The price for wheat is driving the expansion of agricultural monocropping practices, which in turn facilitate the spread of phytopathogenic fungal infestation. Inoculating plants/ seeds with beneficial bacteria offers a sustainable alternative to chemical fungicides. Predatory myxobacteria are characterized by their production of various lytic secondary metabolites including fungicides. While their modes of action with prey are well studied, the direct application of myxobacteria in plants remains largely unexplored. The aim of this study was to assess whether seed inoculation with myxobacteria has a general effect on plant growth and can control take-all disease caused by Gaeumannomyces tritici (Ggt) in wheat in vivo. The inhibition potential of five different myxobacteria on ten soil-borne phytopathogenic fungi was examined in vitro. The effective myxobacteria, Haliangium ochraceum and Myxococcus virescens, were used as seed inoculants of winter wheat, which was grown for four weeks in soil infected with the pathogen Ggt in an in vivo greenhouse experiment. Myxobacteria inoculation did not impair plant growth (with and without Ggt infection), and M. virescens even showed a slight growth-promoting effect. Furthermore, myxobacteria inoculated plants showed a 50

Summer drought significantly affects soil microbiome diversity and functioning, and metabolic interactions in wetland ecosystems, yet its effects on the microbial food web are less understood. We investigated the dynamics of bacterivorous microorganisms and prey bacteria (PBac) in a rewetted fen peatland before, during, and after a summer drought using small subunit ribosomal RNA (SSU rRNA) gene sequencing and quantitative metatranscriptomics. We identified bacterivores including bacteria (e.g. Myxobacteria), protists, and nematodes, as well as Ca. Patescibacteria and Diapherotrites, Parvarchaeota, Aenigmarchaeota, Nanoarchaeota, and Nanohaloarchaeota archaea (PD) having a host-dependent lifestyle. In addition, bacteriophage transcripts were enumerated. The summer drought increased the diversity of bacterivorous protists (BPro) and bacteria (BBac) in the fen, along with decreases in the water level and increases in redox potential. The SSU rRNA transcripts abundance for all bacterivores (with the exception of the PD) increased during the drought, reaching a peak in October. The transcript abundance of all bacterivores remained above predrought level for at least four months after the drought ended. This became more evident when assessing the bacterivore-to-prey-bacteria ratios, with the ratios of aerobic BBac, BPro, and nematode to PBac increasing by more than two-fold compared to predrought. In contrast, the PD and bacteriophage-to-prey ratios remained rather stable. This study provides a holistic view of the diversity and composition of bacterivores in a fen peat microbiome, including the PD and bacteriophages, and suggests a long-lasting impact of summer drought on bacterivore and food web dynamics. The strong responses of aerobic bacterivores may lead to changes in ecosystem functioning through modulated trophic interactions in a changing climate.

Forest soils in the Alps play a crucial role in global carbon cycling. Soil (micro-)organisms drive carbon cycling in soils through biogeochemical transformations and predator-prey interactions. These relations are dynamic and depend on environmental factors such as elevation and season. Here we present a quantitative metatranscriptomic study on forest ecosystems, comparing 24 metatranscriptomes of a submontane and a subalpine site in Italy with different vegetation covers (deciduous and coniferous) and seasons (spring and autumn). By parallel analysis of ribosomal RNA and transcribed mRNA, we conducted community composition analysis and linked the putative contribution of different taxonomic groups (rRNA) to different functional processes (mRNA) mainly focusing on carbon-cycling. We studied the presence of bacteria, microeukaryotes and viruses. A seasonal influence on the functional profile (mRNA) was observed showing a change in cellular activity. Among the transcripts encoding decomposition enzymes, the transcripts of Terrabacteria were most abundant and Alveolata and Amoebozoa were detected as potential cellulose and lignin biodegradation contributors. Taxonomic profiles showed only site-specific signatures, with no clear seasonal change. The biggest difference was found within Acidobacteriota which were more abundant at higher elevation whereas predatory protists and predatory bacteria were more abundant at lower elevation.

