Seasonal and diel changes in photosynthetic activity of the snow alga Chlamydomonas nivalis (Chlorophyceae) from Svalbard determined by pulse amplitude modulation fluorometry

The role of subsurface ice in sustaining bacteria in continental and maritime glaciers

In supraglacial environments, surface and subsurface ices are two distinct and connected microhabitats in terms of physicochemical and biological aspects. At the frontline of climate change, glaciers lose tremendous ice masses to downstream ecosystems, serving as crucial sources of both biotic and abiotic materials. In this study, we studied the disparities and relationships of microbial communities between surface and subsurface ices collected from a maritime and a continental glacier during summer. The results showed that surface ices had significantly higher nutrients and were more physiochemically different than subsurface ices. Despite lower nutrients, subsurface ices had higher alpha-diversity with more unique and enriched operational taxonomic units (OTUs) than surface ices, indicating the potential role of subsurface as a bacterial refuge. Sorensen dissimilarity between bacterial communities in surface ices and subsurface ices was mainly contributed by the turnover component, suggesting strong species replacement from surface to subsurface ices due to large environmental gradients. For different glaciers, the maritime glacier had significantly higher alpha-diversity than the continental glacier. The difference between surface and subsurface communities was more pronounced in the maritime glacier than in the continental glacier. The network analysis revealed that surface-enriched and subsurface-enriched OTUs formed independent modules, with surface-enriched OTUs having closer interconnections and greater importance in the network of the maritime glacier. This study highlights the important role of subsurface ice as a bacterial refuge and enriches our knowledge of microbial properties in glaciers.

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.

Adaptation versus plastic responses to temperature, light, and nitrate availability in cultured snow algal strains

Snow algal blooms are widespread, dominating low temperature, high light, and oligotrophic melting snowpacks. Here, we assessed the photophysiological and cellular stoichiometric responses of snow algal genera Chloromonas spp. and Microglena spp. in their vegetative life stage isolated from the Arctic and Antarctic to gradients in temperature (5 - 15°C), nitrate availability (1 - 10 µmol L-1), and light (50 and 500 µmol photons m-2 s-1). When grown under gradients in temperature, measured snow algal strains displayed Fv/Fm values increased by ∼115% and electron transport rates decreased by ∼50% at 5°C compared to 10 and 15°C, demonstrating how low temperatures can mimic high light impacts to photophysiology. When using carrying capacity as opposed to growth rate as a metric for determining the temperature optima, these snow algal strains can be defined as psychrophilic, with carrying capacities ∼90% higher at 5°C than warmer temperatures. All strains approached Redfield C:N stoichiometry when cultured under nutrient replete conditions regardless of temperature (5.7 ± 0.4 across all strains), whereas significant increases in C:N were apparent when strains were cultured under nitrate concentrations that reflected in situ conditions (17.8 ± 5.9). Intra-specific responses in photophysiology were apparent under high light with Chloromonas spp. more capable of acclimating to higher light intensities. These findings suggest that in situ conditions are not optimal for the studied snow algal strains, but they are able to dynamically adjust both their photochemistry and stoichiometry to acclimate to these conditions.

Pigment signatures of algal communities and their implications for glacier surface darkening

Blooms of pigmented algae darken the surface of glaciers and ice sheets, thereby enhancing solar energy absorption and amplifying ice and snow melt. The impacts of algal pigment and community composition on surface darkening are still poorly understood. Here, we characterise glacier ice and snow algal pigment signatures on snow and bare ice surfaces and study their role in photophysiology and energy absorption on three glaciers in Southeast Greenland. Purpurogallin and astaxanthin esters dominated the glacier ice and snow algal pigment pools (mass ratios to chlorophyll a of 32 and 56, respectively). Algal biomass and pigments impacted chromophoric dissolved organic matter concentrations. Despite the effective absorption of astaxanthin esters at wavelengths where incoming irradiance peaks, the cellular energy absorption of snow algae was 95% lower than anticipated from their pigmentation, due to pigment packaging. The energy absorption of glacier ice algae was consequently ~ 5 × higher. On bare ice, snow algae may have locally contributed up to 13% to total biological radiative forcing, despite contributing 44% to total biomass. Our results give new insights into the impact of algal community composition on bare ice energy absorption and biomass accumulation during snow melt.

