Atmospheric trace gases support primary production in Antarctic desert surface soil

Unearthing terrestrial extreme microbiomes for searching terrestrial-like life in the Solar System

The possibility of life elsewhere in the universe has fascinated humankind for ages. To the best of our knowledge, life, as we know it, is limited to planet Earth; yet current investigation suggests that life might be more common than previously thought. In this review, we explore extreme terrestrial analogue environments in the search for some notable examples of extreme organisms, including overlooked microbial groups such as viruses, fungi, and protists, associated with limits of life on Earth. This knowledge is integral to provide the foundational principles needed to predict what sort of Earth-like organisms we might find in the Solar System and beyond, and to understand the future and origins of life on Earth.

Microbial Community Structure and Metabolic Potential at the Initial Stage of Soil Development of the Glacial Forefields in Svalbard

Microbial communities have been identified as the primary inhabitants of Arctic forefields. However, the metabolic potential of microbial communities in these newly exposed soils remains underexplored due to limited access. Here, we sampled the very edge of the glacial forefield in Svalbard and performed the 16S rRNA genes and metagenomic analysis to illustrate the ecosystem characteristics. Burkholderiales and Micrococcales were the dominant bacterial groups at the initial stage of soil development of glacial forefields. 214 metagenome-assembled genomes were recovered from glacier forefield microbiome datasets, including only 2 belonging to archaea. Analysis of these metagenome-assembled genomes revealed that 41% of assembled genomes had the genetic potential to use nitrate and nitrite as electron acceptors. Metabolic pathway reconstruction for these microbes suggested versatility for sulfide and thiosulfate oxidation, H₂ and CO utilization, and CO₂ fixation. Our results indicate the importance of anaerobic processes in elemental cycling in the glacial forefields. Besides, a range of genes related to adaption to low temperature and other stresses were detected, which revealed the presence of diverse mechanisms of adaption to the extreme environment of Svalbard. This research provides ecological insight into the initial stage of the soil developed during the retreating of glaciers.

Insights into the microbial life in silica-rich subterranean environments: microbial communities and ecological interactions in an orthoquartzite cave (Imawarì Yeuta, Auyan Tepui, Venezuela)

Microbial communities inhabiting caves in quartz-rich rocks are still underexplored, despite their possible role in the silica cycle. The world’s longest orthoquartzite cave, Imawarì Yeuta, represents a perfect arena for the investigation of the interactions between microorganisms and silica in non-thermal environments due to the presence of extraordinary amounts of amorphous silica speleothems of different kinds. In this work, the microbial diversity of Imawarì Yeuta was dissected by analyzing nineteen samples collected from different locations representative of different silica amorphization phases and types of samples. Specifically, we investigated the major ecological patterns in cave biodiversity, specific taxa enrichment, and the main ecological clusters through co-occurrence network analysis. Water content greatly contributed to the microbial communities’ composition and structures in the cave leading to the sample clustering into three groups DRY, WET, and WATER. Each of these groups was enriched in members of Actinobacteriota, Acidobacteriota, and Gammaproteobacteria, respectively. Alpha diversity analysis showed the highest value of diversity and richness for the WET samples, while the DRY group had the lowest. This was accompanied by the presence of correlation patterns including either orders belonging to various phyla from WET samples or orders belonging to the Actinobacteriota and Firmicutes phyla from DRY group samples. The phylogenetic analysis of the dominant species in WET and DRY samples showed that Acidobacteriota and Actinobacteriota strains were affiliated with uncultured bacteria retrieved from various oligotrophic and silica/quartz-rich environments, not only associated with subterranean sites. Our results suggest that the water content greatly contributes to shaping the microbial diversity within a subterranean quartzite environment. Further, the phylogenetic affiliation between Imawarì Yeuta dominant microbes and reference strains retrieved from both surface and subsurface silica- and/or CO₂/CO-rich environments, underlines the selective pressure applied by quartz as rock substrate. Oligotrophy probably in association with the geochemistry of silica/quartz low pH buffering activity and alternative energy sources led to the colonization of specific silica-associated microorganisms. This study provides clues for a better comprehension of the poorly known microbial life in subsurface and surface quartz-dominated environments.

Crop-type-driven changes in polyphenols regulate soil nutrient availability and soil microbiota

Crop rotation is a typical agronomic practice to mitigate soil deterioration caused by continuous cropping. However, the mechanisms of soil biotic and abiotic factors in response to different cropping patterns in acidic and polyphenol-rich tea nurseries remain unclear. In this study, the composition and function of microbial communities were comparatively investigated in soils of tea seedlings continuously planted for 2 years (AC: autumn-cutting; SC: summer-cutting) and in soils rotation with strawberries alternately for 3 years (AR: autumn-cutting). The results showed that AR significantly improved the survival of tea seedlings but greatly reduced the contents of soil polyphenols. The lower soil polyphenol levels in AR were associated with the decline of nutrients (SOC, TN, Olsen-P) availability, which stimulates the proliferation of nutrient cycling-related bacteria and mixed-trophic fungi, endophytic fungi and ectomycorrhizal fungi, thus further satisfying the nutrient requirements of tea seedlings. Moreover, lower levels of polyphenols facilitated the growth of plant beneficial microorganisms (Bacillus, Mortierella, etc.) and suppressed pathogenic fungi (Pseudopestalotiopsis, etc.), creating a more balanced microbial community that is beneficial to plant health. Our study broadens the understanding of the ecological role of plant secondary metabolites and provides new insights into the sustainability of tea breeding.

Atmospheric chemosynthesis is phylogenetically and geographically widespread and contributes significantly to carbon fixation throughout cold deserts

Cold desert soil microbiomes thrive despite severe moisture and nutrient limitations. In Eastern Antarctic soils, bacterial primary production is supported by trace gas oxidation and the light-independent RuBisCO form IE. This study aims to determine if atmospheric chemosynthesis is widespread within Antarctic, Arctic and Tibetan cold deserts, to identify the breadth of trace gas chemosynthetic taxa and to further characterize the genetic determinants of this process. H₂ oxidation was ubiquitous, far exceeding rates reported to fulfill the maintenance needs of similarly structured edaphic microbiomes. Atmospheric chemosynthesis occurred globally, contributing significantly (p < 0.05) to carbon fixation in Antarctica and the high Arctic. Taxonomic and functional analyses were performed upon 18 cold desert metagenomes, 230 dereplicated medium-to-high-quality derived metagenome-assembled genomes (MAGs) and an additional 24,080 publicly available genomes. Hydrogenotrophic and carboxydotrophic growth markers were widespread. RuBisCO IE was discovered to co-occur alongside trace gas oxidation enzymes in representative Chloroflexota, Firmicutes, Deinococcota and Verrucomicrobiota genomes. We identify a novel group of high-affinity [NiFe]-hydrogenases, group 1m, through phylogenetics, gene structure analysis and homology modeling, and reveal substantial genetic diversity within RuBisCO form IE (rbcL1E), and high-affinity 1h and 1l [NiFe]-hydrogenase groups. We conclude that atmospheric chemosynthesis is a globally-distributed phenomenon, extending throughout cold deserts, with significant implications for the global carbon cycle and bacterial survival within environmental reservoirs.

The Microbiomes in Lichen and Moss Biocrust Contribute Differently to Carbon and Nitrogen Cycles in Arid Ecosystems

Biological soil crusts (biocrusts) are distributed in arid and semiarid regions across the globe. Microorganisms are an essential component in biocrusts. They add and accelerate critical biochemical processes. However, little is known about the functional genes and metabolic processes of microbiomes in lichen and moss biocrust. This study used shotgun metagenomic sequencing to compare the microbiomes of lichen-dominated and moss-dominated biocrust and reveal the microbial genes and metabolic pathways involved in carbon and nitrogen cycling. The results showed that Actinobacteria, Bacteroidetes, and Acidobacteria were more abundant in moss biocrust than lichen biocrust, while Proteobacteria and Cyanobacteria were more abundant in lichen biocrust than moss biocrust. The relative abundance of carbohydrate-active enzymes and enzymes associated with carbon and nitrogen metabolism differed significantly between microbiomes of the two biocrust types. However, in the microbial communities of both biocrust types, respiration pathways dominated over carbon fixation pathways. The genes encoding carbon monoxide dehydrogenase were more abundant than those encoding ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCo) involved in carbon fixation. Similarly, metabolic N-pathway diversity was dominated by nitrogen reduction, followed by denitrification, with nitrogen fixation the lowest proportion. Gene diversity involved in N cycling differed between the microbiomes of the two biocrust types. Assimilatory nitrate reduction genes had higher relative abundance in lichen biocrust, whereas dissimilatory nitrate reduction genes had higher relative abundance in moss biocrust. As dissolved organic carbon and soil organic carbon are considered the main drivers of the community structure in the microbiome of biocrust, these results indicate that biocrust type has a pivotal role in microbial diversity and related biogeochemical cycling.

