Search NASA⌕ Search

SEARCH · Search NASA

Results for “methanogen”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Microbial community dynamics and cycling of plutonium and iron in a seasonally stratified and radiologically contaminated pond

Plutonium (Pu) cycling and mobility in the environment can be impacted by the iron cycle and microbial community dynamics. We investigated the spatial and temporal changes of the microbiome in an iron (Fe)-rich, plutonium-contaminated, monomictic reservoir (Pond B, Savannah River Site, South Carolina, USA). The microbial community composition varied with depth during seasonal thermal stratification and was strongly correlated with redox. During stratification, Fe(II) oxidizers (e.g., Ferrovum, Rhodoferax, Chlorobium) were most abundant in the hypoxic/anoxic zones, while Fe(III) reducers (e.g., Geothrix, Geobacter) dominated the deep, anoxic zone. Sulfate reducers and methanogens were present in the anoxic layer, likely contributing to iron and plutonium cycling. Multinomial regression of predicted functions/pathways identified metabolisms highly associated with stratification (within the top 5%), including iron reduction, methanogenesis, C1 compound utilization, fermentation, and aromatic compound degradation. Two sediment cores collected at the Inlet and Outlet of the pond were dominated by putative fermenters and organic matter (OM) degraders. Overall, microbiome analyses revealed the potential for three microbial impacts on the plutonium and iron biogeochemical cycles: (1) plutonium bioaccumulation throughout the water column, (2) Pu–Fe-OM-aggregate formation by Fe(II) oxidizers under microaerophilic/aerobic conditions, and (3) Pu–Fe-OM-aggregate or sediment reductive dissolution and organic matter degradation in the deep, anoxic waters.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2024

Reactive transport modeling of organic carbon degradation in marine methane hydrate systems

Abstract Natural methane hydrate has often been observed in sand layers that contain no particulate organic carbon (POC), but are surrounded by organic-rich, fine-grained marine muds. In this paper, we develop a reactive transport model (RTM) of a microbially-mediated set of POC degradation reactions, including hydrolysis of POC driven by extracellular enzymes, fermentation of the resulting high-molecular weight dissolved organic carbon (HMW-DOC), and methanogenesis that consumes low-molecular weight dissolved organic carbon (LMW-DOC). These processes are mediated by two groups of microbes, fermenters and methanogens that are heterogeneously distributed in different lithologies, with the largest numbers of microbes in the large pores of coarse-grained layers. We find that the RTM can reproduce methane hydrate occurrences observed in two different geological environments, at Walker Ridge Site 313-H (Gulf of Mexico) and IODP Site U1325 (Cascadia Margin). We also find that microbes can degrade POC even if they are physically separated, as extracellular enzymes and DOC can diffuse away from where they are produced by microbes. Microbial activity is highest at relatively early times after burial at shallow depths and near lithological boundaries, where concentration gradients transport solutes to intervals that contain the most microbes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Dynamic soil columns simulate Arctic redox biogeochemistry and carbon release during changes in water saturation

Thawing Arctic permafrost can induce hydrologic change and alter redox conditions, shifting the balance of soil organic matter (SOM) decomposition. There remains uncertainty about how soil saturation and redox transitions impact dissolved and gas phase carbon fluxes, and efforts to link hydrobiogeochemical processes to ecosystem-scale models are limited. This study evaluates SOM decomposition of Arctic tundra soils using column experiments, water chemistry measurements, microbial community analysis, and a PFLOTRAN reactive transport model. Soil columns from a thermokarst channel (TC) and an upland tundra (UC) were exposed to cycles of saturation and drainage, which controlled carbon emissions. During saturation, an outflow of dissolved organic carbon from the UC soil correlated with elevated reduced iron and decreased pH; during drainage, UC carbon dioxide fluxes were 70% higher than TC fluxes. Intermittent methane release was observed for TC, consistent with higher methanogen abundance. Slower drainage in the TC soil correlated with more subtle biogeochemical changes. PFLOTRAN simulations captured experimental trends in soil carbon fluxes, oxygen concentrations, and water contents. The model was then used to evaluate additional soil water drainage rates. This study emphasizes the importance of considering hydrologic change when evaluating and simulating SOM decomposition in dynamic Arctic tundra environments.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2023

Insights into pyrrolysine function from structures of a trimethylamine methyltransferase and its corrinoid protein complex