Morphological and spectroscopic analysis of snow and glacier algae and their parasitic fungi on different glaciers of Svalbard

The results show the morphological analyses and spectroscopic studies of snow and glacier algae and their parasitic fungi in Svalbard (High Arctic). Fixed algal cells of two species, Sanguina nivaloides and Ancylonema nordenskioeldii, were imaged using light microscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). Fluorescence microscopy using Calcofluor white stain supported the observations of parasitic fungi on the algal cells. Images in brightfield microscopy showed chytrid-like fungi penetrating the cells of both algal species. Parasites were found to colonize the cells of A. nordenskioeldii and hypnozygotes of S. nivaloides, while no fungi infected the cyst stages of S. nivaloides. The autofluorescence analysis revealed the ability of S. nivaloides to glow when excited with different wavelengths, while A. nordenskioeldii did not fluoresce. The hypnozygotes of S. nivaloides emitted brighter fluorescence than the cysts, and the most intense luminosity was observed in the UV range. The Fourier-transform infrared spectroscopy (FTIR) and energy-dispersive X-ray spectroscopy (EDS) spectroscopic analysis showed differences in the chemical composition between samples collected from three different sites. Samples dominated by cyst cells were characterized by the presence of an abundant polysaccharide envelope.

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.

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Morphological and physicochemical diversity of snow algae from Alaska

Snow algae are photosynthetic microbes growing in thawing snow. They usually show various morphological cell types. The aim of this study was to carry out microscopic and spectroscopic analysis of different forms of cells of snow algae collected on glaciers in Alaska. Four different shapes of algal cells were observed with the use of bright field LM (Light Microscopy), DIC (Differential Interference Contrast), EDF (Extended Depth Focus), fluorescence microscopy, and SEM (Scanning Electron Microscopy). The cells exhibited the strongest autofluorescence after the exposure to 365-nm excitation light, and the intensity differed among the cell types. Zygotes (cysts) showed the most intense fluorescence. Acridine orange staining revealed the acid nature of the algal cells. The use of Congo red and Calcofluor white fluorochromes indicated differences in the structure of polysaccharides in the cell wall in the individual types of algal cells. FTIR (Fourier-Transform Infrared Spectroscopy) analyses showed the presence of polysaccharides not only in the algal cells but also in the fixative solution. The presence of polysaccharides in the extracellular algal fraction was confirmed by X-ray dispersion spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy imaging (SEM). The differences observed in the structure of the cell wall of the different forms of red snow algae prompt further analysis of this structure.

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.

The microbiome of alpine snow algae shows a specific inter-kingdom connectivity and algae-bacteria interactions with supportive capacities

Mutualistic interactions within microbial assemblages provide a survival strategy under extreme conditions; however, little is known about the complexity of interaction networks in multipartite, free-living communities. In the present study, the interplay within algae-dominated microbial communities exposed to harsh environmental influences in the Austrian Alps was assessed in order to reveal the interconnectivity of eukaryotic and prokaryotic inhabitants. All analyzed snowfields harbored distinct microbial communities. Network analyses revealed that mutual exclusion prevailed among microalgae in the alpine environment, while bacteria were mainly positively embedded in the interaction networks. Especially members of Proteobacteria, with a high prevalence of Oxalobacteraceae, Pseudomonadaceae, and Sphingomonadaceae showed genus-specific co-occurrences with distinct microalgae. Co-cultivation experiments with algal and bacterial isolates confirmed beneficial interactions that were predicted based on the bioinformatic analyses; they resulted in up to 2.6-fold more biomass for the industrially relevant microalga Chlorella vulgaris, and up to 4.6-fold increase in biomass for the cryophilic Chloromonas typhlos. Our findings support the initial hypothesis that microbial communities exposed to adverse environmental conditions in alpine systems harbor inter-kingdom supportive capacities. The insights into mutualistic inter-kingdom interactions and the ecology of microalgae within complex microbial communities provide explanations for the prevalence and resilience of such assemblages in alpine environments.

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.

Ecophysiological Features of Polar Soil Unicellular Microalgae1

Due to their ecological, physiological, and molecular adaptations to low and varying temperatures, as well as varying seasonal irradiances, polar non-marine eukaryotic microalgae could be suitable for low-temperature biotechnology. Adaptations include the synthesis of compounds from different metabolic pathways that protect them against stress. Production of biological compounds and various biotechnological applications, for instance, water treatment technology, are of interest to humans. To select prospective strains for future low-temperature biotechnology in polar regions, temperature and irradiance of growth requirements (Q₁₀ and Ea of 10 polar soil unicellular strains) were evaluated. In terms of temperature, three groups of strains were recognized: (i) cold-preferring where temperature optima ranged between 10.1 and 18.4°C, growth rate 0.252 and 0.344 · d^-1 , (ii) cold- and warm-tolerating with optima above 10°C and growth rate 0.162-0.341 · d^-1 , and (iii) warm-preferring temperatures above 20°C and growth rate 0.249-0.357 · d^-1 . Their light requirements were low. Mean values Q₁₀ for specific growth rate ranged from 0.7 to 3.1. The lowest Ea values were observed on cold-preferring and the highest in the warm-preferring strains. One strain from each temperature group was selected for P_N and R_D measurements. The P_N :R_D ratio of the warm-preferring strains was less affected by temperature similarly as Q₁₀ and Ea. For future biotechnological applications, the strains with broad temperature tolerance (i.e., the group of cold- and warm-tolerating and warm-preferring strains) will be most useful.

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.