Soil substrate culturing approaches recover diverse members of Actinomycetota from desert soils of Herring Island, East Antarctica

Antimicrobial resistance is an escalating health crisis requiring urgent action. Most antimicrobials are natural products (NPs) sourced from Actinomycetota, particularly the Streptomyces. Underexplored and extreme environments are predicted to harbour novel microorganisms with the capacity to synthesise unique metabolites. Herring Island is a barren and rocky cold desert in East Antarctica, remote from anthropogenic impact. We aimed to recover rare and cold-adapted NP-producing bacteria, by employing two culturing methods which mimic the natural environment: direct soil culturing and the soil substrate membrane system. First, we analysed 16S rRNA gene amplicon sequencing data from 18 Herring Island soils and selected the soil sample with the highest Actinomycetota relative abundance (78%) for culturing experiments. We isolated 166 strains across three phyla, including novel and rare strains, with 94% of strains belonging to the Actinomycetota. These strains encompassed thirty-five ‘species’ groups, 18 of which were composed of Streptomyces strains. We screened representative strains for genes which encode polyketide synthases and non-ribosomal peptide synthetases, indicating that 69% have the capacity to synthesise polyketide and non-ribosomal peptide NPs. Fourteen Streptomyces strains displayed antimicrobial activity against selected bacterial and yeast pathogens using an in situ assay. Our results confirm that the cold-adapted bacteria of the harsh East Antarctic deserts are worthy targets in the search for bioactive compounds.

Salt flat microbial diversity and dynamics across salinity gradient

Sabkhas are hypersaline, mineral-rich, supratidal mudflats that harbor microbes that are adapted to high salt concentration. Sabkha microbial diversity is generally studied for their community composition, but less is known about their genetic structure and heterogeneity. In this study, we analyzed a coastal sabkha for its microbial composition using 16S rDNA and whole metagenome, as well as for its population genetic structure. Our 16S rDNA analysis show high alpha diversity in both inner and edge sabkha than outer sabkha. Beta diversity result showed similar kind of microbial composition between inner and edge sabkha, while outer sabkha samples show different microbial composition. At phylum level, Bacteroidetes (~ 22 to 34%), Euryarchaeota (~ 18 to ~ 30%), unclassified bacteria (~ 24 to ~ 35%), Actinobacteria (~ 0.01 to ~ 11%) and Cyanobacteria (less than 1%) are predominantly found in both inside and edge sabkha regions, whereas Proteobacteria (~ 92 to ~ 97%) and Parcubacteria (~ 1 to ~ 2%) are predominately found in outer sabkha. Our 225 metagenomes assembly from this study showed similar bacterial community profile as observed in 16S rDNA-based analysis. From the assembled genomes, we found important genes that are involved in biogeochemical cycles and secondary metabolite biosynthesis. We observed a dynamic, thriving ecosystem that engages in metabolic activity that shapes biogeochemical structure via carbon fixation, nitrogen, and sulfur cycling. Our results show varying degrees of horizontal gene transfers (HGT) and homologous recombination, which correlates with the observed high diversity for these populations. Moreover, our pairwise population differentiation (Fst) for the abundance of species across the salinity gradient of sabkhas identified genes with strong allelic differentiation, lower diversity and elevated nonsynonymous to synonymous ratio of variants, which suggest selective sweeps for those gene variants. We conclude that the process of HGT, combined with recombination and gene specific selection, constitute the driver of genetic variation in bacterial population along a salinity gradient in the unique sabkha ecosystem.

Active lithoautotrophic and methane-oxidizing microbial community in an anoxic, sub-zero, and hypersaline High Arctic spring

Lost Hammer Spring, located in the High Arctic of Nunavut, Canada, is one of the coldest and saltiest terrestrial springs discovered to date. It perennially discharges anoxic (<1 ppm dissolved oxygen), sub-zero (~-5 °C), and hypersaline (~24% salinity) brines from the subsurface through up to 600 m of permafrost. The sediment is sulfate-rich (1 M) and continually emits gases composed primarily of methane (~50%), making Lost Hammer the coldest known terrestrial methane seep and an analog to extraterrestrial habits on Mars, Europa, and Enceladus. A multi-omics approach utilizing metagenome, metatranscriptome, and single-amplified genome sequencing revealed a rare surface terrestrial habitat supporting a predominantly lithoautotrophic active microbial community driven in part by sulfide-oxidizing Gammaproteobacteria scavenging trace oxygen. Genomes from active anaerobic methane-oxidizing archaea (ANME-1) showed evidence of putative metabolic flexibility and hypersaline and cold adaptations. Evidence of anaerobic heterotrophic and fermentative lifestyles were found in candidate phyla DPANN archaea and CG03 bacteria genomes. Our results demonstrate Mars-relevant metabolisms including sulfide oxidation, sulfate reduction, anaerobic oxidation of methane, and oxidation of trace gases (H₂, CO₂) detected under anoxic, hypersaline, and sub-zero ambient conditions, providing evidence that similar extant microbial life could potentially survive in similar habitats on Mars.

Unique Geothermal Chemistry Shapes Microbial Communities on Mt. Erebus, Antarctica

Mt. Erebus, Antarctica, is the world's southernmost active volcano and is unique in its isolation from other major active volcanic systems and its distinctive geothermal systems. Using 16S rRNA gene amplicon sequencing and physicochemical analyses, we compared samples collected at two contrasting high-temperature (50°C-65°C) sites on Mt. Erebus: Tramway Ridge, a weather-protected high biomass site, and Western Crater, an extremely exposed low biomass site. Samples were collected along three thermal gradients, one from Western Crater and two within Tramway Ridge, which allowed an examination of the heterogeneity present at Tramway Ridge. We found distinct soil compositions between the two sites, and to a lesser extent within Tramway Ridge, correlated with disparate microbial communities. Notably, pH, not temperature, showed the strongest correlation with these differences. The abundance profiles of several microbial groups were different between the two sites; class Nitrososphaeria amplicon sequence variants (ASVs) dominated the community profiles at Tramway Ridge, whereas Acidobacteriotal ASVs were only found at Western Crater. A co-occurrence network, paired with physicochemical analyses, allowed for finer scale analysis of parameters correlated with differential abundance profiles, with various parameters (total carbon, total nitrogen, soil moisture, soil conductivity, sulfur, phosphorous, and iron) showing significant correlations. ASVs assigned to Chloroflexi classes Ktedonobacteria and Chloroflexia were detected at both sites. Based on the known metabolic capabilities of previously studied members of these groups, we predict that chemolithotrophy is a common strategy in this system. These analyses highlight the importance of conducting broader-scale metagenomics and cultivation efforts at Mt. Erebus to better understand this unique environment.

Microbial oxidation of atmospheric trace gases

The atmosphere has recently been recognized as a major source of energy sustaining life. Diverse aerobic bacteria oxidize the three most abundant reduced trace gases in the atmosphere, namely hydrogen (H₂), carbon monoxide (CO) and methane (CH₄). This Review describes the taxonomic distribution, physiological role and biochemical basis of microbial oxidation of these atmospheric trace gases, as well as the ecological, environmental, medical and astrobiological importance of this process. Most soil bacteria and some archaea can survive by using atmospheric H₂ and CO as alternative energy sources, as illustrated through genetic studies on Mycobacterium cells and Streptomyces spores. Certain specialist bacteria can also grow on air alone, as confirmed by the landmark characterization of Methylocapsa gorgona, which grows by simultaneously consuming atmospheric CH₄, H₂ and CO. Bacteria use high-affinity lineages of metalloenzymes, namely hydrogenases, CO dehydrogenases and methane monooxygenases, to utilize atmospheric trace gases for aerobic respiration and carbon fixation. More broadly, trace gas oxidizers enhance the biodiversity and resilience of soil and marine ecosystems, drive primary productivity in extreme environments such as Antarctic desert soils and perform critical regulatory services by mitigating anthropogenic emissions of greenhouse gases and toxic pollutants.

Bacterial microbiomes from mucus and breath of southern resident killer whales (Orcinus orca)

Opportunities to assess odontocete health are restricted due to their limited time at the surface, relatively quick movements and large geographic ranges. For endangered populations such as the southern resident killer whales (SKRWs) of the northeast Pacific Ocean, taking advantage of non-invasive samples such as expelled mucus and exhaled breath is appealing. Over the past 12 years, such samples were collected, providing a chance to analyse and assess their bacterial microbiomes using amplicon sequencing. Based on operational taxonomic units, microbiome communities from SRKW and transient killer whales showed little overlap between mucus, breath and seawater from SRKW habitats and six bacterial phyla were prominent in expelled mucus but not in seawater. Mollicutes and Fusobacteria were common and abundant in mucus, but not in breath or seawater, suggesting these bacterial classes may be normal constituents of the SRKW microbiome. Out of 134 bacterial families detected, 24 were unique to breath and mucus, including higher abundances of Burkholderiaceae, Moraxellaceae and Chitinophagaceae. Although there were multiple bacterial genera in breath or mucus that include pathogenic species (e.g. Campylobacter, Hemophilus, Treponema), the presence of these bacteria is not necessarily evidence of disease or infection. Future emphasis on genotyping mucus samples to the individual animal will allow further assessment in the context of that animal's history, including body condition index and prior contaminants burden. This study is the first to examine expelled mucus from cetaceans for microbiomes and demonstrates the value of analysing these types of non-invasive samples.

Shifts in Bacterial Community Composition and Functional Traits at Different Time Periods Post-deglaciation of Gangotri Glacier, Himalaya

Climate change causes an unprecedented increase in glacial retreats. The melting ice exposes land for colonization and diversification of bacterial communities leading to soil development, changes in plant community composition, and ecosystem functioning. Although a few studies have focused on macro-level deglaciation impacts, little is known about such effects on the bacterial community succession. Here, we provide meta-barcoding-based insight into the ecological attributes of bacterial community across different retreating periods of the Gangotri glacier, western Himalaya. We selected three sites along a terminal moraine representing recent (~ 20 yrs), intermediate (~ 100 yrs), and late (~ 300 yrs) deglaciation periods. Results showed that the genus Mycobacterium belonging to phylum Actinobacteria dominated recently deglaciated land. Relative abundance of these pioneer bacterial taxa decreased by 20-50% in the later stages with the emergence of new and rising of the less abundant members of the phyla Proteobacteria, Firmicutes, Planctomycetes, Acidobacteria, Verrucomicrobia, Candidatus TM6, and Chloroflexi. The community in the recent stage was less rich and harbored competitive interactions, while the later stages experienced a surge in bacterial diversity with cooperative interactions. The shift in α-diversity and composition was strongly influenced by soil organic carbon, carbon to nitrogen ratio, and soil moisture content. The functional analyses revealed a progression from a metabolism focused to a functionally progressive community required for bacterial co-existence and succession in plant communities. Overall, the findings indicate that the bacterial communities inhabit, diversify, and develop specialized functions post-deglaciation leading to nutrient inputs to soil and vegetation development, which may provide feedback to climate change.