BACKGROUND:Oil contamination in soils causes significant environmental impacts and risks to human health. Oil components can be naturally reduced by indigenous microorganisms, that are able to degrade such substrates. We used culture-independent and culture-dependent methods to examine the prokaryotic and the eukaryotic microbiome of different heavily oil contaminated soils in Kazakhstan. Bacteria and fungi were enriched and isolated from four soils contaminated with crude oil or hydrocarbons. Aliphatic, aromatic and condensed aromatic model hydrocarbons of crude oil and crude oil itself were used as substrates for the enrichment and the isolation experiments. The enrichment process was accompanied by culture-independent tests. RESULTS:The results of the Illumina sequencing of the contaminated soils and the enrichment cultures were compared with the results of the culture-dependent isolation and determination of bacterial, yeast and filamentous fungal strains. The majority of these 110 strains from 45 different genera belong to well-described hydrocarbon degraders like Bacilli and Rhodococci as well as to Achromobacter, Gordonia, Pseudomonas, Stenotrophomonas, Aspergillus, Exophiala, Fusarium, Meyerozyma, Penicillium and Trichoderma species. The most abundant species was the ascomycetal yeast Meyerozyma guilliermondii followed by strains of the bacterial genus Peribacillus. Furthermore, we combined the microbiome insights on the enrichment procedures and the isolation of bacteria, yeasts and filamentous fungi with the in-fact degradation potential of the isolated species based on substrate consumption and metabolite formation. In addition to the well-described hydrocarbon degraders, the utilization spectrum of less-studied strains of the genera Leifsonia, Neorhizobium, Purpureocillium, Rhodotorula and Sarocladium could be broadened. CONCLUSION:In the end a complex overview of the indigenous microorganisms and their degradation ability of crude oil components emerged and demonstrates the great potential of bioremediation for Kazakhstan soils.

Permafrost degradation creates hydrologically distinct landscapes, including dry soils with enhanced drainage and wet soils with poor drainage. In Fairbanks (Alaska, USA), we compared the active layer of dry and wet degraded permafrost landscapes with that of a non-degraded intact permafrost landscape to assess how hydrological conditions shape microbial community composition, extracellular enzyme activities, and microbial extracellular polymeric substances (EPS). Based on 16S rRNA sequencing, the alpha diversity of the prokaryotic community was significantly lower in the dry site compared to the wet and intact sites, with soil organic carbon (SOC) and pH identified as the primary environmental drivers. The active layer of the dry site was dominated by Thermoleophilia and Acidobacteriae, whereas the wet site was dominated by Gammaproteobacteria and Gemmatimonadetes. The relative proportions of most bacterial classes differed significantly across depths in the dry and wet sites, while the intact site exhibited less vertical variation. Hydrolytic enzyme activities were significantly higher in the topsoil across all sites, whereas oxidative enzyme activities showed relatively uniform patterns with depth but generally exceeded hydrolytic activities. EPS content varied among sites and depths, with the highest EPS-sugar content observed in the wet site overall, whereas topsoil EPS-sugar and EPS-protein contents were highest in the wet site, followed by the dry and intact sites, and subsoil EPS content was highest in the intact site. This study demonstrates that hydrological shifts in degraded permafrost soils shape microbial community structure, enzymatic activity patterns, and EPS content, with potential implications for SOC loss and stabilization.

Abstract Bacterial symbionts were shown to have significant beneficial impacts on the fitness of their host in insects, but little is known on the symbionts of spiders and their interactions with their hosts. Here, we assembled and investigated the circular 575 kb genome of Candidatus Argioplasma dusa, a novel bacterial symbiont of the wasp spider Argiope bruennichi . Phylogenomic analysis placed this species within the phylum Tenericutes, in a poorly characterized clade that may represent a new order-level lineage or affiliate with the Mycoplasmatales order. With 559 predicted genes, the genome is relatively small compared to other Tenericutes genomes (on average 969 genes) and has a low GC content of ~24%. While the genome encodes genes for proteins involved in glycolysis and fermentative acetate production, it revealed minimal biosynthetic capabilities with pathways for nucleotide, amino acid and vitamin biosynthesis being absent in Ca . Argioplasma dusa. This suggests an intracellular endosymbiotic lifestyle within the spider host. The symbiont was detected in A. bruennichi populations across the distribution range of the spider but appears to be absent in certain populations.