The diversity of ice algal communities on the Greenland Ice Sheet as revealed by oligotyping

The Arctic is being disproportionally affected by climate change compared with other geographic locations, and is currently experiencing unprecedented melt rates. The Greenland Ice Sheet (GrIS) can be regarded as the largest supraglacial ecosystem on Earth, and ice algae are the dominant primary producers on bare ice surfaces throughout the course of a melt season. Ice-algal-derived pigments cause a darkening of the ice surface, which in turn decreases albedo and increases melt rates. The important role of ice algae in changing melt rates has only recently been recognized, and we currently know little about their community compositions and functions. Here, we present the first analysis of ice algal communities across a 100 km transect on the GrIS by high-throughput sequencing and subsequent oligotyping of the most abundant taxa. Our data reveal an extremely low algal diversity with Ancylonema nordenskiöldii and a Mesotaenium species being by far the dominant taxa at all sites. We employed an oligotyping approach and revealed a hidden diversity not detectable by conventional clustering of operational taxonomic units and taxonomic classification. Oligotypes of the dominant taxa exhibit a site-specific distribution, which may be linked to differences in temperatures and subsequently the extent of the melting. Our results help to better understand the distribution patterns of ice algal communities that play a crucial role in the GrIS ecosystem.

Photoacclimation by Arctic cryoconite phototrophs

Cryoconite is a matrix of sediment, biogenic polymer and a microbial community that resides on glacier surfaces. The phototrophic component of this community is well adapted to this extreme environment, including high light stress. Photoacclimation of the cryoconite phototrophic community on Longyearbreen, Svalbard, was investigated using in situ variable chlorophyll fluorescence. Rapid light curves (RLCs) and induction-recovery curves were used to analyse photosystem II quantum efficiency, relative electron transport rate and forms of downregulation including non-photochemical quenching (NPQ) and state transitions in cyanobacteria. Phototrophs used a combination of behavioural and physiological photochemical downregulation. Behavioural downregulation is hypothesised to incorporate chloroplast movement and cell or filament positioning within the sediment matrix in order to shade from high light, which resulted in a lack of saturation of RLCs and hence overestimation of productivity. Physiological downregulation likely consisted of biphasic NPQ, comprising a steadily induced light-dependent form and a light-independent NPQ that was not reversed with decreasing light intensity. State transitions by cyanobacteria were the most likely physiological downregulation employed by cyanobacteria within the mixed phototroph community. These findings demonstrate that cryoconite phototrophs combine multiple forms of physiological and behavioural downregulation to optimise light exposure and maximise photosynthetic productivity. This plasticity of photoacclimation enables them to survive productively in the high-light stress environment on the ice surface.

Ecophysiology, secondary pigments and ultrastructure of Chlainomonas sp. (Chlorophyta) from the European Alps compared with Chlamydomonas nivalis forming red snow

Red snow is a well-known phenomenon caused by microalgae thriving in alpine and polar regions during the melting season. The ecology and biodiversity of these organisms, which are adapted to low temperatures, high irradiance and freeze-thaw events, are still poorly understood. We compared two different snow habitats containing two different green algal genera in the European Alps, namely algae blooming in seasonal rock-based snowfields (Chlamydomonas nivalis) and algae dominating waterlogged snow bedded over ice (Chlainomonassp.). Despite the morphological similarity of the red spores found at the snow surface, we found differences in intracellular organization investigated by light and transmission electron microscopy and in secondary pigments investigated by chromatographic analysis in combination with mass spectrometry. Spores ofChlainomonassp. show clear differences fromChlamydomonas nivalisin cell wall arrangement and plastid organization. Active photosynthesis at ambient temperatures indicates a high physiological activity, despite no cell division being present. Lipid bodies containing the carotenoid astaxanthin, which produces the red color, dominate cells of both species, but are modified differently. While inChlainomonassp. astaxanthin is mainly esterified with two fatty acids and is more apolar, inChamydomonas nivalis, in contrast, less apolar monoesters prevail.

Metagenomic and satellite analyses of red snow in the Russian Arctic

Cryophilic algae thrive in liquid water within snow and ice in alpine and polar regions worldwide. Blooms of these algae lower albedo (reflection of sunlight), thereby altering melting patterns (Kohshima, Seko & Yoshimura, 1993; Lutz et al., 2014; Thomas & Duval, 1995). Here metagenomic DNA analysis and satellite imaging were used to investigate red snow in Franz Josef Land in the Russian Arctic. Franz Josef Land red snow metagenomes confirmed that the communities are composed of the autotroph Chlamydomonas nivalis that is supporting a complex viral and heterotrophic bacterial community. Comparisons with white snow communities from other sites suggest that white snow and ice are initially colonized by fungal-dominated communities and then succeeded by the more complex C. nivalis-heterotroph red snow. Satellite image analysis showed that red snow covers up to 80% of the surface of snow and ice fields in Franz Josef Land and globally. Together these results show that C. nivalis supports a local food web that is on the rise as temperatures warm, with potential widespread impacts on alpine and polar environments worldwide.