Out of Thin Air? Astrobiology and Atmospheric Chemotrophy

The emerging understanding of microbial trace gas chemotrophy as a metabolic strategy to support energy and carbon acquisition for microbial survival and growth has significant implications in the search for past, and even extant, life beyond Earth. The use of trace gases, including hydrogen and carbon monoxide as substrates for microbial oxidation, potentially offers a viable strategy with which to support life on planetary bodies that possess a suitable atmospheric composition, such as Mars and Titan. Here, we discuss the current state of knowledge of this process and explore its potential in the field of astrobiological exploration.

Endolithic Bacterial Diversity in Lichen-Dominated Communities Is Shaped by Sun Exposure in McMurdo Dry Valleys, Antarctica

The diversity and composition of endolithic bacterial diversity of several locations in McMurdo Dry Valleys (Continental Antarctica) were explored using amplicon sequencing, targeting the V3 and V4 of the 16S region. Despite the increasing interest in edaphic factors that drive bacterial community composition in Antarctic rocky communities, few researchers focused attention on the direct effects of sun exposure on bacterial diversity; we herein reported significant differences in the northern and southern communities. The analysis of β-diversity showed significant differences among sampled localities. For instance, the most abundant genera found in the north-exposed rocks were Rhodococcus and Blastococcus in Knobhead Mt.; Ktedonobacter and Cyanobacteria Family I Group I in Finger Mt.; Rhodococcus and Endobacter in University Valley; and Segetibacter and Tetrasphaera in Siegfried Peak samples. In south-exposed rocks, instead, the most abundant genera were Escherichia/Shigella and Streptococcus in Knobhead Mt.; Ktedonobacter and Rhodococcus in Finger Mt.; Ktedonobacter and Roseomonas in University Valley; and Blastocatella, Cyanobacteria Family I Group I and Segetibacter in Siegfried Peak. Significant biomarkers, detected by the Linear discriminant analysis Effect Size, were also found among north- and south-exposed communities. Besides, the large number of positive significant co-occurrences may suggest a crucial role of positive associations over competitions under the harsher conditions where these rock-inhabiting microorganisms spread. Although the effect of geographic distances in these extreme environments play a significant role in shaping biodiversity, the study of an edaphic factor, such as solar exposure, adds an important contribution to the mosaic of microbial biodiversity of Antarctic bacterial cryptoendolithic communities.

Geochemically Defined Space-for-Time Transects Successfully Capture Microbial Dynamics Along Lacustrine Chronosequences in a Polar Desert

The space-for-time substitution approach provides a valuable empirical assessment to infer temporal effects of disturbance from spatial gradients. Applied to predict the response of different ecosystems under current climate change scenarios, it remains poorly tested in microbial ecology studies, partly due to the trophic complexity of the ecosystems typically studied. The McMurdo Dry Valleys (MDV) of Antarctica represent a trophically simple polar desert projected to experience drastic changes in water availability under current climate change scenarios. We used this ideal model system to develop and validate a microbial space-for-time sampling approach, using the variation of geochemical profiles that follow alterations in water availability and reflect past changes in the system. Our framework measured soil electrical conductivity, pH, and water activity in situ to geochemically define 17 space-for-time transects from the shores of four dynamic and two static Dry Valley lakes. We identified microbial taxa that are consistently responsive to changes in wetness in the soils and reliably associated with long-term dry or wet edaphic conditions. Comparisons between transects defined at static (open-basin) and dynamic (closed-basin) lakes highlighted the capacity for geochemically defined space-for-time gradients to identify lasting deterministic impacts of historical changes in water presence on the structure and diversity of extant microbial communities. We highlight the potential for geochemically defined space-for-time transects to resolve legacy impacts of environmental change when used in conjunction with static and dynamic scenarios, and to inform future environmental scenarios through changes in the microbial community structure, composition, and diversity.

Large-scale characterization of the macrolide resistome reveals high diversity and several new pathogen-associated genes

Macrolides are broad-spectrum antibiotics used to treat a range of infections. Resistance to macrolides is often conferred by mobile resistance genes encoding Erm methyltransferases or Mph phosphotransferases. New erm and mph genes keep being discovered in clinical settings but their origins remain unknown, as is the type of macrolide resistance genes that will appear in the future. In this study, we used optimized hidden Markov models to characterize the macrolide resistome. Over 16 terabases of genomic and metagenomic data, representing a large taxonomic diversity (11 030 species) and diverse environments (1944 metagenomic samples), were searched for the presence of erm and mph genes. From this data, we predicted 28 340 macrolide resistance genes encoding 2892 unique protein sequences, which were clustered into 663 gene families (<70 % amino acid identity), of which 619 (94 %) were previously uncharacterized. This included six new resistance gene families, which were located on mobile genetic elements in pathogens. The function of ten predicted new resistance genes were experimentally validated in Escherichia coli using a growth assay. Among the ten tested genes, seven conferred increased resistance to erythromycin, with five genes additionally conferring increased resistance to azithromycin, showing that our models can be used to predict new functional resistance genes. Our analysis also showed that macrolide resistance genes have diverse origins and have transferred horizontally over large phylogenetic distances into human pathogens. This study expands the known macrolide resistome more than ten-fold, provides insights into its evolution, and demonstrates how computational screening can identify new resistance genes before they become a significant clinical problem.

Elevational Constraints on the Composition and Genomic Attributes of Microbial Communities in Antarctic Soils

The inland soils found on the Antarctic continent represent one of the more challenging environments for microbial life on Earth. Nevertheless, Antarctic soils harbor unique bacterial and archaeal (prokaryotic) communities able to cope with extremely cold and dry conditions. These communities are not homogeneous, and the taxonomic composition and functional capabilities (genomic attributes) of these communities across environmental gradients remain largely undetermined. We analyzed the prokaryotic communities in soil samples collected from across the Shackleton Glacier region of Antarctica by coupling quantitative PCR, marker gene amplicon sequencing, and shotgun metagenomic sequencing. We found that elevation was the dominant factor explaining differences in the structures of the soil prokaryotic communities, with the drier and saltier soils found at higher elevations harboring less diverse communities and unique assemblages of cooccurring taxa. The higher-elevation soil communities also had lower maximum potential growth rates (as inferred from metagenome-based estimates of codon usage bias) and an overrepresentation of genes associated with trace gas metabolism. Together, these results highlight the utility of assessing community shifts across pronounced environmental gradients to improve our understanding of the microbial diversity found in Antarctic soils and the strategies used by soil microbes to persist at the limits of habitability.

IMPORTANCE Antarctic soils represent an ideal system to study how environmental properties shape the taxonomic and functional diversity of microbial communities given the relatively low diversity of Antarctic soil microbial communities and the pronounced environmental gradients that occur across soils located in reasonable proximity to one another. Moreover, the challenging environmental conditions typical of most Antarctic soils present an opportunity to investigate the traits that allow soil microbes to persist in some of the most inhospitable habitats on Earth. We used cultivation-independent methods to study the bacterial and archaeal communities found in soil samples collected from across the Shackleton Glacier region of the Transantarctic Mountains. We show that those environmental characteristics associated with elevation have the greatest impact on the structure of these microbial communities, with the colder, drier, and saltier soils found at higher elevations sustaining less diverse communities that were distinct from those in more hospitable soils with respect to their composition, genomic attributes, and overall life-history strategies. Notably, the harsher conditions found in higher-elevation soils likely select for taxa with lower maximum potential growth rates and an increased reliance on trace gas metabolism to support growth.

Phylogenetically and functionally diverse microorganisms reside under the Ross Ice Shelf

Throughout coastal Antarctica, ice shelves separate oceanic waters from sunlight by hundreds of meters of ice. Historical studies have detected activity of nitrifying microorganisms in oceanic cavities below permanent ice shelves. However, little is known about the microbial composition and pathways that mediate these activities. In this study, we profiled the microbial communities beneath the Ross Ice Shelf using a multi-omics approach. Overall, beneath-shelf microorganisms are of comparable abundance and diversity, though distinct composition, relative to those in the open meso- and bathypelagic ocean. Production of new organic carbon is likely driven by aerobic lithoautotrophic archaea and bacteria that can use ammonium, nitrite, and sulfur compounds as electron donors. Also enriched were aerobic organoheterotrophic bacteria capable of degrading complex organic carbon substrates, likely derived from in situ fixed carbon and potentially refractory organic matter laterally advected by the below-shelf waters. Altogether, these findings uncover a taxonomically distinct microbial community potentially adapted to a highly oligotrophic marine environment and suggest that ocean cavity waters are primarily chemosynthetically-driven systems.

