Atmospheric methane oxidizers are present and active in Canadian high Arctic soils

One Step Closer to Enigmatic USCα Methanotrophs: Isolation of a Methylocapsa-like Bacterium from a Subarctic Soil

The scavenging of atmospheric trace gases has been recognized as one of the lifestyle-defining capabilities of microorganisms in terrestrial polar ecosystems. Several metagenome-assembled genomes of as-yet-uncultivated methanotrophic bacteria, which consume atmospheric CH4 in these ecosystems, have been retrieved in cultivation-independent studies. In this study, we isolated and characterized a representative of these methanotrophs, strain D3K7, from a subarctic soil of northern Russia. Strain D3K7 grows on methane and methanol in a wide range of temperatures, between 5 and 30 °C. Weak growth was also observed on acetate. The presence of acetate in the culture medium stimulated growth at low CH4 concentrations (~100 p.p.m.v.). The finished genome sequence of strain D3K7 is 4.15 Mb in size and contains about 3700 protein-encoding genes. According to the result of phylogenomic analysis, this bacterium forms a common clade with metagenome-assembled genomes obtained from the active layer of a permafrost thaw gradient in Stordalen Mire, Abisco, Sweden, and the mineral cryosol at Axel Heiberg Island in the Canadian High Arctic. This clade occupies a phylogenetic position in between characterized Methylocapsa methanotrophs and representatives of the as-yet-uncultivated upland soil cluster alpha (USCα). As shown by the global distribution analysis, D3K7-like methanotrophs are not restricted to polar habitats but inhabit peatlands and soils of various climatic zones.

Microbial roles in cave biogeochemical cycling

Among fundamental research questions in subterranean biology, the role of subterranean microbiomes playing in key elements cycling is a top-priority one. Karst caves are widely distributed subsurface ecosystems, and cave microbes get more and more attention as they could drive cave evolution and biogeochemical cycling. Research have demonstrated the existence of diverse microbes and their participance in biogeochemical cycling of elements in cave environments. However, there are still gaps in how these microbes sustain in caves with limited nutrients and interact with cave environment. Cultivation of novel cave bacteria with certain functions is still a challenging assignment. This review summarized the role of microbes in cave evolution and mineral deposition, and intended to inspire further exploration of microbial performances on C/N/S biogeocycles.

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.

Sink or Source: Alternative Roles of Glacier Foreland Meadow Soils in Methane Emission Is Regulated by Glacier Melting on the Tibetan Plateau

Glacier foreland soils have long been considered as methane (CH₄) sinks. However, they are flooded by glacial meltwater annually during the glacier melting season, altering their redox potential. The impacts of this annual flooding on CH₄ emission dynamics and methane-cycling microorganisms are not well understood. Herein, we measured in situ methane flux in glacier foreland soils during the pre-melting and melting seasons on the Tibetan Plateau. In addition, high-throughput sequencing and qPCR were used to investigate the diversity, taxonomic composition, and the abundance of methanogenic archaea and methanotrophic bacteria. Our results showed that the methane flux ranged from -10.11 to 4.81 μg·m^-2·h^-1 in the pre-melting season, and increased to 7.48-22.57 μg·m^-2·h^-1 in the melting season. This indicates that glacier foreland soils change from a methane sink to a methane source under the impact of glacial meltwater. The extent of methane flux depends on methane production and oxidation conducted by methanogens and methanotrophs. Among all the environmental factors, pH (but not moisture) is dominant for methanogens, while both pH and moisture are not that strong for methanotrophs. The dominant methanotrophs were Methylobacter and Methylocystis, whereas the methanogens were dominated by methylotrophic Methanomassiliicoccales and hydrogenotrophic Methanomicrobiales. Their distributions were also affected by microtopography and environmental factor differences. This study reveals an alternative role of glacier foreland meadow soils as both methane sink and source, which is regulated by the annual glacial melt. This suggests enhanced glacial retreat may positively feedback global warming by increasing methane emission in glacier foreland soils in the context of climate change.

