Global and local-scale variation in bacterial community structure of snow from the Swiss and Australian Alps

Lichen-associated microbial members are prevalent in the snow microbiome of a sub-arctic alpine tundra

Snow is the largest component of the cryosphere, with its cover and distribution rapidly decreasing over the last decade due to climate warming. It is imperative to characterize the snow (nival) microbial communities to better understand the role of microorganisms inhabiting these rapidly changing environments. Here, we investigated the core nival microbiome, the cultivable microbial members, and the microbial functional diversity of the remote Uapishka mountain range, a massif of alpine sub-arctic tundra and boreal forest. Snow samples were taken over a two-month interval along an altitude gradient with varying degree of anthropogenic traffic and vegetation cover. The core snow alpine tundra/boreal microbiome, which was present across all samples, constituted of Acetobacterales, Rhizobiales and Acidobacteriales bacterial orders, and of Mycosphaerellales and Lecanorales fungal orders, with the dominant fungal taxa being associated with lichens. The snow samples had low active functional diversity, with Richness values ranging from 0 to 19.5. The culture-based viable microbial enumeration ranged from 0 to 8.05 × 103 CFUs/mL. We isolated and whole-genome sequenced five microorganisms which included three fungi, one alga, and one potentially novel bacterium of the Lichenihabitans genus; all of which appear to be part of lichen-associated taxonomic clades.

Life from a Snowflake: Diversity and Adaptation of Cold-Loving Bacteria among Ice Crystals

Incredible as it is, researchers have now the awareness that even the most extreme environment includes special habitats that host several forms of life. Cold environments cover different compartments of the cryosphere, as sea and freshwater ice, glaciers, snow, and permafrost. Although these are very particular environmental compartments in which various stressors coexist (i.e., freeze–thaw cycles, scarce water availability, irradiance conditions, and poorness of nutrients), diverse specialized microbial communities are harbored. This raises many intriguing questions, many of which are still unresolved. For instance, a challenging focus is to understand if microorganisms survive trapped frozen among ice crystals for long periods of time or if they indeed remain metabolically active. Likewise, a look at their site-specific diversity and at their putative geochemical activity is demanded, as well as at the equally interesting microbial activity at subzero temperatures. The production of special molecules such as strategy of adaptations, cryoprotectants, and ice crystal-controlling molecules is even more intriguing. This paper aims at reviewing all these aspects with the intent of providing a thorough overview of the main contributors in investigating the microbial life in the cryosphere, touching on the themes of diversity, adaptation, and metabolic potential.

Spatial and Annual Variation in Microbial Abundance, Community Composition, and Diversity Associated With Alpine Surface Snow

Understanding microbial community dynamics in the alpine cryosphere is an important step toward assessing climate change impacts on these fragile ecosystems and meltwater-fed environments downstream. In this study, we analyzed microbial community composition, variation in community alpha and beta diversity, and the number of prokaryotic cells and virus-like particles (VLP) in seasonal snowpack from two consecutive years at three high altitude mountain summits along a longitudinal transect across the European Alps. Numbers of prokaryotic cells and VLP both ranged around 104 and 105 per mL of snow meltwater on average, with variation generally within one order of magnitude between sites and years. VLP-to-prokaryotic cell ratios spanned two orders of magnitude, with median values close to 1, and little variation between sites and years in the majority of cases. Estimates of microbial community alpha diversity inferred from Hill numbers revealed low contributions of common and abundant microbial taxa to the total taxon richness, and thus low community evenness. Similar to prokaryotic cell and VLP numbers, differences in alpha diversity between years and sites were generally relatively modest. In contrast, community composition displayed strong variation between sites and especially between years. Analyses of taxonomic and phylogenetic community composition showed that differences between sites within years were mainly characterized by changes in abundances of microbial taxa from similar phylogenetic clades, whereas shifts between years were due to significant phylogenetic turnover. Our findings on the spatiotemporal dynamics and magnitude of variation of microbial abundances, community diversity, and composition in surface snow may help define baseline levels to assess future impacts of climate change on the alpine cryosphere.