Integrated 'Omics', Targeted Metabolite and Single-cell Analyses of Arctic Snow Algae Functionality and Adaptability

Snow algae are poly-extremophilic microalgae and important primary colonizers and producers on glaciers and snow fields. Depending on their pigmentation they cause green or red mass blooms during the melt season. This decreases surface albedo and thus further enhances snow and ice melting. Although the phenomenon of snow algal blooms has been known for a long time, large aspects of their physiology and ecology sill remain cryptic. This study provides the first in-depth and multi-omics investigation of two very striking adjacent green and red snow fields on a glacier in Svalbard. We have assessed the algal community composition of green and red snow including their associated microbiota, i.e., bacteria and archaea, their metabolic profiles (targeted and non-targeted metabolites) on the bulk and single-cell level, and assessed the feedbacks between the algae and their physico-chemical environment including liquid water content, pH, albedo, and nutrient availability. We demonstrate that green and red snow clearly vary in their physico-chemical environment, their microbial community composition and their metabolic profiles. For the algae this likely reflects both different stages of their life cycles and their adaptation strategies. Green snow represents a wet, carbon and nutrient rich environment and is dominated by the algae Microglena sp. with a metabolic profile that is characterized by key metabolites involved in growth and proliferation. In contrast, the dry and nutrient poor red snow habitat is colonized by various Chloromonas species with a high abundance of storage and reserve metabolites likely to face upcoming severe conditions. Combining a multitude of techniques we demonstrate the power of such complementary approaches in elucidating the function and ecology of extremophiles such as green and red snow algal blooms, which play crucial roles in glacial ecosystems.

Snow and ice ecosystems: not so extreme

Snow and ice environments cover up to 21% of the Earth's surface. They have been regarded as extreme environments because of their low temperatures, high UV irradiation, low nutrients and low water availability, and thus, their microbial activity has not been considered relevant from a global microbial ecology viewpoint. In this review, we focus on why snow and ice habitats might not be extreme from a microbiological perspective. Microorganisms interact closely with the abiotic conditions imposed by snow and ice habitats by having diverse adaptations, that include genetic resistance mechanisms, to different types of stresses in addition to inhabiting various niches where these potential stresses might be reduced. The microbial communities inhabiting snow and ice are not only abundant and taxonomically diverse, but complex in terms of their interactions. Altogether, snow and ice seem to be true ecosystems with a role in global biogeochemical cycles that has likely been underestimated. Future work should expand past resistance studies to understanding the function of these ecosystems.

Variations of algal communities cause darkening of a Greenland glacier

We have assessed the microbial ecology on the surface of Mittivakkat glacier in SE-Greenland during the exceptional high melting season in July 2012 when the so far most extreme melting rate for the Greenland Ice Sheet has been recorded. By employing a complementary and multi-disciplinary field sampling and analytical approach, we quantified the dramatic changes in the different microbial surface habitats (green snow, red snow, biofilms, grey ice, cryoconite holes). The observed clear change in dominant algal community and their rapidly changing cryo-organic adaptation inventory was linked to the high melting rate. The changes in carbon and nutrient fluxes between different microbial pools (from snow to ice, cryoconite holes and glacial forefronts) revealed that snow and ice algae dominate the net primary production at the onset of melting, and that they have the potential to support the cryoconite hole communities as carbon and nutrient sources. A large proportion of algal cells is retained on the glacial surface and temporal and spatial changes in pigmentation contribute to the darkening of the snow and ice surfaces. This implies that the fast, melt-induced algal growth has a high albedo reduction potential, and this may lead to a positive feedback speeding up melting processes.

Temperature dependence of photosynthesis and thylakoid lipid composition in the red snow alga Chlamydomonas cf. nivalis (Chlorophyceae)

Here, we report an effect of short acclimation to a wide span of temperatures on photosynthetic electron transfer, lipid and fatty acid composition in the snow alga Chlamydomonas cf. nivalis. The growth and oxygen evolution capacity were low at 2 °C yet progressively enhanced at 10 °C and were significantly higher at temperatures from 5 to 15 °C in comparison with the mesophilic control Chlamydomonas reinhardtii. In search of the molecular mechanisms responsible for the adaptation of photosynthesis to low temperatures, we have found unprecedented high rates of QA to QB electron transfer. The thermodynamics of the process revealed the existence of an increased structural flexibility that we explain with the amino acid changes in the D1 protein combined with the physico-chemical characteristics of the thylakoid membrane composed of > 80% negatively charged phosphatidylglycerol.