While the link between carbon (C) and energy flow in soil microbial assemblages has recently gained attention, it remains unclear how microbial grazing affects these interactions. Nematode grazers modify the community composition and activity of their prey, altering the process rates and fluxes in decomposition pathways. In a 32-day microcosm experiment, bottom-up control in the micro-food web was fostered by amending soil with [[EQUATION]]-labelled maize litter, while top-down control was induced by increasing the density of bacterial- or fungal-feeding nematodes. Changes in micro-food web structure (based on phospholipid fatty acids and nematode community) were related to carbon pools in biomass (Cmic, Cnematode) and fluxes (13Cmic, 13Cnematode, release of CO2 and 13CO2) as well as energy loss (heat release).Amendment with maize litter generally fostered microbial activity and accelerated the fungal decomposition pathway. In combination with the inoculation of bacterial or fungal grazers, however, this initial stimulation was followed by a decline in soil respiration of 5-18%. In soils without resource amendment, higher grazing pressure on bacteria increased the calorespirometric ratio, probably due to lower carbon use efficiency or lower oxidation states of substrates. In fungal feeder dominated soil, carbon and energy fluxes diverged, as litter-induced respiration decreased with time, while litter-induced heat release stabilized. This decoupling of carbon and energy flow at the first trophic step of the micro-food web, i.e. between microorganisms and their grazers, highlights the importance of this trophic link for the energy balance of the entire soil food web.

The methane-cycling microbiomes play crucial roles in methane dynamics. However, little is known about their distributions on a pan-Arctic scale as well as their responses to the widespread permafrost degradation. Based on 621 datasets of 16S rRNA gene amplicons from intact permafrost soils across the pan-Arctic, we identified only 22 methanogen and 26 methanotroph phylotypes. Their relative abundances varied significantly between sites and soil horizons. Only four methanogen phylotypes were detected at all locations. Remarkably, the permafrost soil methane filter was almost exclusively dominated by some obligate methanotroph (Methylobacter-like) phylotypes. However, a case study in Alaska suggests that atmospheric methane oxidizing bacteria (Methylocapsa-like phylotypes) dominated methanotrophs in a drier condition after permafrost degradation. These findings point towards a few key microbes particularly relevant for future studies on Arctic methane dynamics in a warming climate and that under future dry conditions, increased atmospheric methane uptake in Arctic upland soils may occur.

The redox status of permafrost soils is a decisive factor for their nutrient cycling, organic matter decomposition, and greenhouse gas emissions. Although being associated with a variety of processes, data availability of continuous redox measurements in permafrost soils is scarce. Here, we provide a unique dataset covering three years of soil redox potential measurements, obtained from a monitoring approach at three research sites near Fairbanks, Alaska. Redox potential pattern in the permafrost soil active layer showed large seasonal differences, with reducing conditions in the short summer/autumn to largely oxidizing conditions in winter and spring. However, conditions for methane production were at no time recorded in the three years. Especially the freezing and thawing had substantial impact on the redox status, highlighting that assessment of redox conditions in permafrost soils should be extended beyond the typical summer observation periods.