Microbial community analysis of biopiles in Antarctica provides evidence of successful hydrocarbon biodegradation and initial soil ecosystem recovery

Microorganisms comprise the bulk of biodiversity and biomass in Antarctic terrestrial ecosystems. To effectively protect and manage the Antarctic environment from anthropogenic impacts including contamination, the response and recovery of microbial communities should be included in soil remediation efficacy and environmental risk assessments. This is the first investigation into the microbial dynamics associated with large scale bioremediation of hydrocarbon contaminated soil in Antarctica. Over five years of active management, two significant shifts in the microbial community were observed. The initial shift at 12-24 months was significantly correlated with the highest hydrocarbon degradation rates, increased microbial loads, and significant increases in alkB gene abundances. ANCOM analysis identified bacterial genera most likely responsible for the bulk of degradation including Alkanindiges, Arthrobacter, Dietzia and Rhodococcus. The second microbial community shift occurring from 36 to 60 months was associated with further reductions in hydrocarbons and a recovery of amoA nitrification genes, but also increasing pH, accumulation of nitrite and a reduction of oligotrophic bacterial species. Over time, the addition of inorganic fertilisers altered the soil chemistry and led to a disruption of the nitrogen cycle, most likely decoupling ammonia oxidisers from nitrite oxidisers, resulting in nitrite accumulation. The results from this study provide key insights to the long-term management of hydrocarbon bioremediation in Antarctic soils.

Succession of Microbial Communities in Waste Soils of an Iron Mine in Eastern China

The reclamation of mine dump is largely centered on the role played by microorganisms. However, the succession of microbial community structure and function in ecological restoration of the mine soils is still poorly understood. In this study, soil samples with different stacking time were collected from the dump of an iron mine in China and the physicochemical characteristics and microbial communities of these samples were comparatively investigated. The results showed that the fresh bare samples had the lowest pH, highest ion concentration, and were the most deficient in nutrients while the acidity and ion concentration of old bare samples decreased significantly, and the nutritional conditions improved remarkably. Vegetated samples had the weakest acidity, lowest ion concentration, and the highest nutrient concentration. In the fresh mine soils, the iron/sulfur-oxidizers such as Acidiferrobacter and Sulfobacillus were dominant, resulting in the strongest acidity. Bacteria from genera Acidibacter, Metallibacterium, and phyla Cyanobacteria, WPS-2 were abundant in the old bare samples, which contributed to the pH increase and TOC accumulation respectively. Acidobacteriota predominated in the vegetated samples and promoted nutrient enrichment and plant growth significantly. The microbial diversity and evenness of the three types of soils increased gradually, with more complex microbial networks, suggesting that the microbial community became more mature with time and microorganisms co-evolved with the mine soils.

Remarkably coherent population structure for a dominant Antarctic Chlorobium species

BACKGROUND

In Antarctica, summer sunlight enables phototrophic microorganisms to drive primary production, thereby "feeding" ecosystems to enable their persistence through the long, dark winter months. In Ace Lake, a stratified marine-derived system in the Vestfold Hills of East Antarctica, a Chlorobium species of green sulphur bacteria (GSB) is the dominant phototroph, although its seasonal abundance changes more than 100-fold. Here, we analysed 413 Gb of Antarctic metagenome data including 59 Chlorobium metagenome-assembled genomes (MAGs) from Ace Lake and nearby stratified marine basins to determine how genome variation and population structure across a 7-year period impacted ecosystem function.

RESULTS

A single species, Candidatus Chlorobium antarcticum (most similar to Chlorobium phaeovibrioides DSM265) prevails in all three aquatic systems and harbours very little genomic variation (≥ 99% average nucleotide identity). A notable feature of variation that did exist related to the genomic capacity to biosynthesize cobalamin. The abundance of phylotypes with this capacity changed seasonally ~ 2-fold, consistent with the population balancing the value of a bolstered photosynthetic capacity in summer against an energetic cost in winter. The very high GSB concentration (> 10⁸ cells ml^-1 in Ace Lake) and seasonal cycle of cell lysis likely make Ca. Chlorobium antarcticum a major provider of cobalamin to the food web. Analysis of Ca. Chlorobium antarcticum viruses revealed the species to be infected by generalist (rather than specialist) viruses with a broad host range (e.g., infecting Gammaproteobacteria) that were present in diverse Antarctic lakes. The marked seasonal decrease in Ca. Chlorobium antarcticum abundance may restrict specialist viruses from establishing effective lifecycles, whereas generalist viruses may augment their proliferation using other hosts.

CONCLUSION

The factors shaping Antarctic microbial communities are gradually being defined. In addition to the cold, the annual variation in sunlight hours dictates which phototrophic species can grow and the extent to which they contribute to ecosystem processes. The Chlorobium population studied was inferred to provide cobalamin, in addition to carbon, nitrogen, hydrogen, and sulphur cycling, as critical ecosystem services. The specific Antarctic environmental factors and major ecosystem benefits afforded by this GSB likely explain why such a coherent population structure has developed in this Chlorobium species. Video abstract.

Leave no stone unturned: individually adapted xerotolerant Thaumarchaeota sheltered below the boulders of the Atacama Desert hyperarid core

BACKGROUND

The hyperarid core of the Atacama Desert is an extremely harsh environment thought to be colonized by only a few heterotrophic bacterial species. Current concepts for understanding this extreme ecosystem are mainly based on the diversity of these few species, yet a substantial area of the Atacama Desert hyperarid topsoil is covered by expansive boulder accumulations, whose underlying microbiomes have not been investigated so far. With the hypothesis that these sheltered soils harbor uniquely adapted microbiomes, we compared metagenomes and geochemistry between soils below and beside boulders across three distantly located boulder accumulations in the Atacama Desert hyperarid core.

RESULTS

Genome-resolved metagenomics of eleven samples revealed substantially different microbial communities in soils below and beside boulders, despite the presence of shared species. Archaea were found in significantly higher relative abundance below the boulders across all samples within distances of up to 205 km. These key taxa belong to a novel genus of ammonia-oxidizing Thaumarchaeota, Candidatus Nitrosodeserticola. We resolved eight mid-to-high quality genomes of this genus and used comparative genomics to analyze its pangenome and site-specific adaptations. Ca. Nitrosodeserticola genomes contain genes for ammonia oxidation, the 3-hydroxypropionate/4-hydroxybutyrate carbon fixation pathway, and acetate utilization indicating a chemolithoautotrophic and mixotrophic lifestyle. They also possess the capacity for tolerating extreme environmental conditions as highlighted by the presence of genes against oxidative stress and DNA damage. Site-specific adaptations of the genomes included the presence of additional genes for heavy metal transporters, multiple types of ATP synthases, and divergent genes for aquaporins.

CONCLUSION

We provide the first genomic characterization of hyperarid soil microbiomes below the boulders in the Atacama Desert, and report abundant and highly adapted Thaumarchaeaota with ammonia oxidation and carbon fixation potential. Ca. Nitrosodeserticola genomes provide the first metabolic and physiological insight into a thaumarchaeal lineage found in globally distributed terrestrial habitats characterized by various environmental stresses. We consequently expand not only the known genetic repertoire of Thaumarchaeota but also the diversity and microbiome functioning in hyperarid ecosystems. Video Abstract.

Multiple energy sources and metabolic strategies sustain microbial diversity in Antarctic desert soils

Numerous diverse microorganisms reside in the cold desert soils of continental Antarctica, though we lack a holistic understanding of the metabolic processes that sustain them. Here, we profile the composition, capabilities, and activities of the microbial communities in 16 physicochemically diverse mountainous and glacial soils. We assembled 451 metagenome-assembled genomes from 18 microbial phyla and inferred through Bayesian divergence analysis that the dominant lineages present are likely native to Antarctica. In support of earlier findings, metagenomic analysis revealed that the most abundant and prevalent microorganisms are metabolically versatile aerobes that use atmospheric hydrogen to support aerobic respiration and sometimes carbon fixation. Surprisingly, however, hydrogen oxidation in this region was catalyzed primarily by a phylogenetically and structurally distinct enzyme, the group 1l [NiFe]-hydrogenase, encoded by nine bacterial phyla. Through gas chromatography, we provide evidence that both Antarctic soil communities and an axenic Bacteroidota isolate (Hymenobacter roseosalivarius) oxidize atmospheric hydrogen using this enzyme. Based on ex situ rates at environmentally representative temperatures, hydrogen oxidation is theoretically sufficient for soil communities to meet energy requirements and, through metabolic water production, sustain hydration. Diverse carbon monoxide oxidizers and abundant methanotrophs were also active in the soils. We also recovered genomes of microorganisms capable of oxidizing edaphic inorganic nitrogen, sulfur, and iron compounds and harvesting solar energy via microbial rhodopsins and conventional photosystems. Obligately symbiotic bacteria, including Patescibacteria, Chlamydiae, and predatory Bdellovibrionota, were also present. We conclude that microbial diversity in Antarctic soils reflects the coexistence of metabolically flexible mixotrophs with metabolically constrained specialists.