Unique high Arctic methane metabolizing community revealed through in situ 13CH4-DNA-SIP enrichment in concert with genome binning

Greenhouse gas (GHG) emissions from Arctic permafrost soils create a positive feedback loop of climate warming and further GHG emissions. Active methane uptake in these soils can reduce the impact of GHG on future Arctic warming potential. Aerobic methane oxidizers are thought to be responsible for this apparent methane sink, though Arctic representatives of these organisms have resisted culturing efforts. Here, we first used in situ gas flux measurements and qPCR to identify relative methane sink hotspots at a high Arctic cytosol site, we then labeled the active microbiome in situ using DNA Stable Isotope Probing (SIP) with heavy 13CH4 (at 100 ppm and 1000 ppm). This was followed by amplicon and metagenome sequencing to identify active organisms involved in CH4 metabolism in these high Arctic cryosols. Sequencing of 13C-labeled pmoA genes demonstrated that type II methanotrophs (Methylocapsa) were overall the dominant active methane oxidizers in these mineral cryosols, while type I methanotrophs (Methylomarinovum) were only detected in the 100 ppm SIP treatment. From the SIP-13C-labeled DNA, we retrieved nine high to intermediate quality metagenome-assembled genomes (MAGs) belonging to the Proteobacteria, Gemmatimonadetes, and Chloroflexi, with three of these MAGs containing genes associated with methanotrophy. A novel Chloroflexi MAG contained a mmoX gene along with other methane oxidation pathway genes, identifying it as a potential uncultured methane oxidizer. This MAG also contained genes for copper import, synthesis of biopolymers, mercury detoxification, and ammonia uptake, indicating that this bacterium is strongly adapted to conditions in active layer permafrost and providing new insights into methane biogeochemical cycling. In addition, Betaproteobacterial MAGs were also identified as potential cross-feeders with methanotrophs in these Arctic cryosols. Overall, in situ SIP labeling combined with metagenomics and genome binning demonstrated to be a useful tool for discovering and characterizing novel organisms related to specific microbial functions or biogeochemical cycles of interest. Our findings reveal a unique and active Arctic cryosol microbial community potentially involved in CH4 cycling.

Methylomonas albis sp. nov. and Methylomonas fluvii sp. nov.: Two cold-adapted methanotrophs from the river Elbe and emended description of the species Methylovulum psychrotolerans

Three strains of methanotrophic bacteria (EbA^T, EbB^T and Eb1) were isolated from the River Elbe, Germany. These Gram-negative, rod-shaped or coccoid cells contain intracytoplasmic membranes perpendicular to the cell surface. Colonies and liquid cultures appeared bright-pink. The major cellular fatty acids were 12:0 and 14:0, in addition in Eb1 the FA 16:1ω5t was also dominant. Methane and methanol were utilized as sole carbon sources by EbB^T and Eb1, while EbA^T could not use methanol. All strains oxidize methane using the particulate methane monooxygenase. Both strains contain an additional soluble methane monooxygenase. The strains grew optimally at 15-25 °C and at pH 6 and 8. Based on 16S rRNA gene analysis recovered from the full genome, the phylogenetic position of EbA^T is robustly outside any species clade with its closest relatives being Methylomonas sp. MK1 (98.24%) and Methylomonas sp. 11b (98.11%). Its closest type strain is Methylomonas methanica NCIMB11130 (97.91%). The 16S rRNA genes of EbB^T are highly similar to Methylomonas methanica strains with Methylomonas methanica R-45371 as the closest relative (99.87% sequence identity). However, average nucleotide identity (ANI) and digital DNA-DNA-hybridization (dDDH) values reveal it as distinct species. The DNA G + C contents were 51.07 mol% and 51.5 mol% for EbA^T and EbB^T, and 50.7 mol% for Eb1, respectively. Strains EbA^T and EbB^T are representing two novel species within the genus Methylomonas. For strain EbA^T we propose the name Methylomonas albis sp. nov (LMG 29958, JCM 32282) and for EbB^T, we propose the name Methylomonas fluvii sp. nov (LMG 29959, JCM 32283). Eco-physiological descriptions for both strains are provided. Strain Eb1 (LMG 30323, JCM 32281) is a member of the species Methylovulum psychrotolerans. This genus is so far only represented by two isolates but Eb1 is the first isolate from a temperate environment; so, an emended description of the species is given.