Soil is one of the most complex ecosystems, being an elementary source for food and resources needed by humankind. Despite its pivotal role in shaping and transforming this complex ecosystem, the soil microbial food-web is still poorly understood, which stands also true for matter and energy fluxes to higher trophic levels. A group of microbiome predators that has recently come more into focus are the Myxobacteria. They are famous for their predatory life style and might thus influence microbial death, growth and turnover in the soil environment via predation and an elaborate arsenal of secondary metabolites. To shed light on their role in the soil food-web, we investigated their predatory behavior and interactions with potential prey and predators using in vitro and in situ approaches. We tested the predation spectra in vitro with binary interaction assays of four different Myxobacteria (Haliangium ochraceum, Myxococcus virescens, Myxococcus fulvus, Corallococcus coralloides) with 16 different prey bacteria isolated from soils. The in vitro assay showed that each Myxobacterium had species-specific prey spectra. While Haliangium ochraceum and Myxococcus virescens showed the strongest predation effects on prey bacteria, the Corallococcus coralloides strain lysed the fewest prey bacteria. Taken together, not a single bacterium of the tested ones was resistant to lysis. Remarkably, also strains of the never before tested phyla Gemmatimonadota and Veruccomicrobiota can be lysed by Myxobacteria.To shed light on (inter)actions of Myxobacteria with microbiome members in situ, a 32 day long microcosm study with an agricultural soil was performed, manipulating (a) the carbon source and (b) the grazing pressure of higher trophic levels, via the addition of fungivorous and bacterivorous nematodes, respectively. We applied quantitative PCR and quantitative metatranscriptomics microbiome profiling of 80 samples to shed light of the impact of aforementioned manipulations of C-source and grazers on the microbiome. Three-Domain SSU rRNA profiling showed that myxobacteria were highly abundant (up to 20%) in the used agricultural soil. In contrast, fungal abundance was much lower (1.5 %), while Protozoa, i.e. dominated by Amoebozoa and Cercozoa, were much more abundant than fungi (5 %). This suggests a microbial food-web dynamic in this agricultural soil that was heavily dominated by a bacterial, and not fungal channel. As expected, the abundance and composition of Myxobacteria were not affected by addition of fungivorous Nematodes, but it was surprising that this was also true for adding bacterivorous nematodes. We speculate that Myxobacteria reduce predation pressure of nematodes by utilizing secondary metabolites, while in turn killing enough prey bacteria for their own metabolism and growth. Consequently, the high abundance of Myxobacteria suggests a substantial contribution of their (predatory) activity on matter and energy fluxes in these microcosms. In a next step, we will integrate the metatranscriptomics results with organic matter and energy fluxes via flux-web modelling.In conclusion, the in vitro assays showed that Myxobacteria killed all prey bacteria. This, together with their high abundance and resistance to predation from higher trophic levels in the in situ microcosm suggest Myxobacteria as important players in the agricultural soil food web.

Permafrost degradation, freezing and thawing processes, and poor drainage due to underlain frozen ground have far-reaching consequences on soil hydrology and biology and, thus, on the redox dynamic in soils of the Arctic. Assessing the redox status of these soils is essential for understanding soil organic matter decomposition processes and can be done by temporal measurements in the field, analyses of redox-sensitive elements, or identification of microbial species or enzymes in redox process chains. While such approaches provide snippets of the complex redox dynamic, publications reporting long-term in-situ redox potential (EH) measurements in arctic permafrost soils are scarce. Limited accessibility to study sites and technical limitations in measuring the redox potential in a frozen environment may be two reasons for this research gap.But how does the redox potential develop in permafrost soils at different depths in the active layer during the summer? What happens during freezing and thawing? Finally, do thawing/degrading permafrost soils show different patterns compared to intact permafrost?We approached these research questions by installation of a unique soil monitoring setup at 3 sites near Fairbanks, Alaska, in August 2021. An intact permafrost soil (active layer depth about 50 cm) was equipped with 3 redox electrodes (for EH) and 3 hydra probes (for water content and soil temperature) in the topsoil and subsoil, respectively, and connected to a logger unit allowing continuous measurement of these parameters in both depths every 15 minutes. In addition, two sites with advanced permafrost degradation (permafrost level below 100 cm) were equipped in the same way. One degraded site featured large water contents, representing a wet thaw scenario, while the other site was well-drained, representing a dry thaw scenario, thus representing different endmembers of the ongoing climate-change induced permafrost thaw.Here, we present the first 2 years of soil monitoring in a discontinuous permafrost area in Interior Alaska from 09/2021 to 09/2023. Overall, pH values of all soils varied between 4.5-6.3. The dry thaw scenario showed oxic conditions (i.e., EH >600 mV) in top- and subsoil, while water contents were low. The wet thaw scenario exhibited high topsoil redox potentials (i.e., EH >500 mV), while subsoil redox potential was lower (i.e., EH 400 mV in the summer period (August to October), suggesting less oxygen consumption in this recently thawed permafrost subsoil.