Chemosynthetic and photosynthetic bacteria contribute differentially to primary production across a steep desert aridity gradient

Desert soils harbour diverse communities of aerobic bacteria despite lacking substantial organic carbon inputs from vegetation. A major question is therefore how these communities maintain their biodiversity and biomass in these resource-limiting ecosystems. Here, we investigated desert topsoils and biological soil crusts collected along an aridity gradient traversing four climatic regions (sub-humid, semi-arid, arid, and hyper-arid). Metagenomic analysis indicated these communities vary in their capacity to use sunlight, organic compounds, and inorganic compounds as energy sources. Thermoleophilia, Actinobacteria, and Acidimicrobiia were the most abundant and prevalent bacterial classes across the aridity gradient in both topsoils and biocrusts. Contrary to the classical view that these taxa are obligate organoheterotrophs, genome-resolved analysis suggested they are metabolically flexible, with the capacity to also use atmospheric H2 to support aerobic respiration and often carbon fixation. In contrast, Cyanobacteria were patchily distributed and only abundant in certain biocrusts. Activity measurements profiled how aerobic H2 oxidation, chemosynthetic CO2 fixation, and photosynthesis varied with aridity. Cell-specific rates of atmospheric H2 consumption increased 143-fold along the aridity gradient, correlating with increased abundance of high-affinity hydrogenases. Photosynthetic and chemosynthetic primary production co-occurred throughout the gradient, with photosynthesis dominant in biocrusts and chemosynthesis dominant in arid and hyper-arid soils. Altogether, these findings suggest that the major bacterial lineages inhabiting hot deserts use different strategies for energy and carbon acquisition depending on resource availability. Moreover, they highlight the previously overlooked roles of Actinobacteriota as abundant primary producers and trace gases as critical energy sources supporting productivity and resilience of desert ecosystems.

Metabolic flexibility allows bacterial habitat generalists to become dominant in a frequently disturbed ecosystem

Ecological theory suggests that habitat disturbance differentially influences distributions of habitat generalist and specialist species. While well-established for macroorganisms, this theory has rarely been explored for microorganisms. Here we tested these principles in permeable (sandy) sediments, ecosystems with much spatiotemporal variation in resource availability and physicochemical conditions. Microbial community composition and function were profiled in intertidal and subtidal sediments using 16S rRNA gene amplicon sequencing and metagenomics, yielding 135 metagenome-assembled genomes. Community composition and metabolic traits modestly varied with sediment depth and sampling date. Several taxa were highly abundant and prevalent in all samples, including within the orders Woeseiales and Flavobacteriales, and classified as habitat generalists; genome reconstructions indicate these taxa are highly metabolically flexible facultative anaerobes and adapt to resource variability by using different electron donors and acceptors. In contrast, obligately anaerobic taxa such as sulfate reducers and candidate lineage MBNT15 were less abundant overall and only thrived in more stable deeper sediments. We substantiated these findings by measuring three metabolic processes in these sediments; whereas the habitat generalist-associated processes of sulfide oxidation and fermentation occurred rapidly at all depths, the specialist-associated process of sulfate reduction was restricted to deeper sediments. A manipulative experiment also confirmed habitat generalists outcompete specialist taxa during simulated habitat disturbance. Together, these findings show metabolically flexible habitat generalists become dominant in highly dynamic environments, whereas metabolically constrained specialists are restricted to narrower niches. Thus, an ecological theory describing distribution patterns for macroorganisms likely extends to microorganisms. Such findings have broad ecological and biogeochemical ramifications.

Diversity of microbial communities and genes involved in nitrous oxide emissions in Antarctic soils impacted by marine animals as revealed by metagenomics and 100 metagenome-assembled genomes

Antarctic soils generally have low temperatures and limited availability of liquid water and nutrients. However, animals can increase the nutrient availability of ice-free areas by transferring nutrients from marine to terrestrial ecosystems, mainly through their excreta. In this study, we employed shotgun metagenomics and population genome binning techniques to study the diversity of microbial communities in Antarctic soils impacted by marine pinnipeds and birds relative to soils with no evident animal presence. We obtained ~285,000 16S rRNA gene-carrying metagenomic reads representing ~60 phyla and 100 metagenome-assembled genomes (MAGs) representing eight phyla. Only nine of these 100 MAGs represented previously described species, revealing that these soils harbor extensive novel diversity. Proteobacteria, Actinobacteria, and Bacteroidetes were the most abundant phyla in all samples, with Rhodanobacter being one of the most abundant genera in the bird-impacted soils. Further, the relative abundance of genes related to denitrification was at least double in soils impacted by birds than soils without animal influence. These results advance our understanding of the microbial populations and their genes involved in nitrous oxide emissions in ice-free coastal Antarctic soils impacted by marine animals and reveal novel microbial diversity associated with these ecosystems.

Simulated Microgravity Promotes Horizontal Gene Transfer of Antimicrobial Resistance Genes between Bacterial Genera in the Absence of Antibiotic Selective Pressure

Bacteria are able to adapt and survive in harsh and changing environments through many mechanisms, with one of them being horizontal gene transfer (HGT). This process is one of the leading culprits in the spread of antimicrobial resistance (AMR) within bacterial communities and could pose a significant health threat to astronauts if they fell ill, especially on long-duration space missions. In order to better understand the degree of HGT activity that could occur in space, biosafety level-2, donor and recipient bacteria were co-cultured under simulated microgravity (SMG) on Earth with concomitant 1G controls. Two AMR genes, bla_OXA-500 and ISAba1, from the donor Acinetobacter pittii, were tracked in four recipient strains of Staphylococcus aureus (which did not harbor those genes) using polymerase chain reaction. All four S. aureus strains that were co-cultured with A. pittii under SMG had a significantly higher number of isolates that were now bla_OXA-500- and ISAba1-positive compared to growth at 1G. The acquisition of these genes by the recipient induced a phenotypic change, as these isolates were now resistant to oxacillin, which they were previously susceptible to. This is a novel study, presenting, for the first time, increased HGT activity under SMG and the potential impact of the space environment in promoting increased gene dissemination within bacterial communities.

Candidatus Eremiobacterota, a metabolically and phylogenetically diverse terrestrial phylum with acid-tolerant adaptations

Candidatus phylum Eremiobacterota (formerly WPS-2) is an as-yet-uncultured bacterial clade that takes its name from Ca. Eremiobacter, an Antarctic soil aerobe proposed to be capable of a novel form of chemolithoautotrophy termed atmospheric chemosynthesis, that uses the energy derived from atmospheric H₂-oxidation to fix CO₂ through the Calvin-Benson-Bassham (CBB) cycle via type 1E RuBisCO. To elucidate the phylogenetic affiliation and metabolic capacities of Ca. Eremiobacterota, we analysed 63 public metagenome-assembled genomes (MAGs) and nine new MAGs generated from Antarctic soil metagenomes. These MAGs represent both recognized classes within Ca. Eremiobacterota, namely Ca. Eremiobacteria and UBP9. Ca. Eremiobacteria are inferred to be facultatively acidophilic with a preference for peptides and amino acids as nutrient sources. Epifluorescence microscopy revealed Ca. Eremiobacteria cells from Antarctica desert soil to be coccoid in shape. Two orders are recognized within class Ca. Eremiobacteria: Ca. Eremiobacterales and Ca. Baltobacterales. The latter are metabolically versatile, with individual members having genes required for trace gas driven autotrophy, anoxygenic photosynthesis, CO oxidation, and anaerobic respiration. UBP9, here renamed Ca. Xenobia class. nov., are inferred to be obligate heterotrophs with acidophilic adaptations, but individual members having highly divergent metabolic capacities compared to Ca. Eremiobacteria, especially with regard to respiration and central carbon metabolism. We conclude Ca. Eremiobacterota to be an ecologically versatile phylum with the potential to thrive under an array of "extreme" environmental conditions.

Hydrogen is a major lifeline for aerobic bacteria

Molecular hydrogen (H₂) is available in trace amounts in most ecosystems through atmospheric, biological, geochemical, and anthropogenic sources. Aerobic bacteria use this energy-dense gas, including at atmospheric concentrations, to support respiration and carbon fixation. While it was thought that aerobic H₂ consumers are rare community members, here we summarize evidence suggesting that they are dominant throughout soils and other aerated ecosystems. Bacterial cultures from at least eight major phyla can consume atmospheric H₂. At the ecosystem scale, H₂ consumers are abundant, diverse, and active across diverse soils and are key primary producers in extreme environments such as hyper-arid deserts. On this basis, we propose that H₂ is a universally available energy source for the survival of aerobic bacteria.

Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase (RubisCO) Is Essential for Growth of the Methanotroph Methylococcus capsulatus Strain Bath

The ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) enzyme found in plants, algae, and an array of autotrophic bacteria is also encoded by a subset of methanotrophs, but its role in these microbes has largely remained elusive. In this study, we showed that CO₂ was requisite for RubisCO-encoding Methylococcus capsulatus strain Bath growth in a bioreactor with continuous influent and effluent gas flow. RNA sequencing identified active transcription of several carboxylating enzymes, including key enzymes of the Calvin and serine cycles, that could mediate CO₂ assimilation during cultivation with both CH₄ and CO₂ as carbon sources. Marker exchange mutagenesis of M. capsulatus Bath genes encoding key enzymes of potential CO₂-assimilating metabolic pathways indicated that a complete serine cycle is not required, whereas RubisCO is essential for growth of this bacterium. ¹³CO₂ tracer analysis showed that CH₄ and CO₂ enter overlapping anaplerotic pathways and implicated RubisCO as the primary enzyme mediating CO₂ assimilation in M. capsulatus Bath. Notably, we quantified the relative abundance of 3-phosphoglycerate and ribulose-1,5-bisphosphate ¹³C isotopes, which supported that RubisCO-produced 3-phosphoglycerate is primarily converted to ribulose-1-5-bisphosphate via the oxidative pentose phosphate pathway in M. capsulatus Bath. Collectively, our data establish that RubisCO and CO₂ play essential roles in M. capsulatus Bath metabolism. This study expands the known capacity of methanotrophs to fix CO₂ via RubisCO, which may play a more pivotal role in the Earth's biogeochemical carbon cycling and greenhouse gas regulation than previously recognized. Further, M. capsulatus Bath and other CO₂-assimilating methanotrophs represent excellent candidates for use in the bioconversion of biogas waste streams that consist of both CH₄ and CO₂. IMPORTANCE The importance of RubisCO and CO₂ in M. capsulatus Bath metabolism is unclear. In this study, we demonstrated that both CO₂ and RubisCO are essential for M. capsulatus Bath growth. ¹³CO₂ tracing experiments supported that RubisCO mediates CO₂ fixation and that a noncanonical Calvin cycle is active in this organism. Our study provides insights into the expanding knowledge of methanotroph metabolism and implicates dually CH₄/CO₂-utilizing bacteria as more important players in the biogeochemical carbon cycle than previously appreciated. In addition, M. capsulatus and other methanotrophs with CO₂ assimilation capacity represent candidate organisms for the development of biotechnologies to mitigate the two most abundant greenhouse gases, CH₄ and CO₂.