USCγ Dominated Community Composition and Cooccurrence Network of Methanotrophs and Bacteria in Subterranean Karst Caves

Karst caves have recently been demonstrated to act as a sink for atmospheric methane, due in part to consumption by microbes residing in caves that can oxidize methane at atmospheric levels. However, our knowledge about the responsible atmospheric methane-oxidizing bacteria (atmMOB) in this vast habitat remains limited to date. To address this issue, weathered rock samples from three karst caves were collected in Guilin City and subjected to high-throughput sequencing of pmoA and 16S rRNA genes. The results showed that members of the high-affinity upland soil cluster (USC), especially upland soil cluster gamma (USCγ), with absolute abundances of 104 to 109 copies · g-1 dry sample, dominated the atmMOB communities, while Proteobacteria and Actinobacteria dominated the overall bacterial communities. Moreover, USCγ was a keystone taxon in cooccurrence networks of both the atmMOB and the total bacterial community, whereas keystone taxa in the bacterial network also included Gaiella and Aciditerrimonas. Positive links overwhelmingly dominated the cooccurrence networks of both atmMOB and the total bacterial community, indicating a consistent response to environmental disturbances. Our study shed new insights on the diversity and abundances underlining atmMOB and total bacterial communities and on microbial interactions in subterranean karst caves, which increased our understanding about USC and supported karst caves as a methane sink. IMPORTANCE Karst caves have recently been demonstrated to be a potential atmospheric methane sink, presumably due to consumption by methane-oxidizing bacteria. However, the sparse knowledge about the diversity, distribution, and community interactions of methanotrophs requires us to seek further understanding of the ecological significance of methane oxidation in these ecosystems. Our pmoA high-throughput results from weathered rock samples from three karst caves in Guilin City confirm the wide occurrence of atmospheric methane-oxidizing bacteria in this habitat, especially those affiliated with the upland soil cluster, with a gene copy number of 104 to 109 copies per gram dry sample. Methanotrophs and the total bacterial communities had more positive than negative interactions with each other as indicated by the cooccurrence network, suggesting their consistent response to environmental disturbance. Our results solidly support caves as an atmospheric methane sink, and they contribute to a comprehensive understanding of the diversity, distribution, and interactions of microbial communities in subsurface karst caves.

Bacterial Number and Genetic Diversity in a Permafrost Peatland (Western Siberia): Testing a Link with Organic Matter Quality and Elementary Composition of a Peat Soil Profile

Permafrost peatlands, containing a sizable amount of soil organic carbon (OC), play a pivotal role in soil (peat) OC transformation into soluble and volatile forms and greatly contribute to overall natural CO2 and CH4 emissions to the atmosphere under ongoing permafrost thaw and soil OC degradation. Peat microorganisms are largely responsible for the processing of this OC, yet coupled studies of chemical and bacterial parameters in permafrost peatlands are rather limited and geographically biased. Towards testing the possible impact of peat and peat pore water chemical composition on microbial population and diversity, here we present results of a preliminary study of the western Siberia permafrost peatland discontinuous permafrost zone. The quantitative evaluation of microorganisms and determination of microbial diversity along a 100 cm thick peat soil column, which included thawed and frozen peat and bottom mineral horizon, was performed by RT-PCR and 16S rRNA gene-based metagenomic analysis, respectively. Bacteria (mainly Proteobacteria, Acidobacteria, Actinobacteria) strongly dominated the microbial diversity (99% sequences), with a negligible proportion of archaea (0.3–0.5%). There was a systematic evolution of main taxa according to depth, with a maximum of 65% (Acidobacteria) encountered in the active layer, or permafrost boundary (50–60 cm). We also measured C, N, nutrients and ~50 major and trace elements in peat (19 samples) as well as its pore water and dispersed ice (10 samples), sampled over the same core, and we analyzed organic matter quality in six organic and one mineral horizon of this core. Using multiparametric statistics (PCA), we tested the links between the total microbial number and 16S rRNA diversity and chemical composition of both the solid and fluid phase harboring the microorganisms. Under climate warming and permafrost thaw, one can expect a downward movement of the layer of maximal genetic diversity following the active layer thickening. Given a one to two orders of magnitude higher microbial number in the upper (thawed) layers compared to bottom (frozen) layers, an additional 50 cm of peat thawing in western Siberia may sizably increase the total microbial population and biodiversity of active cells.