Bacterial exopolysaccharides (EPS) act as natural biopolymers that bind soil particles together, promoting structural stability and creating protective microenvironments for microbial survival. This study aimed to isolate and characterize potential EPS-producing bacteria from the active layer of two degraded permafrost soils with different hydrological landscapes, and from non-degraded permafrost soil. A total of 64 bacterial isolates were obtained, representing three phyla: Firmicutes, Actinomycetota, and Pseudomonadota. EPS production was assessed by determining the polysaccharide content measured as glucose equivalent, and 26 bacterial isolates were identified as potential EPS producers. Among the bacterial isolates, Curtobacterium oceanosedimentum, Frigoribacterium faeni, Streptomyces strains, Neobacillus bataviensis, and Mesobacillus subterraneus exhibited high polysaccharide concentrations. Uronic acids were present in EPS produced by C. oceanosedimentum and N. bataviensis, while amino sugars were identified in EPS from isolates of Bacillus, Streptomyces, Luteimonas, and Phyllobacterium. Based on 16S rRNA gene sequence similarities, the relative proportion of taxa associated with EPS-producing bacterial isolates such as Bacillus, Peribacillus, and Streptomyces was higher in the dry site, while Neobacillus, Pseudarthrobacter, and Microbacterium were more abundant in the wet and intact sites. This study suggests that EPS production with diverse carbohydrate compositions primarily promotes structural stability in degrading permafrost soils by binding soil particles together and forming protective microenvironments. Additionally, EPS may contribute to nutrient retention and microbial protection under fluctuating environmental conditions, complementing their primary role in soil stability.

Volcanic ash is widely held responsible for fertilizing soils, but the processes and conditions leading to volcanic soil fertility remain poorly understood. We report here the results of a greenhouse volcanic ash fertilization (VAF) experiment aimed to explore the impact of basaltic ash addition (0-10 wt%) to soil on plant growth, nutrient uptake, and the soil microbiome. Our four-month experiment with the Coleus species Plectranthus scutellarioides on forest soil revealed a non-linear growth response with distinct growth patterns below and above 3 wt% ash addition, respectively. Low ash VAF (<3 wt% ash) had a negligible growth impact, while high ash addition yielded a threefold increase in biomass production, reproductive effort, and total nutrient uptake in aboveground plant biomass. An increased uptake of nutrients that are not part of the ash itself (e.g., nitrogen) implies that VAF was not a direct nutrient addition process but rather acts indirectly. The soil microbiome composition (16S rRNA; Bacteria/Archaea, and 18S rRNA; Eukaryota, rRNA gene markers) also changed at >3 wt% ash concentrations, with no apparent further change with increasing ash content. Key changes were not only an increased relative abundance of several potentially plant-growth-promoting rhizobacteria and fungi but also a decreased nematode abundance, suggesting that changes in the microbiome are likely a major factor for the substantial VAF effect observed in our experiment. We conclude that VAF with basaltic ash offers substantial potential benefits for agriculture as well as post-eruptive plant productivity on soils possibly associated with significant CO2 sequestration from the atmosphere. However, VAF is a complex process that does not act by simple nutrient addition from its chemical inventory but rather acts by restructuring the soil growth environment.

The upland soil microbiome is dominated by aerobic bacteria that oxidize atmospheric trace gases, including CO, H2, and CH4. As a result, soils are the largest biological sink for these climate-active gases. Whether global warming will enhance or suppress these processes remains unclear. Here, we studied the warming responses of soil trace gas oxidizers by profiling natural geothermal gradients in a subarctic grassland with over 60 years of field warming at +6°C. We integrate field flux measurements, ex situ biogeochemical assays, metagenomics, and metatranscriptomics to determine ecosystem and cellular-level responses. Our results show that the oxidation of atmospheric CO and H2, but not CH4, increased with long-term warming due to higher cell numbers. However, at the cellular level, trace gas oxidizers, especially methanotrophs, tended to reduce gas consumption and transcription of gas-metabolizing enzymes in response to long-term warming. Our findings suggest that soils may remain a robust sink for trace gases despite lower per-cell activity. This work establishes a framework for interpreting the relationships between temperature and microbial trace gas oxidation on timescales relevant to Earth’s climate system. ### Competing Interest Statement The authors have declared no competing interest. Czech Science Foundation, https://ror.org/01pv73b02, 21-17322M Deutsche Forschungsgemeinschaft, https://ror.org/018mejw64, 433256088 The Research Council of Norway, 315129, 344999 Australian Research Council, DE250101210, FT240100502 Research Council of Finland, 354501 FWF Austrian Science Fund, https://doi.org/10.55776/COE7