Eukaryotic organisms in extreme acidic environments, the río tinto case

A major issue in microbial ecology is to identify the limits of life for growth and survival, and to understand the molecular mechanisms that define these limits. Thus, interest in the biodiversity and ecology of extreme environments has grown in recent years for several reasons. Some are basic and revolve around the idea that extreme environments are believed to reflect early Earth conditions. Others are related to the biotechnological potential of extremophiles. In this regard, the study of extremely acidic environments has become increasingly important since environmental acidity is often caused by microbial activity. Highly acidic environments are relatively scarce worldwide and are generally associated with volcanic activity or mining operations. For most acidic environments, low pH facilitates metal solubility, and therefore acidic waters tend to have high concentrations of heavy metals. However, highly acidic environments are usually inhabited by acidophilic and acidotolerant eukaryotic microorganisms such as algae, amoebas, ciliates, heliozoan and rotifers, not to mention filamentous fungi and yeasts. Here, we review the general trends concerning the diversity and ecophysiology of eukaryotic acidophilic microorganims, as well as summarize our latest results on this topic in one of the largest extreme acidic rivers, Río Tinto (SW, Spain).

The dynamic arctic snow pack: an unexplored environment for microbial diversity and activity

The Arctic environment is undergoing changes due to climate shifts, receiving contaminants from distant sources and experiencing increased human activity. Climate change may alter microbial functioning by increasing growth rates and substrate use due to increased temperature. This may lead to changes of process rates and shifts in the structure of microbial communities. Biodiversity may increase as the Arctic warms and population shifts occur as psychrophilic/psychrotolerant species disappear in favor of more mesophylic ones. In order to predict how ecological processes will evolve as a function of global change, it is essential to identify which populations participate in each process, how they vary physiologically, and how the relative abundance, activity and community structure will change under altered environmental conditions. This review covers aspects of the importance and implication of snowpack in microbial ecology emphasizing the diversity and activity of these critical members of cold zone ecosystems.

Elemental and fatty acid composition of snow algae in Arctic habitats

Red, orange or green snow is the macroscopic phenomenon comprising different eukaryotic algae. Little is known about the ecology and nutrient regimes in these algal communities. Therefore, eight snow algal communities from five intensively tinted snow fields in western Spitsbergen were analysed for nutrient concentrations and fatty acid (FA) composition. To evaluate the importance of a shift from green to red forms on the FA-variability of the field samples, four snow algal strains were grown under nitrogen replete and moderate light (+N+ML) or N-limited and high light (−N+HL) conditions. All eight field algal communities were dominated by red and orange cysts. Dissolved nutrient concentration of the snow revealed a broad range of NH⁺₄ (<0.005–1.2 mg N l⁻¹) and only low PO³⁻₄ (<18 μg P l⁻¹) levels. The external nutrient concentration did not reflect cellular nutrient ratios as C:N and C:P ratios of the communities were highest at locations containing relatively high concentrations of NH⁺₄ and PO³⁻₄. Molar N:P ratios ranged from 11 to 21 and did not suggest clear limitation of a single nutrient. On a per carbon basis, we found a 6-fold difference in total FA content between the eight snow algal communities, ranging from 50 to 300 mg FA g C⁻¹. In multivariate analyses total FA content opposed the cellular N:C quota and a large part of the FA variability among field locations originated from the abundant FAs C18:1n-9, C18:2n-6, and C18:3n-3. Both field samples and snow algal strains grown under −N+HL conditions had high concentrations of C18:1n-9. FAs possibly accumulated due to the cessation of growth. Differences in color and nutritional composition between patches of snow algal communities within one snow field were not directly related to nutrient conditions. We propose that the highly patchy distribution of snow algae within and between snow fields may also result from differences in topographical and geological parameters such as slope, melting water rivulets, and rock formation.

Photophysiology and albedo-changing potential of the ice algal community on the surface of the Greenland ice sheet

Darkening of parts of the Greenland ice sheet surface during the summer months leads to reduced albedo and increased melting. Here we show that heavily pigmented, actively photosynthesising microalgae and cyanobacteria are present on the bare ice. We demonstrate the widespread abundance of green algae in the Zygnematophyceae on the ice sheet surface in Southwest Greenland. Photophysiological measurements (variable chlorophyll fluorescence) indicate that the ice algae likely use screening mechanisms to downregulate photosynthesis when exposed to high intensities of visible and ultraviolet radiation, rather than non-photochemical quenching or cell movement. Using imaging microspectrophotometry, we demonstrate that intact cells and filaments absorb light with characteristic spectral profiles across ultraviolet and visible wavelengths, whereas inorganic dust particles typical for these areas display little absorption. Our results indicate that the phototrophic community growing directly on the bare ice, through their photophysiology, most likely have an important role in changing albedo, and subsequently may impact melt rates on the ice sheet.