Methane-Oxidizing Communities in Lichen-Dominated Forested Tundra Are Composed Exclusively of High-Affinity USCα Methanotrophs

Upland soils of tundra function as a constant sink for atmospheric CH₄ but the identity of methane oxidizers in these soils remains poorly understood. Methane uptake rates of -0.4 to -0.6 mg CH₄-C m^-2 day^-1 were determined by the static chamber method in a mildly acidic upland soil of the lichen-dominated forested tundra, North Siberia, Russia. The maximal CH₄ oxidation activity was localized in an organic surface soil layer underlying the lichen cover. Molecular identification of methanotrophic bacteria based on retrieval of the pmoA gene revealed Upland Soil Cluster Alpha (USCα) as the only detectable methanotroph group. Quantification of these pmoA gene fragments by means of specific qPCR assay detected ~10⁷pmoA gene copies g^-1 dry soil. The pmoA diversity was represented by seven closely related phylotypes; the most abundant phylotype displayed 97.5% identity to pmoA of Candidatus Methyloaffinis lahnbergensis. Further analysis of prokaryote diversity in this soil did not reveal 16S rRNA gene fragments from well-studied methanotrophs of the order Methylococcales and the family Methylocystaceae. The largest group of reads (~4% of all bacterial 16S rRNA gene fragments) that could potentially belong to methanotrophs was classified as uncultivated Beijerinckiaceae bacteria. These reads displayed 96-100 and 95-98% sequence similarity to 16S rRNA gene of Candidatus Methyloaffinis lahnbergensis and "Methylocapsa gorgona" MG08, respectively, and were represented by eight species-level operational taxonomic units (OTUs), two of which were highly abundant. These identification results characterize subarctic upland soils, which are exposed to atmospheric methane concentrations only, as a unique habitat colonized mostly by USCα methanotrophs.

Extreme environments: microbiology leading to specialized metabolites

The prevalence of multidrug-resistant microbial pathogens due to the continued misuse and overuse of antibiotics in agriculture and medicine is raising the prospect of a return to the preantibiotic days of medicine at the time of diminishing numbers of drug leads. The good news is that an increased understanding of the nature and extent of microbial diversity in natural habitats coupled with the application of new technologies in microbiology and chemistry is opening up new strategies in the search for new specialized products with therapeutic properties. This review explores the premise that harsh environmental conditions in extreme biomes, notably in deserts, permafrost soils and deep-sea sediments select for micro-organisms, especially actinobacteria, cyanobacteria and fungi, with the potential to synthesize new druggable molecules. There is evidence over the past decade that micro-organisms adapted to life in extreme habitats are a rich source of new specialized metabolites. Extreme habitats by their very nature tend to be fragile hence there is a need to conserve those known to be hot-spots of novel gifted micro-organisms needed to drive drug discovery campaigns and innovative biotechnology. This review also provides an overview of microbial-derived molecules and their biological activities focusing on the period from 2010 until 2018, over this time 186 novel structures were isolated from 129 representatives of microbial taxa recovered from extreme habitats.

Upland soil cluster γ dominates the methanotroph communities in the karst Heshang Cave

Microorganisms are thought to play a critical role in methane (CH4) consumption in karst caves and yet the presence and diversity of methane-oxidizing bacteria (MOB) remain a mystery. In Heshang Cave, CH4 concentration decreases from 1.9 ppm at the entrance to 0.65 ppm inside the cave. To explore the presence and diversity of MOB in this cave, weathered rocks and sediment samples were collected from the cave and subjected to molecular analysis. The abundances of MOB were 107-108 copies g-1 dry sample via quantification of the pmoA gene, which are comparable to or even higher than those reported in other terrestrial environments, and account for up to 20% of the total microbial communities. Phylogenetically, MOB communities were dominated by the 'high-affinity' upland soil cluster γ (USCγ), although the predominance of Type Ia MOB was also detected in the permanently waterlogged stream sediment. The estimated CH4 oxidation potential varied dramatically among samples in the range of 0.6-80 CH4 m-3 d-1. Collectively, this study provides compelling evidence that the high-affinity MOB capable of oxidizing CH4 at the atmospheric level are present in Heshang Cave, which may play an important role in the CH4 consumption, and supports karst caves as important atmospheric CH4 sinks.