Microbial anhydrobiosis

The loss of cellular water (desiccation) and the resulting low cytosolic water activity are major stress factors for life. Numerous prokaryotic and eukaryotic taxa have evolved molecular and physiological adaptions to periods of low water availability or water-limited environments that occur across the terrestrial Earth. The changes within cells during the processes of desiccation and rehydration, from the activation (and inactivation) of biosynthetic pathways to the accumulation of compatible solutes, have been studied in considerable detail. However, relatively little is known on the metabolic status of organisms in the desiccated state; that is, in the sometimes extended periods between the drying and rewetting phases. During these periods, which can extend beyond decades and which we term 'anhydrobiosis', organismal survival could be dependent on a continued supply of energy to maintain the basal metabolic processes necessary for critical functions such as macromolecular repair. Here, we review the state of knowledge relating to the function of microorganisms during the anhydrobiotic state, highlighting substantial gaps in our understanding of qualitative and quantitative aspects of molecular and biochemical processes in desiccated cells.

Reticulibacter mediterranei gen. nov., sp. nov., within the new family Reticulibacteraceae fam. nov., and Ktedonospora formicarum gen. nov., sp. nov., Ktedonobacter robiniae sp. nov., Dictyobacter formicarum sp. nov. and Dictyobacter arantiisoli sp. nov., belonging to the class Ktedonobacteria

The aerobic, Gram-positive, mesophilic Ktedonobacteria strains, Uno17^T, SOSP1-1^T, 1-9^T, 1-30^T and 150040^T, formed mycelia of irregularly branched filaments, produced spores or sporangia, and numerous secondary metabolite biosynthetic gene clusters. The five strains grew at 15-40 °C (optimally at 30 °C) and pH 4.0-8.0 (optimally at pH 6.0-7.0), and had 7.21-12.67 Mb genomes with 49.7-53.7 mol% G+C content. They shared MK9(H₂) as the major menaquinone and C_16 : 1-2OH and iso-C_17 : 0 as the major cellular fatty acids. Phylogenetic and phylogenomic analyses showed that Uno17^T and SOSP1-9^T were most closely related to members of the genus Dictyobacter, with 94.43-96.21 % 16S rRNA gene similarities and 72.16-81.56% genomic average nucleotide identity. The strain most closely related to SOSP1-1^T and SOSP1-30^T was Ktedonobacter racemifer SOSP1-21^T, with 91.33 and 98.84 % 16S rRNA similarities, and 75.13 and 92.35% average nucleotide identities, respectively. Strain 150040^T formed a distinct clade within the order Ktedonobacterales, showing <90.47 % 16S rRNA gene similarity to known species in this order. Based on these results, we propose: strain 150040^T as Reticulibacter mediterranei gen. nov., sp. nov. (type strain 150 040^T=CGMCC 1.17052^T=BCRC 81202^T) within the family Reticulibacteraceae fam. nov. in the order Ktedonobacterales; strain SOSP1-1^T as Ktedonospora formicarum gen. nov., sp. nov. (type strain SOSP1-1^T=CGMCC 1.17205^T=BCRC 81203^T) and strain SOSP1-30^T as Ktedonobacter robiniae sp. nov. (type strain SOSP1-30^T=CGMCC 1.17733^T=BCRC 81205^T) within the family Ktedonobacteraceae; strain Uno17^T as Dictyobacter arantiisoli sp. nov. (type strain Uno17^T=NBRC 113155^T=BCRC 81116^T); and strain SOSP1-9^T as Dictyobacter formicarum sp. nov. (type strain SOSP1-9^T=CGMCC 1.17206^T=BCRC 81204^T) within the family Dictyobacteraceae.

Quantifying the effects of hydrogen on carbon assimilation in a seafloor microbial community associated with ultramafic rocks

Thermodynamic models predict that H₂ is energetically favorable for seafloor microbial life, but how H₂ affects anabolic processes in seafloor-associated communities is poorly understood. Here, we used quantitative ¹³C DNA stable isotope probing (qSIP) to quantify the effect of H₂ on carbon assimilation by microbial taxa synthesizing ¹³C-labeled DNA that are associated with partially serpentinized peridotite rocks from the equatorial Mid-Atlantic Ridge. The rock-hosted seafloor community was an order of magnitude more diverse compared to the seawater community directly above the rocks. With added H₂, peridotite-associated taxa increased assimilation of ¹³C-bicarbonate and ¹³C-acetate into 16S rRNA genes of operational taxonomic units by 146% (±29%) and 55% (±34%), respectively, which correlated with enrichment of H₂-oxidizing NiFe-hydrogenases encoded in peridotite-associated metagenomes. The effect of H₂ on anabolism was phylogenetically organized, with taxa affiliated with Atribacteria, Nitrospira, and Thaumarchaeota exhibiting the most significant increases in ¹³C-substrate assimilation in the presence of H₂. In SIP incubations with added H₂, an order of magnitude higher number of peridotite rock-associated taxa assimilated ¹³C-bicarbonate, ¹³C-acetate, and ¹³C-formate compared to taxa that were not associated with peridotites. Collectively, these findings indicate that the unique geochemical nature of the peridotite-hosted ecosystem has selected for H₂-metabolizing, rock-associated taxa that can increase anabolism under high H₂ concentrations. Because ultramafic rocks are widespread in slow-, and ultraslow-spreading oceanic lithosphere, continental margins, and subduction zones where H₂ is formed in copious amounts, the link between H₂ and carbon assimilation demonstrated here may be widespread within these geological settings.

Termite gas emissions select for hydrogenotrophic microbial communities in termite mounds

Organoheterotrophs are the dominant bacteria in most soils worldwide. While many of these bacteria can subsist on atmospheric hydrogen (H₂), levels of this gas are generally insufficient to sustain hydrogenotrophic growth. In contrast, bacteria residing within soil-derived termite mounds are exposed to high fluxes of H₂ due to fermentative production within termite guts. Here, we show through community, metagenomic, and biogeochemical profiling that termite emissions select for a community dominated by diverse hydrogenotrophic Actinobacteriota and Dormibacterota. Based on metagenomic short reads and derived genomes, uptake hydrogenase and chemosynthetic RuBisCO genes were significantly enriched in mounds compared to surrounding soils. In situ and ex situ measurements confirmed that high- and low-affinity H₂-oxidizing bacteria were highly active in the mounds, such that they efficiently consumed all termite-derived H₂ emissions and served as net sinks of atmospheric H₂ Concordant findings were observed across the mounds of three different Australian termite species, with termite activity strongly predicting H₂ oxidation rates (R ² = 0.82). Cell-specific power calculations confirmed the potential for hydrogenotrophic growth in the mounds with most termite activity. In contrast, while methane is produced at similar rates to H₂ by termites, mounds contained few methanotrophs and were net sources of methane. Altogether, these findings provide further evidence of a highly responsive terrestrial sink for H₂ but not methane and suggest H₂ availability shapes composition and activity of microbial communities. They also reveal a unique arthropod-bacteria interaction dependent on H₂ transfer between host-associated and free-living microbial communities.

Sasso Pisano Geothermal Field Environment Harbours Diverse Ktedonobacteria Representatives and Illustrates Habitat-Specific Adaptations

The hydrothermal steam environment of Sasso Pisano (Italy) was selected to investigate the associated microbial community and its metabolic potential. In this context, 16S and 18S rRNA gene partial sequences of thermophilic prokaryotes and eukaryotes inhabiting hot springs and fumaroles as well as mesophilic microbes colonising soil and water were analysed by high-throughput amplicon sequencing. The eukaryotic and prokaryotic communities from hot environments clearly differ from reference microbial communities of colder soil sites, though Ktedonobacteria showed high abundances in various hot spring samples and a few soil samples. This indicates that the hydrothermal steam environments of Sasso Pisano represent not only a vast reservoir of thermophilic but also mesophilic members of this Chloroflexi class. Metabolic functional profiling revealed that the hot spring microbiome exhibits a higher capability to utilise methane and aromatic compounds and is more diverse in its sulphur and nitrogen metabolism than the mesophilic soil microbial consortium. In addition, heavy metal resistance-conferring genes were significantly more abundant in the hot spring microbiome. The eukaryotic diversity at a fumarole indicated high abundances of primary producers (unicellular red algae: Cyanidiales), consumers (Arthropoda: Collembola sp.), and endoparasite Apicomplexa (Gregarina sp.), which helps to hypothesise a simplified food web at this hot and extremely nutrient-deprived acidic environment.