Denitrification is the key process leading to production and loss of nitrogen gases from soils. Its main drivers are N availability and soil water content, but interactions with other elements, such as carbon and phosphorus, can also influence N2O formation. So far, robust information on the effects of P and the historical context of P addition on N2O sources remains limited. To address this knowledge gap, we conducted a mesocosm chamber experiment using isotopic approaches to investigate N transformations and N2O sources following P fertilizer addition in soils with varying histories of P fertilization (low and high P). Differences in long-term fertilization affected C, N, and P availability as well as microbial community composition and nutrient cycling processes. Initially, microbes in both soils were C-limited with slightly higher C availability and microbial respiration in high P soils. Low P availability in low P soil did not restrict denitrification. In contrast, long-term P-unfertilized soil had higher N2O losses compared to high P soil, which were further stimulated with P addition. Glucose addition alleviated C limitation and strongly promoted microbial growth and respiration, but did not affect N2O emissions among treatments. Bacterial denitrification and nitrifier denitrification were the main N2O forming processes, while dissimilatory nitrate reduction to ammonium (DNRA) contributed to NO3− reduction, but only slightly to N2O formation.

The impact of drought on ammonia oxidizing microbes (AOM) in peatlands remains unclear, despite their role in facilitating a rate-limiting step in nitrification and the increasing prevalence of drought. This study aims to identify trends in ammonia oxidizing archaea (AOA) and bacteria (AOB) responses to summer drought in a rewetted percolation (PW) and coastal fen (CW) in northeastern Germany. AOA and AOB abundances were evaluated using 16S rRNA amplicon sequences and SSU RNA metatranscriptomes. We further quantified amoA gene copies with qPCR and transcript copies with reverse-transcription (RT-)qPCR from in situ peat soil sampled bi-monthly between April 2018 and February 2019. The magnitude of AOM drought response was unexpectedly more pronounced in PW than CW, despite PW exhibiting higher hydrological stability. RT-qPCR showed that PW had an increase in bacterial and archaeal amoA transcript abundances during drought. Additionally, there was evidence in the PW mRNA metatranscriptome for shifts in soil nitrogen cycling, first from a decrease in nitrogen fixation after drought onset, then due to a late-drought increase in assimilatory nitrate reduction to ammonium. In contrast, CW had no significant shifts in RT-qPCR amoA transcripts or the nitrogen cycling functional metatranscriptome during the drought. These results suggest that AOM react significantly to drought, responding to changes in soil hydrology and amplifying shifts in nitrogen cycling gene transcription. More frequent droughts will increase the role of AOM in rewetted fens; however, this response may vary between peatland types based on whether the ecosystem is frequently exposed to hydrological changes (such as in coastal fens) or is accustomed to hydrological stability (percolation fens).