Isolation and characterization of fast-growing green snow bacteria from coastal East Antarctica

Snow microorganisms play a significant role in climate change and affecting the snow melting rate in the Arctic and Antarctic regions. While research on algae inhabiting green and red snow has been performed extensively, bacteria dwelling in this biotope have been studied to a much lesser extent. In this study, we performed 16S rRNA gene amplicon sequencing of two green snow samples collected from the coastal area of the eastern part of Antarctica and conducted genotypic and phenotypic profiling of 45 fast-growing bacteria isolated from these samples. 16S rRNA gene amplicon sequencing of two green snow samples showed that bacteria inhabiting these samples are mostly represented by families Burkholderiaceae (46.31%), Flavobacteriaceae (22.98%), and Pseudomonadaceae (17.66%). Identification of 45 fast-growing bacteria isolated from green snow was performed using 16S rRNA gene sequencing. We demonstrated that they belong to the phyla Actinobacteria and Proteobacteria, and are represented by the genera Arthrobacter, Cryobacterium, Leifsonia, Salinibacterium, Paeniglutamicibacter, Rhodococcus, Polaromonas, Pseudomonas, and Psychrobacter. Nearly all bacterial isolates exhibited various growth temperatures from 4°C to 25°C, and some isolates were characterized by a high level of enzymatic activity. Phenotyping using Fourier transform infrared (FTIR) spectroscopy revealed a possible accumulation of intracellular polymer polyhydroxyalkanoates (PHA) or lipids in some isolates. The bacteria showed different lipids/PHA and protein profiles. It was shown that lipid/PHA and protein spectral regions are the most discriminative for differentiating the isolates.

Short-term effects of snow cover manipulation on soil bacterial diversity and community composition

Winter snow cover is a major driver of soil microbial processes in high-latitude and high-altitude ecosystems. Warming-induced reduction in snow cover as predicted under future climate scenarios may shift soil bacterial communities with consequences for soil carbon and nutrient cycling. The underlying mechanisms, however, remain elusive. In the present study, we conducted a snow manipulation experiment in a Tibetan spruce forest to explore the immediate and intra-annual legacy effects of snow exclusion on soil bacterial communities. We analyzed bacterial diversity and community composition in the winter (i.e., the deep snow season), in the transitional thawing period, and in the middle of the growing season. Proteobacteria, Acidobacteria, and Actinobacteria were dominant phyla across the seasons and snow regimes. Bacterial diversity was generally not particularly sensitive to the absence of snow cover. However, snow exclusion positively affected Simpson diversity in the winter but not in the thawing period and the growing season. Bacterial diversity further tended to be higher in winter than in the growing season. In the winter, the taxonomic composition shifted in response to snow exclusion, while composition did not differ between exclusion and control plots in the thawing period and the growing season. Soil bacterial communities strongly varied across seasons, and the variations differed in specific groups. Both soil climatic factors (i.e., temperature and moisture) and soil biochemical variables partly accounted for the seasonal dynamics of bacterial communities. Taken together, our study indicates that soil bacterial communities in Tibetan forests are rather resilient to change in snow cover, at least at an intra-annual scale.

Snow microbiome functional analyses reveal novel aspects of microbial metabolism of complex organic compounds

Microbes active in extreme cold are not as well explored as those of other extreme environments. Studies have revealed a substantial microbial diversity and identified cold-specific microbiome molecular functions. We analyzed the metagenomes and metatranscriptomes of 20 snow samples collected in early and late spring in Svalbard, Norway using mi-faser, our read-based computational microbiome function annotation tool. Our results reveal a more diverse microbiome functional capacity and activity in the early- vs. late-spring samples. We also find that functional dissimilarity between the same-sample metagenomes and metatranscriptomes is significantly higher in early than late spring samples. These findings suggest that early spring samples may contain a larger fraction of DNA of dormant (or dead) organisms, while late spring samples reflect a new, metabolically active community. We further show that the abundance of sequencing reads mapping to the fatty acid synthesis-related microbial pathways in late spring metagenomes and metatranscriptomes is significantly correlated with the organic acid levels measured in these samples. Similarly, the organic acid levels correlate with the pathway read abundances of geraniol degradation and inversely correlate with those of styrene degradation, suggesting a possible nutrient change. Our study thus highlights the activity of microbial degradation pathways of complex organic compounds previously unreported at low temperatures.