The 22nd genetically encoded amino acid, pyrrolysine, plays a unique role in the key step in the growth of methanogens on mono-, di-, and tri-methylamines by activating the methyl group of these substrates for transfer to a corrinoid cofactor. Previous crystal structures of the Methanosarcina barkeri monomethylamine methyltransferase elucidated the structure of pyrrolysine and provide insight into its role in monomethylamine activation. Herein, we report the second structure of a pyrrolysine-containing protein, the M. barkeri trimethylamine methyltransferase MttB, and its structure bound to sulfite, a substrate analog of trimethylamine. We also report the structure of MttB in complex with its cognate corrinoid protein MttC, which specifically receives the methyl group from the pyrrolysine-activated trimethylamine substrate during methanogenesis. Together these structures provide key insights into the role of pyrrolysine in methyl group transfer from trimethylamine to the corrinoid cofactor in MttC.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2020

Anaerobic respiration pathways and response to increased substrate availability of Arctic wetland soils

The availability of labile carbon (C) compounds in Arctic wetland soils is expected to increase due to thawing permafrost and increased fermentation as a result of decomposition of organic matter with warming. How microbial communities respond to this change will affect the balance of CO 2 and CH 4 emitted during anaerobic organic matter decomposition, and ultimately the net radiative forcing of greenhouse gas emissions from these soils. While soil water content limits aerobic respiration, the factors controlling methanogenesis and anaerobic respiration are poorly defined in suboxic Arctic soils. In this study, we conducted incubation experiments on two tundra soils from field sites on the Seward Peninsula, Alaska, with contrasting pH and geochemistry to determine the pathways of anaerobic microbial respiration and changes with increasing substrate availability upon warming. In incubation of soils from the circumneutral Teller site, the ratio of CO 2 to CH 4 dropped from 10 to <2 after 60 days, indicating rapid depletion of alternative terminal electron acceptors (TEAs). Addition of acetate stimulated production of CO 2 and CH 4 in a nearly 1:1 ratio, consistent with methanogenesis, and the composition of the microbial community shifted to favor clades capable of utilizing the added acetate such as the Fe(III)-reducing Geobacter and the methanogenic archaea Methanosarcina. In contrast, both CO 2 and CH 4 production declined with acetate addition during incubation of soils from the more acidic Council site, and fermentative microorganisms increased in abundance despite the high availability of fermentation products. These results demonstrate that the degree to which increasing substrate availability stimulates greenhouse gas production in tundra wetlands will vary widely depending on soil pH and geochemistry.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

Incorporating concentration-dependent sediment microbial activity into methylmercury production kinetics modeling

We report in anoxic environments, anaerobic microorganisms carrying the hgcAB gene cluster can mediate the transformation of inorganic mercury (Hg(II)) to monomethylmercury (MMHg). The kinetics of Hg(II) transformation to MMHg in periphyton from East Fork Poplar Creek (EFPC) in Oak Ridge, TN have previously been modeled using a transient availability model (TAM). The TAM for Hg(II) methylation combines methylation/demethylation kinetics with kinetic expressions for processes that decrease Hg(II) and MMHg availability for methylation and demethylation (multisite sorption of Hg(II) and MMHg, Hg(II) reduction/Hg(0) oxidation). In this study, the TAM is used for the first time to describe MMHg production in sediment. We assessed MMHg production in sediment microcosms using two different sediment types from EFPC: a relatively anoxic, carbon-rich sediment with higher microbial activity (higher CO 2 production from sediment) and a relatively oxic, sandy, carbon-poor sediment with lower microbial activity (lower CO 2 production from sediment). Based on 16s rRNA sequencing, the overall microbial community structure in the two sediments was retained during the incubations. However, the hgcA containing methanogenic Euryarchaeota communities differed between sediment types and their growth followed different trajectories over the course of incubations, potentially contributing to the distinct patterns of MMHg production observed. The general TAM paradigm performed well in describing MMHg production in the sediments. However, the MMHg production and ancillary data suggested the need to revise the model structure to incorporate terms for concentration-dependent microbial activity over the course of the incubations. We modified the TAM to include Monod-type kinetics for methylation and demethylation and observed an improved fit for the carbon-rich, microbially active sediment. Overall our work shows that the TAM can be applied to describe Hg(II) methylation in sediments and that including expressions accounting for concentration-dependent microbial activity can improve the accuracy of the model description of the data in some cases.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