Persistence and resistance: survival mechanisms of Candidatus Dormibacterota from nutrient-poor Antarctic soils

Candidatus Dormibacterota is an uncultured bacterial phylum found predominantly in soil that is present in high abundances within cold desert soils. Here, we interrogate nine metagenome-assembled genomes (MAGs), including six new MAGs derived from soil metagenomes obtained from two eastern Antarctic sites. Phylogenomic and taxonomic analyses revealed these MAGs represent four genera and five species, representing two order-level clades within Ca. Dormibacterota. Metabolic reconstructions of these MAGs revealed the potential for aerobic metabolism, and versatile adaptations enabling persistence in the 'extreme' Antarctic environment. Primary amongst these adaptations were abilities to scavenge atmospheric H₂ and CO as energy sources, as well as using the energy derived from H₂ oxidation to fix atmospheric CO₂ via the Calvin-Bassham-Benson cycle, using a RuBisCO type IE. We propose that these allow Ca. Dormibacterota to persist using H₂ oxidation and grow using atmospheric chemosynthesis in terrestrial Antarctica. Fluorescence in situ hybridization revealed Ca. Dormibacterota to be coccoid cells, 0.3-1.4 μm in diameter, with some cells exhibiting the potential for a symbiotic or syntrophic lifestyle.

Thiocyanate and Organic Carbon Inputs Drive Convergent Selection for Specific Autotrophic Afipia and Thiobacillus Strains Within Complex Microbiomes

Thiocyanate (SCN^–) contamination threatens aquatic ecosystems and pollutes vital freshwater supplies. SCN^–-degrading microbial consortia are commercially adapted for remediation, but the impact of organic amendments on selection within SCN^–-degrading microbial communities has not been investigated. Here, we tested whether specific strains capable of degrading SCN^– could be reproducibly selected for based on SCN^– loading and the presence or absence of added organic carbon. Complex microbial communities derived from those used to treat SCN^–-contaminated water were exposed to systematically increased input SCN concentrations in molasses-amended and -unamended reactors and in reactors switched to unamended conditions after establishing the active SCN^–-degrading consortium. Five experiments were conducted over 790 days, and genome-resolved metagenomics was used to resolve community composition at the strain level. A single Thiobacillus strain proliferated in all reactors at high loadings. Despite the presence of many Rhizobiales strains, a single Afipia variant dominated the molasses-free reactor at moderately high loadings. This strain is predicted to break down SCN^– using a novel thiocyanate desulfurase, oxidize resulting reduced sulfur, degrade product cyanate to ammonia and CO₂ via cyanate hydratase, and fix CO₂ via the Calvin–Benson–Bassham cycle. Removal of molasses from input feed solutions reproducibly led to dominance of this strain. Although sustained by autotrophy, reactors without molasses did not stably degrade SCN^– at high loading rates, perhaps due to loss of biofilm-associated niche diversity. Overall, convergence in environmental conditions led to convergence in the strain composition, although reactor history also impacted the trajectory of community compositional change.

Shotgun metagenomics reveals distinct functional diversity and metabolic capabilities between 12 000-year-old permafrost and active layers on Muot da Barba Peider (Swiss Alps)

The warming-induced thawing of permafrost promotes microbial activity, often resulting in enhanced greenhouse gas emissions. The ability of permafrost microorganisms to survive the in situ sub-zero temperatures, their energetic strategies and their metabolic versatility in using soil organic materials determine their growth and functionality upon thawing. Hence, functional characterization of the permafrost microbiome, particularly in the underexplored mid-latitudinal alpine regions, is a crucial first step in predicting its responses to the changing climate, and the consequences for soil-climate feedbacks. In this study, for the first time, the functional potential and metabolic capabilities of a temperate mountain permafrost microbiome from central Europe has been analysed using shotgun metagenomics. Permafrost and active layers from the summit of Muot da Barba Peider (MBP) [Swiss Alps, 2979 m above sea level (a.s.l.)] revealed a strikingly high functional diversity in the permafrost (north-facing soils at a depth of 160 cm). Permafrost metagenomes were enriched in stress-response genes (e.g. cold-shock genes, chaperones), as well as in genes involved in cell defence and competition (e.g. antiviral proteins, antibiotics, motility, nutrient-uptake ABC transporters), compared with active-layer metagenomes. Permafrost also showed a higher potential for the synthesis of carbohydrate-active enzymes, and an overrepresentation of genes involved in fermentation, carbon fixation, denitrification and nitrogen reduction reactions. Collectively, these findings demonstrate the potential capabilities of permafrost microorganisms to thrive in cold and oligotrophic conditions, and highlight their metabolic versatility in carbon and nitrogen cycling. Our study provides a first insight into the high functional gene diversity of the central European mountain permafrost microbiome. Our findings extend our understanding of the microbial ecology of permafrost and represent a baseline for future investigations comparing the functional profiles of permafrost microbial communities at different latitudes.

A genomic catalog of Earth's microbiomes

The reconstruction of bacterial and archaeal genomes from shotgun metagenomes has enabled insights into the ecology and evolution of environmental and host-associated microbiomes. Here we applied this approach to >10,000 metagenomes collected from diverse habitats covering all of Earth's continents and oceans, including metagenomes from human and animal hosts, engineered environments, and natural and agricultural soils, to capture extant microbial, metabolic and functional potential. This comprehensive catalog includes 52,515 metagenome-assembled genomes representing 12,556 novel candidate species-level operational taxonomic units spanning 135 phyla. The catalog expands the known phylogenetic diversity of bacteria and archaea by 44% and is broadly available for streamlined comparative analyses, interactive exploration, metabolic modeling and bulk download. We demonstrate the utility of this collection for understanding secondary-metabolite biosynthetic potential and for resolving thousands of new host linkages to uncultivated viruses. This resource underscores the value of genome-centric approaches for revealing genomic properties of uncultivated microorganisms that affect ecosystem processes.

Innovations to culturing the uncultured microbial majority

Despite the surge of microbial genome data, experimental testing is important to confirm inferences about the cell biology, ecological roles and evolution of microorganisms. As the majority of archaeal and bacterial diversity remains uncultured and poorly characterized, culturing is a priority. The growing interest in and need for efficient cultivation strategies has led to many rapid methodological and technological advances. In this Review, we discuss common barriers that can hamper the isolation and culturing of novel microorganisms and review emerging, innovative methods for targeted or high-throughput cultivation. We also highlight recent examples of successful cultivation of novel archaea and bacteria, and suggest key microorganisms for future cultivation attempts.

Transition from unclassified Ktedonobacterales to Actinobacteria during amorphous silica precipitation in a quartzite cave environment

The orthoquartzite Imawarì Yeuta cave hosts exceptional silica speleothems and represents a unique model system to study the geomicrobiology associated to silica amorphization processes under aphotic and stable physical-chemical conditions. In this study, three consecutive evolution steps in the formation of a peculiar blackish coralloid silica speleothem were studied using a combination of morphological, mineralogical/elemental and microbiological analyses. Microbial communities were characterized using Illumina sequencing of 16S rRNA gene and clone library analysis of carbon monoxide dehydrogenase (coxL) and hydrogenase (hypD) genes involved in atmospheric trace gases utilization. The first stage of the silica amorphization process was dominated by members of a still undescribed microbial lineage belonging to the Ktedonobacterales order, probably involved in the pioneering colonization of quartzitic environments. Actinobacteria of the Pseudonocardiaceae and Acidothermaceae families dominated the intermediate amorphous silica speleothem and the final coralloid silica speleothem, respectively. The atmospheric trace gases oxidizers mostly corresponded to the main bacterial taxa present in each speleothem stage. These results provide novel understanding of the microbial community structure accompanying amorphization processes and of coxL and hypD gene expression possibly driving atmospheric trace gases metabolism in dark oligotrophic caves.

Carbon fixation and rhodopsin systems in microbial mats from hypersaline lakes Brava and Tebenquiche, Salar de Atacama, Chile

In this work, molecular diversity of two hypersaline microbial mats was compared by Whole Genome Shotgun (WGS) sequencing of environmental DNA from the mats. Brava and Tebenquiche are lakes in the Salar de Atacama, Chile, where microbial communities are growing in extreme conditions, including high salinity, high solar irradiance, and high levels of toxic metals and metaloids. Evaporation creates hypersaline conditions in these lakes and mineral precipitation is a characteristic geomicrobiological feature of these benthic ecosystems. The mat from Brava was more rich and diverse, with a higher number of different taxa and with species more evenly distributed. At the phylum level, Proteobacteria, Cyanobacteria, Chloroflexi, Bacteroidetes and Firmicutes were the most abundant, including ~75% of total sequences. At the genus level, the most abundant sequences were affilitated to anoxygenic phototropic and cyanobacterial genera. In Tebenquiche mats, Proteobacteria and Bacteroidetes covered ~70% of the sequences, and 13% of the sequences were affiliated to Salinibacter genus, thus addressing the lower diversity. Regardless of the differences at the taxonomic level, functionally the two mats were similar. Thus, similar roles could be fulfilled by different organisms. Carbon fixation through the Wood-Ljungdahl pathway was well represented in these datasets, and also in other mats from Andean lakes. In spite of presenting less taxonomic diversity, Tebenquiche mats showed increased abundance and variety of rhodopsin genes. Comparison with other metagenomes allowed identifying xantorhodopsins as hallmark genes not only from Brava and Tebenquiche mats, but also for other mats developing at high altitudes in similar environmental conditions.