Reconstruction of the Functional Ecosystem in the High Light, Low Temperature Union Glacier Region, Antarctica

Antarctica is covered by multiple larger glaciers with diverse extreme conditions. Microorganisms in Antarctic regions are primarily responsible for diverse biogeochemical processes. The identity and functionality of microorganisms from polar glaciers are defined. However, little is known about microbial communities from the high elevation glaciers. The Union Glacier, located in the inland of West Antarctica at 79°S, is a challenging environment for life to survive due to the high irradiance and low temperatures. Here, soil and rock samples were obtained from three high mountains (Rossman Cove, Charles Peak, and Elephant Head) adjacent to the Union Glacier. Using metagenomic analyses, the functional microbial ecosystem was analyzed through the reconstruction of carbon, nitrogen and sulfur metabolic pathways. A low biomass but diverse microbial community was found. Although archaea were detected, bacteria were dominant. Taxa responsible for carbon fixation were comprised of photoautotrophs (Cyanobacteria) and chemoautotrophs (mainly Alphaproteobacterial clades: Bradyrhizobium, Sphingopyxis, and Nitrobacter). The main nitrogen fixation taxa were Halothece (Cyanobacteria), Methyloversatilis, and Leptothrix (Betaproteobacteria). Diverse sulfide-oxidizing and sulfate-reducing bacteria, fermenters, denitrifying microbes, methanogens, and methane oxidizers were also found. Putative producers provide organic carbon and nitrogen for the growth of other heterotrophic microbes. In the biogeochemical pathways, assimilation and mineralization of organic compounds were the dominant processes. Besides, a range of metabolic pathways and genes related to high irradiance, low temperature and other stress adaptations were detected, which indicate that the microbial communities had adapted to and could survive in this harsh environment. These results provide a detailed perspective of the microbial functional ecology of the Union Glacier area and improve our understanding of linkages between microbial communities and biogeochemical cycling in high Antarctic ecosystems.

Spatial Variability of Antarctic Surface Snow Bacterial Communities

It was once a long-held view that the Antarctic was a pristine environment with low biomass, low biodiversity and low rates of microbial activity. However, as the intensity of scientific investigation has increased, so these views have started to change. In particular, the role and impact of human activity toward indigenous microbial communities has started to come under more intense scrutiny. During the Subglacial Lake Ellsworth exploration campaign in December 2012, a microbiological survey was conducted to determine the extent and likelihood of exogenous input into the subglacial lake system during the hot-water drilling process. Snow was collected from the surface to represent that used for melt water production for hot-water drilling. The results of this study showed that snow used to provide melt water differed in its microbiological composition from that of the surrounding area and raised the question of how the biogeography of snow-borne microorganisms might influence the potential outcome of scientific analyses. In this study, we investigated the biogeography of microorganisms in snow around a series of Antarctic logistic hubs, where human activity was clearly apparent, and from which scientific investigations have been undertaken. A change in microbial community structure with geographical location was apparent and, notably, a decrease in alpha diversity at more remote southern latitudes. Soil-related microorganisms dominated microbial assemblages suggesting terrestrial input, most likely from long-range aeolian transport into continental Antarctica. We also observed that relic DNA was not a major issue when assessing snow samples. Overall, our observations might have profound implications for future scientific activities in Antarctica, such as the need to establish "no-go" protected areas, the need for better characterization of field sites and improved protocols for sterilization and verification of ice drilling equipment.

Sources and selection of snow-specific microbial communities in a Greenlandic sea ice snow cover

Sea ice and its snow cover are critical for global processes including climate regulation and biogeochemical cycles. Despite an increase in studies focused on snow microorganisms, the ecology of snow inhabitants remains unclear. In this study, we investigated sources and selection of a snowpack-specific microbial community by comparing metagenomes from samples collected in a Greenlandic fjord within a vertical profile including atmosphere, snowpack with four distinct layers of snow, sea ice brine and seawater. Microbial communities in all snow layers derived from mixed sources, both marine and terrestrial, and were more similar to atmospheric communities than to sea ice or seawater communities. The surface snow metagenomes were characterized by the occurrence of genes involved in photochemical stress resistance, primary production and metabolism of diverse carbon sources. The basal saline snow layer that was in direct contact with the sea ice surface harbored a higher abundance of cells than the overlying snow layers, with a predominance of Alteromonadales and a higher relative abundance of marine representatives. However, the overall taxonomic structure of the saline layer was more similar to that of other snow layers and the atmosphere than to underlying sea ice and seawater. The expulsion of relatively nutrient-rich sea ice brine into basal snow might have stimulated the growth of copiotrophic psychro- and halotolerant snow members. Our study indicates that the size, composition and function of snowpack microbial communities over sea ice were influenced primarily by atmospheric deposition and inflow of sea ice brine and that they form a snow-specific assemblage reflecting the particular environmental conditions of the snowpack habitat.