Advances in the in-field detection of microorganisms in ice

The historic view of ice-bound ecosystems has been one of a predominantly lifeless environment, where microorganisms certainly exist but are assumed to be either completely inactive or in a state of long-term dormancy. However, this standpoint has been progressively overturned in the past 20years as studies have started to reveal the importance of microbial life in the functioning of these environments. Our present knowledge of the distribution, taxonomy, and metabolic activity of such microbial life has been derived primarily from laboratory-based analyses of collected field samples. To date, only a restricted range of life detection and characterization techniques have been applied in the field. Specific examples include direct observation and DNA-based techniques (microscopy, specific stains, and community profiling based on PCR amplification), the detection of biomarkers (such as adenosine triphosphate), and measurements of metabolism [through the uptake and incorporation of radiolabeled isotopes or chemical alteration of fluorescent substrates (umbelliferones are also useful here)]. On-going improvements in technology mean that smaller and more robust life detection and characterization systems are continually being designed, manufactured, and adapted for in-field use. Adapting technology designed for other applications is the main source of new methodology, and the range of techniques is currently increasing rapidly. Here we review the current use of technology and techniques to detect and characterize microbial life within icy environments and specifically its deployment to in-field situations. We discuss the necessary considerations, limitations, and adaptations, review emerging technologies, and highlight the future potential. Successful application of these new techniques to in-field studies will certainly generate new insights into the way ice bound ecosystems function.

Osmotic stress in Arctic and Antarctic strains of the green alga Zygnema (Zygnematales, Streptophyta): effects on photosynthesis and ultrastructure

The osmotic potential and effects of plasmolysis on photosynthetic oxygen evolution and chlorophyll fluorescence were studied in two Arctic Zygnema sp. (strain B, strain G) and two Antarctic Zygnema sp. (strain E, strain D). Antarctic strain D was newly characterized by rbcL sequence analysis in the present study. The two Antarctic strains, D and E, are most closely related and may represent different isolates of the same species, in contrast, strain B and G are separate lineages. Incipient plasmolysis in the cells was determined by light microscopy after incubating cells in sorbitol solutions ranging between 200 mM and 1000 mM sorbitol for 3, 6 and 24 h. In Zygnema strain B and G incipient plasmolysis occurred at ∼600 mM sorbitol solution (720 mOsmol kg⁻¹, ψ = −1.67 MPa) and in strains D and E at ∼300 mM (318 mOsmol kg⁻¹, ψ = −0.8 MPa) sorbitol solution. Hechtian strands were visualized in all plasmolysed cells, which is particularly interesting, as these cells lack pores or plasmodesmata. Ultrastructural changes upon osmotic stress were a retraction of the condensed cytoplasm from the cell walls, damages to chloroplast and mitochondrial membranes, increasing numbers of plastoglobules in the chloroplasts and membrane enclosed particles in the extraplasmatic space. Maximum photosynthetic rates (P_max) in light saturated range were between 145.5 μmol O₂ h⁻¹ mg⁻¹ Chl a in Zygnema G and 752.9 μmol O₂ h⁻¹ mg⁻¹ Chl a in Zygnema E. After incubation in 800 mM sorbitol for 3 h P_max decreased to the following percentage of the initial values: B: 16.3%, D: 16.8%, E: 26.1% and G: 35.0%. Osmotic stress (800 mM sorbitol) decreased maximum photochemical quantum yield of photosystem II (F_v/F_m) when compared to controls. Maximum values of relative electron transport rates of photosystem II (rETR_max) decreased after incubation in 400 mM sorbitol in Zygnema D and E, while they decreased in Zygnema B and G only after incubation in 800 mM sorbitol. The kinetics of the rETR curves were similar for the Arctic strains Zygnema B and G, but distinct from the Antarctic strains Zygnema D and E, which were similar when compared with each other. This suggests that the investigated Arctic Zygnema sp. strains might be better adapted to tolerate osmotic water stress than the investigated strains from the Antarctic.

Research on cryosestic communities in Svalbard: the snow algae of temporary snowfields in Petuniabukta, Central Svalbard

Although the observations of the colored snow in Svalbard are dated from 17th century, the research in cryosestic algae during last 50 years was focused to NW part of Svalbard, Hornsund and Sveagruva areas. No observations have been reported for the Central or East Svalbard yet, with exception of Longyearbyen and Sveagruva areas. In Petuniabukta, two types of possible cryoseston habitats were recognized: 1) temporal snow fields that melt-away in summer completely, and 2) perennial glacier surfaces. During summer seasons 2009 - 2011, ten temporary snow field samples were sampled in the Petuniabukta area. The cryosestic communities did not cause visible snow coloration, however the presence of snow algae was revealed by light microscope. In samples, zygospores of Chlamydomonas cf. nivalis, were dominant. Other found species included Chlamydomonas nivalis, Chloromonas nivalis, Cryodactylon glaciale, and cf. Chainomonas sp. No motile stages of snow algae were observed. Due to low cell concentration and association of the cells with dust grains, wind transport of snow algae to the snow fields seems to be more possible than presence of a stable cryosestic community growing in situ.

Characterization of an UV- and VIS-absorbing, purpurogallin-derived secondary pigment new to algae and highly abundant in Mesotaenium berggrenii (Zygnematophyceae, Chlorophyta), an extremophyte living on glaciers

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.