Species interactions and distinct microbial communities in high Arctic permafrost affected cryosols are associated with the CH4 and CO2 gas fluxes

Microbial metabolism of the thawing organic carbon stores in permafrost results in a positive feedback loop of greenhouse gas emissions. CO₂ and CH₄ fluxes and the associated microbial communities in Arctic cryosols are important in predicting future warming potential of the Arctic. We demonstrate that topography had an impact on CH₄ and CO₂ flux at a high Arctic ice-wedge polygon terrain site, with higher CO₂ emissions and lower CH₄ uptake at troughs compared to polygon interior soils. The pmoA sequencing suggested that USCα cluster of uncultured methanotrophs is likely responsible for observed methane sink. Community profiling revealed distinct assemblages across the terrain at different depths. Deeper soils contained higher abundances of Verrucomicrobia and Gemmatimonadetes, whereas the polygon interior had higher Acidobacteria and lower Betaproteobacteria and Deltaproteobacteria abundances. Genome sequencing of isolates from the terrain revealed presence of carbon cycling genes including ones involved in serine and ribulose monophosphate pathways. A novel hybrid network analysis identified key members that had positive and negative impacts on other species. Operational Taxonomic Units (OTUs) with numerous positive interactions corresponded to Proteobacteria, Candidatus Rokubacteria and Actinobacteria phyla, while Verrucomicrobia and Acidobacteria members had negative impacts on other species. Results indicate that topography and microbial interactions impact community composition.

Upland Soil Cluster Gamma dominates methanotrophic communities in upland grassland soils

Aerobic methanotrophs in upland soils consume atmospheric methane, serving as a critical counterbalance to global warming; however, the biogeographic distribution patterns of their abundance and community composition are poorly understood, especial at a large scale. In this study, soils were sampled from 30 grasslands across >2000 km on the Qinghai-Tibetan Plateau to determine the distribution patterns of methanotrophs and their driving factors at a regional scale. Methanotroph abundance and community composition were analyzed using quantitative PCR and Illumina Miseq sequencing of pmoA genes, respectively. The pmoA gene copies ranged from 8.2 × 10⁵ to 1.1 × 10⁸ per gram dry soil. Among the 30 grassland soil samples, Upland Soil Cluster Gamma (USCγ) dominated the methanotroph communities in 26 samples. Jasper Ridge Cluster (JR3) was the most dominant methanotrophic cluster in two samples; while Methylocystis, cluster FWs, and Methylobacter were abundant in other two wet soil samples. Interestingly, reanalyzing the pmoA genes sequencing data from existing publications suggested that USCγ was also the main methanotrophic cluster in grassland soils in other regions, especially when their mean annual precipitation was <500 mm. Canonical Analysis of Principal Coordinates including all soil samples indicated that the methanotrophic community composition was significantly correlated with local environmental factors, among which mean annual precipitation and pH showed the strongest correlations. Variance partitioning analysis showed that environmental factors and spatial distance were significant factors affecting the community structure of methanotrophs, and environmental properties were more important factors. Collectively, these findings indicate that atmospheric methane may be mainly oxidized by USCγ in upland soils. They also highlight the key role of water availability and pH in determining the abundance and community profiles of grassland soil methanotrophs.

