analyses using 18S rDNA, rbcL and ITS2 rDNA sequences indicated that S. cryophila K-1 is related to Chloromonas, known to contain several snow algae. The taxon forms an independent lineage and is clearly genetically distinct from the type strain of Chloromonas rosae var. psychrophila from North America that is supposed to have morphologically identical cysts. For a taxonomic treatment including a species assignment of S. cryophila K-1 from Europe within Chloromonas, flagellates will have to be cultivated from cysts or from acquired field material for a detailed morphological description. Acquisition and genetic analysis of cysts that resemble S. cryophila from America could elucidate their relationship to European samples.