Diversity and ecology of psychrophilic microorganisms

Cold environments represent the majority of the biosphere on Earth and have been successfully colonized by psychrophilic microorganisms that are able to thrive at low temperatures and to survive and even maintain metabolic activity at subzero temperatures. These microorganisms play key ecological roles in their habitats and include a wide diversity of representatives of all three domains (Bacteria, Archaea, Eukarya). In this review, we summarize recent knowledge on the abundance, on the taxonomic and functional biodiversity, on low temperature adaptation and on the biogeography of microbial communities in a range of aquatic and terrestrial cold environments.

Cell physiology of plants growing in cold environments

The life of plants growing in cold extreme environments has been well investigated in terms of morphological, anatomical, and ecophysiological adaptations. In contrast, long-term cellular or metabolic studies have been performed by only a few groups. Moreover, a number of single reports exist, which often represent just a glimpse of plant behavior. The review draws together the literature which has focused on tissue and cellular adaptations mainly to low temperatures and high light. Most studies have been done with European alpine plants; comparably well studied are only two phanerogams found in the coastal Antarctic. Plant adaptation in northern polar regions has always been of interest in terms of ecophysiology and plant propagation, but nowadays, this interest extends to the effects of global warming. More recently, metabolic and cellular investigations have included cold and UV resistance mechanisms. Low-temperature stress resistance in plants from cold environments reflects the climate conditions at the growth sites. It is now a matter of molecular analyses to find the induced genes and their products such as chaperones or dehydrins responsible for this resistance. Development of plants under snow or pollen tube growth at 0 degrees C shows that cell biology is needed to explain the stability and function of the cytoskeleton. Many results in this field are based on laboratory studies, but several publications show that it is not difficult to study cellular mechanisms with the plants adapted to a natural stress. Studies on high light and UV loads may be split in two parts. Many reports describe natural UV as harmful for the plants, but these studies were mainly conducted by shielding off natural UV (as controls). Other experiments apply additional UV in the field and have had practically no negative impact on metabolism. The latter group is supported by the observations that green overwintering plants increase their flavonoids under snow even in the absence of UV. Thus, their defense and antioxidant role dominates. Ultrastructural comparisons were unable to find special light adaptations in plants taken from polar regions vs. high alpine species. The only adaptation found at the subcellular level for most alpine and polar plants are protrusions of the chloroplast envelopes. They are seen as a demand for fast membrane transport requiring additional membrane surface area, whereby the increase in stroma volume may help to support carbohydrate formation. Plants forming such protrusions have to cope with a short vegetation time. These observations are connected to the question as to how photosynthesis works quite well even at or under zero temperatures. The interplay between plastids, mitochondria, and peroxisomes, known as photorespiration, seems to be more intense than in lowland plants. This organelle cooperation serves as a valve for a surplus in solar energy input under cold conditions. Additional metabolic acclimations are under investigation, such as the role of an alternative plastid terminal oxidase. Plants from cold environments may also be seen as ideal objects for studying the combined effects of high light plus cold resistance-from the molecular level to the whole plant adaptation. Modern instrumentation makes it possible to perform vital metabolic measurements under outdoor conditions, and research stations in remote polar and alpine areas provide support for scientists in the preparation of samples for later cellular studies in the home laboratory.

Physiological and morphological processes in the Alpine snow alga Chloromonas nivalis (Chlorophyceae) during cyst formation

Amongst a specialised group of psychrophilic microalgae that have adapted to thrive exclusively in summer snow fields, Chloromonas nivalis has been reported as a species causing green, orange or pink blooms in many alpine and polar regions worldwide. Nevertheless, the cytology, ecophysiology and taxonomy of this species are still unresolved. Intracellular processes during cyst formation, which is the dominant stage on snow fields, were examined with samples from the European Alps to better understand the cellular strategies of a green alga living in this harsh habitat. We show with two different methods, i.e. oxygen optode fluorometry and by chlorophyll fluorescence, that the cysts are photosynthetically highly active, although they do not divide, and that Chloromonas nivalis can cope with low as well as high light conditions. During cyst formation, the chloroplast is fragmented into several smaller parts, enlarging the surface to volume ratio. The pool of xanthophyll-cycle pigments is significantly enlarged, which is different from other snow algae. The cytoplasm is filled with lipid bodies containing astaxanthin, a secondary carotenoid that causes the typical orange colour. The cyst wall surface possesses characteristic elongate flanges, which are assembled extracellulary by accumulation of material in the periplasmatic interspace. Comparison of Chloromonas nivalis samples from different locations (Austrian Alps, Spitsbergen) by molecular methods indicates genetic variations due to spatial isolation, while a North American strain has no close relationship to the taxon.