Widespread soil bacterium that oxidizes atmospheric methane

The global atmospheric level of methane (CH₄), the second most important greenhouse gas, is currently increasing by ∼10 million tons per year. Microbial oxidation in unsaturated soils is the only known biological process that removes CH₄ from the atmosphere, but so far, bacteria that can grow on atmospheric CH₄ have eluded all cultivation efforts. In this study, we have isolated a pure culture of a bacterium, strain MG08 that grows on air at atmospheric concentrations of CH₄ [1.86 parts per million volume (p.p.m.v.)]. This organism, named Methylocapsa gorgona, is globally distributed in soils and closely related to uncultured members of the upland soil cluster α. CH₄ oxidation experiments and ¹³C-single cell isotope analyses demonstrated that it oxidizes atmospheric CH₄ aerobically and assimilates carbon from both CH₄ and CO₂ Its estimated specific affinity for CH₄ (a⁰ _s) is the highest for any cultivated methanotroph. However, growth on ambient air was also confirmed for Methylocapsa acidiphila and Methylocapsa aurea, close relatives with a lower specific affinity for CH₄, suggesting that the ability to utilize atmospheric CH₄ for growth is more widespread than previously believed. The closed genome of M. gorgona MG08 encodes a single particulate methane monooxygenase, the serine cycle for assimilation of carbon from CH₄ and CO₂, and CO₂ fixation via the recently postulated reductive glycine pathway. It also fixes dinitrogen and expresses the genes for a high-affinity hydrogenase and carbon monoxide dehydrogenase, suggesting that atmospheric CH₄ oxidizers harvest additional energy from oxidation of the atmospheric trace gases carbon monoxide (0.2 p.p.m.v.) and hydrogen (0.5 p.p.m.v.).

Activity and diversity of methane-oxidizing bacteria along a Norwegian sub-Arctic glacier forefield

Methane (CH4) is one of the most abundant greenhouse gases in the atmosphere and identification of its sources and sinks is crucial for the reliability of climate model outputs. Although CH4 production and consumption rates have been reported from a broad spectrum of environments, data obtained from glacier forefields are restricted to a few locations. We report the activities of methanotrophic communities and their diversity along a chronosequence in front of a sub-Arctic glacier using high-throughput sequencing and gas flux measurements. CH4 oxidation rates were measured in the field throughout the growing season during three sampling times at eight different sampling points in combination with laboratory incubation experiments. The overall results showed that the methanotrophic community had similar trends of increased CH4 consumption and increased abundance as a function of soil development and time of year. Sequencing results revealed that the methanotrophic community was dominated by a few OTUs and that a short-term increase in CH4 concentration, as performed in the field measurements, altered slightly the relative abundance of the OTUs.

Microbial diversity and biogeography in Arctic soils

Microorganisms dominate terrestrial environments in the polar regions and Arctic soils are known to harbour significant microbial diversity, far more diverse and numerous in the region than was once thought. Furthermore, the geographic distribution and structure of Arctic microbial communities remains elusive, despite their important roles in both biogeochemical cycling and in the generation and decomposition of climate active gases. Critically, Arctic soils are estimated to store over 1500 Pg of carbon and, thus, have the potential to generate positive feedback within the climate system. As the Arctic region is currently undergoing rapid change, the likelihood of faster release of greenhouse gases such as CO₂ , CH₄ and N₂ O is increasing. Understanding the microbial communities in the region, in terms of their diversity, abundance and functional activity, is key to producing accurate models of greenhouse gas release. This review brings together existing data to determine what we know about microbial diversity and biogeography in Arctic soils.

Unravelling the Identity, Metabolic Potential and Global Biogeography of the Atmospheric Methane-Oxidizing Upland Soil Cluster α

Understanding of global methane sources and sinks is a prerequisite for the design of strategies to counteract global warming. Microbial methane oxidation in soils represents the largest biological sink for atmospheric methane. However, still very little is known about the identity, metabolic properties and distribution of the microbial group proposed to be responsible for most of this uptake, the uncultivated upland soil cluster α (USCα). Here, we reconstructed a draft genome of USCα from a combination of targeted cell sorting and metagenomes from forest soil, providing the first insights into its metabolic potential and environmental adaptation strategies. The 16S rRNA gene sequence recovered was distinctive and suggests this crucial group as a new genus within the Beijerinckiaceae, close to Methylocapsa. Application of a fluorescently labelled suicide substrate for the particulate methane monooxygenase enzyme (pMMO) coupled to 16S rRNA fluorescence in situ hybridisation (FISH) allowed for the first time a direct link of the high‐affinity activity of methane oxidation to USCα cells in situ. Analysis of the global biogeography of this group further revealed its presence in previously unrecognized habitats, such as subterranean and volcanic biofilm environments, indicating a potential role of these environments in the biological sink for atmospheric methane.