Microbial connectivity and sorting in a High Arctic watershed

Aquatic ecosystems in the High Arctic are facing unprecedented changes as a result of global warming effects on the cryosphere. Snow pack is a central feature of northern landscapes, but the snow microbiome and its microbial connectivity to adjacent and downstream habitats have been little explored. To evaluate these aspects, we sampled along a hydrologic continuum at Ward Hunt Lake (latitude 83°N) in the Canadian High Arctic, from snow banks, water tracks in the permafrost catchment, the upper and lower strata of the lake, and the lake outlet and its coastal marine mixing zone. The microbial communities were analyzed by high-throughput sequencing of 16 and 18S rRNA to determine the composition of potentially active Bacteria, Archaea and microbial Eukarya. Each habitat had distinct microbial assemblages, with highest species richness in the subsurface water tracks that connected the melting snow to the lake. However, up to 30% of phylotypes were shared along the hydrologic continuum, showing that many taxa originating from the snow can remain in the active fraction of downstream microbiomes. The results imply that changes in snowfall associated with climate warming will affect microbial community structure throughout all spatially connected habitats within snow-fed polar ecosystems.

The future of genomics in polar and alpine cyanobacteria

In recent years, genomic analyses have arisen as an exciting way of investigating the functional capacity and environmental adaptations of numerous micro-organisms of global relevance, including cyanobacteria. In the extreme cold of Arctic, Antarctic and alpine environments, cyanobacteria are of fundamental ecological importance as primary producers and ecosystem engineers. While their role in biogeochemical cycles is well appreciated, little is known about the genomic makeup of polar and alpine cyanobacteria. In this article, we present ways that genomic techniques might be used to further our understanding of cyanobacteria in cold environments in terms of their evolution and ecology. Existing examples from other environments (e.g. marine/hot springs) are used to discuss how methods developed there might be used to investigate specific questions in the cryosphere. Phylogenomics, comparative genomics and population genomics are identified as methods for understanding the evolution and biogeography of polar and alpine cyanobacteria. Transcriptomics will allow us to investigate gene expression under extreme environmental conditions, and metagenomics can be used to complement tradition amplicon-based methods of community profiling. Finally, new techniques such as single cell genomics and metagenome assembled genomes will also help to expand our understanding of polar and alpine cyanobacteria that cannot readily be cultured.

LEA proteins are involved in cyst desiccation resistance and other abiotic stresses in Azotobacter vinelandii

Late embryogenesis abundant (LEA) proteins constitute a large protein family that is closely associated with resistance to abiotic stresses in multiple organisms and protect cells against drought and other stresses. Azotobacter vinelandii is a soil bacterium that forms desiccation-resistant cysts. This bacterium possesses two genes, here named lea1 and lea2, coding for avLEA1 and avLEA2 proteins, both containing 20-mer motifs characteristic of eukaryotic plant LEA proteins. In this study, we found that disruption of the lea1 gene caused a loss of the cysts' viability after 3 months of desiccation, whereas at 6 months, wild-type or lea2 mutant strain cysts remained viable. Vegetative cells of the lea1 mutant were more sensitive to osmotic stress; cysts developed by this mutant were also more sensitive to high temperatures than cysts or vegetative cells of the wild type or of the lea2 mutant. Expression of lea1 was induced several fold during encystment. In addition, the protective effects of these proteins were assessed in Escherichia coli cells. We found that E. coli cells overexpressing avLEA1 were more tolerant to salt stress than control cells; finally, in vitro analysis showed that avLEA1 protein was able to prevent the freeze thaw-induced inactivation of lactate dehydrogenase. In conclusion, avLEA1 is essential for the survival of A. vinelandii in dry conditions and for protection against hyper-osmolarity, two major stress factors that bacteria must cope with for survival in the environment. This is the first report on the role of bacterial LEA proteins on the resistance of cysts to desiccation.