Microbial sequences retrieved from environmental samples from seasonal arctic snow and meltwater from Svalbard, Norway

16S rRNA gene (rrs) clone libraries were constructed from two snow samples (May 11, 2007 and June 7, 2007) and two meltwater samples collected during the spring of 2007 in Svalbard, Norway (79 degrees N). The libraries covered 19 different microbial classes, including Betaproteobacteria (21.3%), Sphingobacteria (16.4%), Flavobacteria (9.0%), Acidobacteria (7.7%) and Alphaproteobacteria (6.5%). Significant differences were detected between the two sets of sample libraries. First, the meltwater libraries had the highest community richness (Chao1: 103.2 and 152.2) and Shannon biodiversity indices (between 3.38 and 3.59), when compared with the snow libraries (Chao1: 14.8 and 59.7; Shannon index: 1.93 and 3.01). Second, integral-LIBSHUFF analyses determined that the bacterial communities in the snow libraries were significantly different from those of the meltwater libraries. Despite these differences, our data also support the theory that a common core group of microbial populations exist within a variety of cryohabitats. Electronic supplementary material The online version of this article (doi:10.1007/s00792-009-0299-2) contains supplementary material, which is available to authorized users.

The vegetative arctic freshwater green alga Zygnema is insensitive to experimental UV exposure

The physiological performance and ultrastructural integrity of the vegetative freshwater green alga Zygnema sp., growing under ambient polar day solar radiation and after exposure to experimentally low radiation, but with high UVR:PAR ratio were investigated. In the laboratory, algae were exposed to low photosynthetic active radiation (PAR=P, 400-700 nm, 20 micromol m(-2) s(-1)), PAR + UV-A = PA (320-400 nm, 4.00 W m(-2) = UV-A) and PAR + UV-A + UV-B = PAB (280-320 nm, 0.42 W m(-2) = UV-B) for 24 h at 7 degrees C. Photosynthetic performance and ultrastructure of ambient solar radiation-exposed (field control) and experimentally treated Zygnema samples were assessed using chlorophyll fluorescence, and transmission electron microscopy (TEM). No significant treatment effect was observed in the photosynthesis-irradiance curve parameters. Exclusion of the UV-B spectrum in the laboratory treatment caused significantly lower effective photosynthetic quantum yield compared to samples exposed to the whole radiation spectrum. TEM revealed no obvious differences in the ultrastructure of field control and laboratory P-, PA- and PAB-exposed samples. Substantial amounts of lipid bodies, visualized by Sudan IV staining, were observed in all samples. Chloroplasts contained numerous plastoglobules. Organelles like mitochondria, Golgi bodies and the nucleus remained unaffected by the radiation exposures. Zygnema is well adapted to ambient solar radiation, enabling the alga to cope with experimental UV exposure and it is expected to persist in a scenario with enhanced UV radiation caused by stratospheric ozone depletion.

Impacts of macroalgal spores on the dynamics of adult macroalgae in a eutrophic estuary: high versus low hydrodynamic seasons and long-term simulations for global warming scenarios

A model describing macroalgal early life stages and adult dynamics was used to quantify the effects of non-adult forms on the productivity of adult macroalgae in a eutrophic estuary. Predictions indicate that during years with mild winters and low rainfall, spring blooms will occur at the expense of the growth and reproduction of overwintering adults and without the contribution of the spore bank. In these circumstances, there is a positive correlation between the intensity of the blooms and the biomass of overwintering adults until a maximum threshold value. On the contrary, in years with high rainfall and low or inexistent biomass of overwintering adults, the onset of adult's growth depends on the biomass of viable dormant spores, the growing season occurs later and adult productivity is very limited. Long-term predictions for climate change scenarios suggest that, in general, global warming will have adverse affects on Ulva intestinalis productivity, with the adults being more affected than the early life stages.

Microbial primary production on an Arctic glacier is insignificant in comparison with allochthonous organic carbon input

Cryoconite holes are unique freshwater environments on glacier surfaces, formed when solar-heated dark debris melts down into the ice. Active photoautotrophic microorganisms are abundant within the holes and fix inorganic carbon due to the availability of liquid water and solar radiation. Cryoconite holes are potentially important sources of organic carbon to the glacial ecosystem, but the relative magnitudes of autochthonous microbial primary production and wind-borne allochthonous organic matter brought are unknown. Here, we compare an estimate of annual microbial primary production in 2006 on Werenskioldbreen, a Svalbard glacier, with the organic carbon content of cryoconite debris. There is a great disparity between annual primary production (4.3 mug C g(-1) year(-1)) and the high content of organic carbon within the debris (1.7-4.5%, equivalent to 8500-22 000 mug C g(-1) debris). Long-term accumulation of autochthonous organic matter is considered unlikely due to ablation dynamics and the surface hydrology of the glacier. Rather, it is more likely that the majority of the organic matter on Werenskioldbreen is allochthonous. Hence, although glacier surfaces can be a significant source of organic carbon for glacial environments on Svalbard, they may be reservoirs rather than oases of high productivity.