Atmospheric CH4 oxidation by Arctic permafrost and mineral cryosols as a function of water saturation and temperature

The response of methanotrophic bacteria capable of oxidizing atmospheric CH₄ to climate warming is poorly understood, especially for those present in Arctic mineral cryosols. The atmospheric CH₄ oxidation rates were measured in microcosms incubated at 4 °C and 10 °C along a 1-m depth profile and over a range of water saturation conditions for mineral cryosols containing type I and type II methanotrophs from Axel Heiberg Island (AHI), Nunavut, Canada. The cryosols exhibited net consumption of ~2 ppmv CH₄ under all conditions, including during anaerobic incubations. Methane oxidation rates increased with temperature and decreased with increasing water saturation and depth, exhibiting the highest rates at 10 °C and 33% saturation at 5 cm depth (260 ± 60 pmol CH₄ gdw^-1 d^-1 ). Extrapolation of the CH₄ oxidation rates to the field yields net CH₄ uptake fluxes ranging from 11 to 73 μmol CH₄  m^-2 d^-1 , which are comparable to field measurements. Stable isotope mass balance indicates ~50% of the oxidized CH₄ is incorporated into the biomass regardless of temperature or saturation. Future atmospheric CH₄ uptake rates at AHI with increasing temperatures will be determined by the interplay of increasing CH₄ oxidation rates vs. water saturation and the depth to the water table during summer thaw.

Microbial diversity and functional capacity in polar soils

Global change is disproportionately affecting cold environments (polar and high elevation regions), with potentially negative impacts on microbial diversity and functional processes. In most cold environments the combination of low temperatures, and physical stressors, such as katabatic wind episodes and limited water availability result in biotic systems, which are in trophic terms very simple and primarily driven by microbial communities. Metagenomic approaches have provided key insights on microbial communities in these systems and how they may adapt to stressors and contribute towards mediating crucial biogeochemical cycles. Here we review, the current knowledge regarding edaphic-based microbial diversity and functional processes in Antarctica, and the Artic. Such insights are crucial and help to establish a baseline for understanding the impact of climate change on Polar Regions.

A temperate river estuary is a sink for methanotrophs adapted to extremes of pH, temperature and salinity

River Tyne (UK) estuarine sediments harbour a genetically and functionally diverse community of methane-oxidizing bacteria (methanotrophs), the composition and activity of which were directly influenced by imposed environmental conditions (pH, salinity, temperature) that extended far beyond those found in situ. In aerobic sediment slurries methane oxidation rates were monitored together with the diversity of a functional gene marker for methanotrophs (pmoA). Under near in situ conditions (4-30°C, pH 6-8, 1-15 g l(-1) NaCl), communities were enriched by sequences affiliated with Methylobacter and Methylomonas spp. and specifically a Methylobacter psychrophilus-related species at 4-21°C. More extreme conditions, namely high temperatures ≥ 40°C, high ≥ 9 and low ≤ 5 pH, and high salinities ≥ 35 g l(-1) selected for putative thermophiles (Methylocaldum), acidophiles (Methylosoma) and haloalkaliphiles (Methylomicrobium). The presence of these extreme methanotrophs (unlikely to be part of the active community in situ) indicates passive dispersal from surrounding environments into the estuary.