In recent years, our understanding of archaeal diversity has greatly expanded, especially with the discovery of new groups like the Asgard archaea. These archaea show diverse phylogenetic and genomic traits, enabling them to thrive in various environments. Due to their close relationship to eukaryotes, a large number of metagenomic studies have been performed on Asgard archaea. Research on the fine scale distribution, diversity and quantification in saline aquatic sediments where they mostly occur, has, however, remained scarce. In this study, we investigated depths of shallow saline sediment cores from three distinct European environments: the Baltic Sea near Hiddensee, the coastal Lake Techirghiol in Romania, and an estuarine canal in Piran, Slovenia. Based on 16S rDNA amplicon sequencing, we observe variation in the relative abundance and occurrence of at least seven different Asgard groups that are distinct between the three environments and in their depth distribution. Lokiarchaeia and Thorarchaeia emerge as dominant Asgard groups across all sites, reaching maximal relative abundances of 2.28 and 2.68% of the total microbial communities respectively, with a maximal abundance of all Asgard reaching approx. 5.21% in Hiddensee. Quantitative PCR assays provide insights into the absolute abundance of Lokiarchaeia, supporting distinct patterns of distribution across depths in different sediments. Co-occurrence network analysis indicates distinct potential microbial partners across different Asgard groups. Overall, our study shows that Asgard archaea are found as a stable component in shallow sediment layers and have considerably diversified on macro- and microscales.

The methane-cycling microbiomes in Arctic permafrost-affected soils play crucial roles in the production and consumption of this important greenhouse gas. However, little is known about the distributions of Arctic methanogens and methanotrophs across the regional scale and along the vertical soil profile, as well as their responses to the widespread permafrost thaw. Using a unique sample set from nine different locations across the pan-Arctic, we identified methanogen and methanotroph phylotypes in 729 datasets of 16S rRNA gene amplicons. In 621 samples of intact permafrost soils across the pan-Arctic, only 22 methanogen and 26 methanotroph phylotypes were identified. Relative abundances of both functional groups varied significantly between sites and soil horizons. Only four methanogen phylotypes were detected at all locations, with the hydrogenotrophic Methanobacterium lacus dominating. Remarkably, the permafrost soil methane filter was almost exclusively comprised of a few phylotypes closely related to the obligate methanotrophic species Methylobacter tundripaludum. In degraded permafrost sites in Alaska, M. tundripaludum also dominated the methanotroph microbiome in the wet site. However, in dry, water-drained former permafrost site, Methylocapsa phylotypes, closely related with the atmospheric methane oxidizing bacteria, were exclusively found and dominant, indicating a massive restructuring of the methanotroph guild that consequently resulted in functional changes from a soil methane filter to an atmospheric methane sink. This study provides first insights into the identity and intricate spatial distribution of methanotrophs and methanogens in permafrost soils at a pan-Arctic scale and their responses to different water status after permafrost degradation. These findings point towards a few key microbes particularly relevant for future studies on Arctic CH4 dynamics in a warming climate and that under future dry conditions more atmospheric CH4 uptake in Arctic upland soils might happen. ### Competing Interest Statement The authors have declared no competing interest.

An aerobic methanotroph was isolated from a biofilm of coal mine Gruve 7 (Svalbard) and designated strain G7T. Cells of strain G7T were Gram-stain-negative, pink-pigmented and motile rods. Strain G7T could grow at pH 6.8 and at temperatures ranging from 4 to 21 °C. The genome size was 4.00 Mb with a (digital) DNA G+C content of 47.7 mol%. Strain G7T represents a member of the family Methylomonadaceae of the class Gammaproteobacteria. It displayed 94.6-99.7% 16S rRNA gene sequence similarity to the type strains of the genus Methylobacter. Whole-genome comparisons based on average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) confirmed that strain G7T represents a novel species. It showed 16S rRNA gene identity of 99.7%, 91.8% ANI and 46% dDDH to the closest type strain, Methylobacter svalbardensis LS7-T4AT, with ANI and dDDH being much lower than the typically used 95 and 70% cutoffs, respectively, to delineate different species. For methane activation, strain G7T carries genes encoding particulate methane monooxygenase (pmoCAB). Also, genes of the methane utilization pathways, i.e. oxidation of methane to carbon dioxide and assimilation of methane-carbon to biomass, were encoded in the genome. Strikingly, compared to all other Methylobacter spp. strains, strain G7T did not have nitrogenase genes for nitrogen fixation. Strain G7T also possessed genes for ectoine production, which was not observed in the genomes of its closest relatives. Based on phenotypic, genetic and phylogenetic data, strain G7T represents a novel species within the genus Methylobacter for which the name Methylobacter arcticus sp. nov. is proposed, with strain G7T (DSM: 117899; LMG: 33632) as the type strain.