Simultaneous Oxidation of Atmospheric Methane, Carbon Monoxide and Hydrogen for Bacterial Growth

The second largest sink for atmospheric methane (CH₄) is atmospheric methane oxidizing-bacteria (atmMOB). How atmMOB are able to sustain life on the low CH₄ concentrations in air is unknown. Here, we show that during growth, with air as its only source for energy and carbon, the recently isolated atmospheric methane-oxidizer Methylocapsa gorgona MG08 (USCα) oxidizes three atmospheric energy sources: CH₄, carbon monoxide (CO), and hydrogen (H₂) to support growth. The cell-specific CH₄ oxidation rate of M. gorgona MG08 was estimated at ~0.7 × 10⁻¹⁸ mol cell⁻¹ h⁻¹, which, together with the oxidation of CO and H₂, supplies 0.38 kJ Cmol⁻¹ h⁻¹ during growth in air. This is seven times lower than previously assumed necessary to support bacterial maintenance. We conclude that atmospheric methane-oxidation is supported by a metabolic flexibility that enables the simultaneous harvest of CH₄, H₂ and CO from air, but the key characteristic of atmospheric CH₄ oxidizing bacteria might be very low energy requirements.

Distribution of Mixotrophy and Desiccation Survival Mechanisms across Microbial Genomes in an Arid Biological Soil Crust Community

Desert surface soils devoid of plant cover are populated by a variety of microorganisms, many with yet unresolved physiologies and lifestyles. Nevertheless, a common feature vital for these microorganisms inhabiting arid soils is their ability to survive long drought periods and reactivate rapidly in rare incidents of rain. Chemolithotrophic processes such as oxidation of atmospheric hydrogen and carbon monoxide are suggested to be a widespread energy source to support dormancy and resuscitation in desert soil microorganisms. Here, we assessed the distribution of chemolithotrophic, phototrophic, and desiccation-related metabolic potential among microbial populations in arid biological soil crusts (BSCs) from the Negev Desert, Israel, via population-resolved metagenomic analysis. While the potential to utilize light and atmospheric hydrogen as additional energy sources was widespread, carbon monoxide oxidation was less common than expected. The ability to utilize continuously available energy sources might decrease the dependency of mixotrophic populations on organic storage compounds and carbon provided by the BSC-founding cyanobacteria. Several populations from five different phyla besides the cyanobacteria encoded CO2 fixation potential, indicating further potential independence from photoautotrophs. However, we also found population genomes with a strictly heterotrophic genetic repertoire. The highly abundant Rubrobacteraceae (Actinobacteriota) genomes showed particular specialization for this extreme habitat, different from their closest cultured relatives. Besides the ability to use light and hydrogen as energy sources, they encoded extensive O2 stress protection and unique DNA repair potential. The uncovered differences in metabolic potential between individual, co-occurring microbial populations enable predictions of their ecological niches and generation of hypotheses on the dynamics and interactions among them.IMPORTANCE This study represents a comprehensive community-wide genome-centered metagenome analysis of biological soil crust (BSC) communities in arid environments, providing insights into the distribution of genes encoding different energy generation mechanisms, as well as survival strategies, among populations in an arid soil ecosystem. It reveals the metabolic potential of several uncultured and previously unsequenced microbial genera, families, and orders, as well as differences in the metabolic potential between the most abundant BSC populations and their cultured relatives, highlighting once more the danger of inferring function on the basis of taxonomy. Assigning functional potential to individual populations allows for the generation of hypotheses on trophic interactions and activity patterns in arid soil microbial communities and represents the basis for future resuscitation and activity studies of the system, e.g., involving metatranscriptomics.

Trace gas oxidizers are widespread and active members of soil microbial communities

Soil microorganisms globally are thought to be sustained primarily by organic carbon sources. Certain bacteria also consume inorganic energy sources such as trace gases, but they are presumed to be rare community members, except within some oligotrophic soils. Here we combined metagenomic, biogeochemical and modelling approaches to determine how soil microbial communities meet energy and carbon needs. Analysis of 40 metagenomes and 757 derived genomes indicated that over 70% of soil bacterial taxa encode enzymes to consume inorganic energy sources. Bacteria from 19 phyla encoded enzymes to use the trace gases hydrogen and carbon monoxide as supplemental electron donors for aerobic respiration. In addition, we identified a fourth phylum (Gemmatimonadota) potentially capable of aerobic methanotrophy. Consistent with the metagenomic profiling, communities within soil profiles from diverse habitats rapidly oxidized hydrogen, carbon monoxide and to a lesser extent methane below atmospheric concentrations. Thermodynamic modelling indicated that the power generated by oxidation of these three gases is sufficient to meet the maintenance needs of the bacterial cells capable of consuming them. Diverse bacteria also encode enzymes to use trace gases as electron donors to support carbon fixation. Altogether, these findings indicate that trace gas oxidation confers a major selective advantage in soil ecosystems, where availability of preferred organic substrates limits microbial growth. The observation that inorganic energy sources may sustain most soil bacteria also has broad implications for understanding atmospheric chemistry and microbial biodiversity in a changing world.

Anaerobic Carbon Monoxide Uptake by Microbial Communities in Volcanic Deposits at Different Stages of Successional Development on O-yama Volcano, Miyake-jima, Japan

Research on Kilauea and O-yama Volcanoes has shown that microbial communities and their activities undergo major shifts in response to plant colonization and that molybdenum-dependent CO oxidizers (Mo-COX) and their activities vary with vegetation and deposit age. Results reported here reveal that anaerobic CO oxidation attributed to nickel-dependent CO oxidizers (Ni-COX) also occurs in volcanic deposits that encompass different developmental stages. Ni-COX at three distinct sites responded rapidly to anoxia and oxidized CO from initial concentrations of about 10 ppm to sub-atmospheric levels. CO was also actively consumed at initial 25% concentrations and 25 °C, and during incubations at 60 °C; however, uptake under the latter conditions was largely confined to an 800-year-old forested site. Analyses of microbial communities based on 16S rRNA gene sequences in treatments with and without 25% CO incubated at 25 °C or 60 °C revealed distinct responses to temperature and CO among the sites and evidence for enrichment of known and potentially novel Ni-COX. The results collectively show that CO uptake by volcanic deposits occurs under a wide range of conditions; that CO oxidizers in volcanic deposits may be more diverse than previously imagined; and that Ni-dependent CO oxidizers might play previously unsuspected roles in microbial succession.

Lithogenic hydrogen supports microbial primary production in subglacial and proglacial environments

Life in environments devoid of photosynthesis, such as on early Earth or in contemporary dark subsurface ecosystems, is supported by chemical energy. How, when, and where chemical nutrients released from the geosphere fuel chemosynthetic biospheres is fundamental to understanding the distribution and diversity of life, both today and in the geologic past. Hydrogen (H₂) is a potent reductant that can be generated when water interacts with reactive components of mineral surfaces such as silicate radicals and ferrous iron. Such reactive mineral surfaces are continually generated by physical comminution of bedrock by glaciers. Here, we show that dissolved H₂ concentrations in meltwaters from an iron and silicate mineral-rich basaltic glacial catchment were an order of magnitude higher than those from a carbonate-dominated catchment. Consistent with higher H₂ abundance, sediment microbial communities from the basaltic catchment exhibited significantly shorter lag times and faster rates of net H₂ oxidation and dark carbon dioxide (CO₂) fixation than those from the carbonate catchment, indicating adaptation to use H₂ as a reductant in basaltic catchments. An enrichment culture of basaltic sediments provided with H₂, CO₂, and ferric iron produced a chemolithoautotrophic population related to Rhodoferax ferrireducens with a metabolism previously thought to be restricted to (hyper)thermophiles and acidophiles. These findings point to the importance of physical and chemical weathering processes in generating nutrients that support chemosynthetic primary production. Furthermore, they show that differences in bedrock mineral composition can influence the supplies of nutrients like H₂ and, in turn, the diversity, abundance, and activity of microbial inhabitants.

Pseudarthrobacter psychrotolerans sp. nov., a cold-adapted bacterium isolated from Antarctic soil

A novel cold-tolerant bacterium, designated strain YJ56T, was isolated from Antarctic soil collected from the Cape Burk area. Phylogenetic analysis through 16S rRNA gene sequence similarity revealed that strain YJ56T was most closely related to the genus Pseudarthrobacter , including Pseudarthrobacter oxydans DSM 20119T (99.06 % similarity), Pseudarthrobacter polychromogenes DSM 20136T (98.98 %) and Pseudarthrobacter sulfonivorans ALLT (98.76 %). The genome size (5.2 Mbp) of strain YJ56T was the largest among all the published genomes of Pseudarthrobacter type strains (4.2–5.0 Mbp). The genomic G+C content of strain YJ56T (64.7 mol%) was found to be consistent with those of other Pseudarthrobacter strains (62.0–71.0 mol%). The average nucleotide identity and average amino acid identity values between strain YJ56T and P. sulfonivorans ALLT were estimated at 84.1 and 84.2 %, respectively. The digital DNA–DNA hybridization value between the two strains was calculated to be 28.0 %. This rod-shaped and obligate aerobic strain exhibited no swimming or swarming motility. It had catalase activity but no oxidase activity. Cells grew at 4–28 °C (optimum, 13 °C) and pH 5.0–11.0 (optimum, pH 7.0) and with 0–6.0 % (w/v) NaCl (optimum, 0%) in Reasoner's 2A medium. MK-9 (H2) was the sole menaquinone. Two-dimensional TLC results revealed that the primary polar lipids were diphosphatidylglycerol, phosphatidylglycerol, two glycolipids and phosphatidylinositol. Fatty acid methyl ester analysis showed that anteiso-C15 : 0, anteiso-C17 : 0, iso-C15 : 0, C16 : 0 and iso-C16 : 0 were the major cellular fatty acids in strain YJ56T. Based on phenotypic and genotypic characteristics, strain YJ56T represents a novel species of the genus Pseudarthrobacter , and thus the name Pseudarthrobacter psychrotolerans sp. nov is proposed. The type strain is YJ56T (=JCM 33881T=KACC 21510T).