Diversity and Habitat Preferences of Cultivated and Uncultivated Aerobic Methanotrophic Bacteria Evaluated Based on pmoA as Molecular Marker

Methane-oxidizing bacteria are characterized by their capability to grow on methane as sole source of carbon and energy. Cultivation-dependent and -independent methods have revealed that this functional guild of bacteria comprises a substantial diversity of organisms. In particular the use of cultivation-independent methods targeting a subunit of the particulate methane monooxygenase (pmoA) as functional marker for the detection of aerobic methanotrophs has resulted in thousands of sequences representing "unknown methanotrophic bacteria." This limits data interpretation due to restricted information about these uncultured methanotrophs. A few groups of uncultivated methanotrophs are assumed to play important roles in methane oxidation in specific habitats, while the biology behind other sequence clusters remains still largely unknown. The discovery of evolutionary related monooxygenases in non-methanotrophic bacteria and of pmoA paralogs in methanotrophs requires that sequence clusters of uncultivated organisms have to be interpreted with care. This review article describes the present diversity of cultivated and uncultivated aerobic methanotrophic bacteria based on pmoA gene sequence diversity. It summarizes current knowledge about cultivated and major clusters of uncultivated methanotrophic bacteria and evaluates habitat specificity of these bacteria at different levels of taxonomic resolution. Habitat specificity exists for diverse lineages and at different taxonomic levels. Methanotrophic genera such as Methylocystis and Methylocaldum are identified as generalists, but they harbor habitat specific methanotrophs at species level. This finding implies that future studies should consider these diverging preferences at different taxonomic levels when analyzing methanotrophic communities.

An active atmospheric methane sink in high Arctic mineral cryosols

Methane (CH4) emission by carbon-rich cryosols at the high latitudes in Northern Hemisphere has been studied extensively. In contrast, data on the CH4 emission potential of carbon-poor cryosols is limited, despite their spatial predominance. This work employs CH4 flux measurements in the field and under laboratory conditions to show that the mineral cryosols at Axel Heiberg Island in the Canadian high Arctic consistently consume atmospheric CH4. Omics analyses present the first molecular evidence of active atmospheric CH4-oxidizing bacteria (atmMOB) in permafrost-affected cryosols, with the prevalent atmMOB genotype in our acidic mineral cryosols being closely related to Upland Soil Cluster α. The atmospheric (atm) CH4 uptake at the study site increases with ground temperature between 0 °C and 18 °C. Consequently, the atm CH4 sink strength is predicted to increase by a factor of 5-30 as the Arctic warms by 5-15 °C over a century. We demonstrate that acidic mineral cryosols are a previously unrecognized potential of CH4 sink that requires further investigation to determine its potential impact on larger scales. This study also calls attention to the poleward distribution of atmMOB, as well as to the potential influence of microbial atm CH4 oxidation, in the context of regional CH4 flux models and global warming.

Site- and horizon-specific patterns of microbial community structure and enzyme activities in permafrost-affected soils of Greenland

Permafrost-affected soils in the Northern latitudes store huge amounts of organic carbon (OC) that is prone to microbial degradation and subsequent release of greenhouse gasses to the atmosphere. In Greenland, the consequences of permafrost thaw have only recently been addressed, and predictions on its impact on the carbon budget are thus still highly uncertain. However, the fate of OC is not only determined by abiotic factors, but closely tied to microbial activity. We investigated eight soil profiles in northeast Greenland comprising two sites with typical tundra vegetation and one wet fen site. We assessed microbial community structure and diversity (SSU rRNA gene tag sequencing, quantification of bacteria, archaea and fungi), and measured hydrolytic and oxidative enzyme activities. Sampling site and thus abiotic factors had a significant impact on microbial community structure, diversity and activity, the wet fen site exhibiting higher potential enzyme activities and presumably being a hot spot for anaerobic degradation processes such as fermentation and methanogenesis. Lowest fungal to bacterial ratios were found in topsoils that had been relocated by cryoturbation ("buried topsoils"), resulting from a decrease in fungal abundance compared to recent ("unburied") topsoils. Actinobacteria (in particular Intrasporangiaceae) accounted for a major fraction of the microbial community in buried topsoils, but were only of minor abundance in all other soil horizons. It was indicated that the distribution pattern of Actinobacteria and a variety of other bacterial classes was related to the activity of phenol oxidases and peroxidases supporting the hypothesis that bacteria might resume the role of fungi in oxidative enzyme production and degradation of phenolic and other complex substrates in these soils. Our study sheds light on the highly diverse, but poorly-studied communities in permafrost-affected soils in Greenland and their role in OC degradation.