Deep-branching acetogens in serpentinized subsurface fluids of Oman

Little is known of acetogens in contemporary serpentinizing systems, despite widely supported theories that serpentinite-hosted environments supported the first life on Earth via acetogenesis. To address this knowledge gap, genome-resolved metagenomics was applied to subsurface fracture water communities from an area of active serpentinization in the Samail Ophiolite, Sultanate of Oman. Two deeply branching putative bacterial acetogen types were identified in the communities belonging to the Acetothermia (hereafter, types I and II) that exhibited distinct distributions among waters with lower and higher water–rock reaction (i.e., serpentinization influence), respectively. Metabolic reconstructions revealed contrasting core metabolic pathways of type I and II Acetothermia, including in acetogenic pathway components (e.g., bacterial- vs. archaeal-like carbon monoxide dehydrogenases [CODH], respectively), hydrogen use to drive acetogenesis, and chemiosmotic potential generation via respiratory (type I) or canonical acetogen ferredoxin-based complexes (type II). Notably, type II Acetothermia metabolic pathways allow for use of serpentinization-derived substrates and implicate them as key primary producers in contemporary hyperalkaline serpentinite environments. Phylogenomic analyses indicate that 1) archaeal-like CODH of the type II genomes and those of other serpentinite-associated Bacteria derive from a deeply rooted horizontal transfer or origin among archaeal methanogens and 2) Acetothermia are among the earliest evolving bacterial lineages. The discovery of dominant and early-branching acetogens in subsurface waters of the largest near-surface serpentinite formation provides insight into the physiological traits that likely facilitated rock-supported life to flourish on a primitive Earth and possibly on other rocky planets undergoing serpentinization.

Science & Technology - Other Topics↗
DOE OSTI2022

Biogeochemical Characteristics of Earth's Volcanic Permafrost: An Analog of Extraterrestrial Environments

This article describes a study of frozen volcanic deposits collected from volcanoes Tolbachik and Bezymianny on the Kamchatka Peninsula, Russia, and Deception Island volcano, Antarctica. In addition, we studied suprasnow ash layers deposited after the 2007 eruptions of volcanoes Shiveluch and Bezymianny on Kamchatka. The main objectives were to characterize the presence and survivability of thermophilic microorganisms in perennially frozen volcanic deposits. As opposed to permafrost from the polar regions, viable thermophiles were detected in volcanic permafrost by cultivation, microscopy, and sequencing. In the permafrost of Tolbachik volcano, we observed methane formation by both psychrophilic and thermophilic methanogenic archaea, while at 37°C, methane production was noticeably lower. Thermophilic bacteria isolated from volcanic permafrost from the Deception Island were 99.93% related to Geobacillus stearothermophilus. Furthermore, our data showed biological sulfur reduction to sulfide at 85°C and even at 130°C, where hyperthermophilic archaea of the genus Thermoproteus were registered. Sequences of hyperthermophilic bacteria of the genus Caldicellulosiruptor were discovered in clone libraries from fresh volcanic ash deposited on snow. Microorganisms found in volcanic terrestrial permafrost may serve as a model for the alien inhabitants of Mars, a cryogenic planet with numerous volcanoes. Thermophiles and hyperthermophiles and their metabolic processes represent a guideline for the future exploration missions on Mars.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2025

Genomic and transcriptomic characterization of carbohydrate-active enzymes in the anaerobic fungus Neocallimastix cameroonii var. constans

Anaerobic gut fungi effectively degrade lignocellulose in the guts of large herbivores, but there remain a limited number of isolated, publicly available, and sequenced strains that impede our understanding of the role of anaerobic fungi within microbial communities. We isolated and characterized a new fungal isolate, Neocallimastix cameroonii var. constans, providing a transcriptomic and genomic understanding of its ability to degrade diverse carbohydrates. This anaerobic fungal strain was stably cultivated for multiple years in vitro among members of an initial enrichment microbial community derived from goat feces, and it demonstrated the ability to pair with other microbial members, namely, archaeal methanogens to produce methane from lignocellulose. Genomic analysis revealed a higher number of predicted carbohydrate-active enzymes encoded in the N. cameroonii var. constans genome compared to most other sequenced anaerobic fungi. The carbohydrate-active enzyme profile for this isolate contained 660 glycoside hydrolases, 160 carbohydrate esterases, 194 glycosyltransferases, and 85 polysaccharide lyases. Differential gene expression analysis showed the upregulation of thousands of genes (including predicted carbohydrate-active enzymes) when N. cameroonii var. constans was grown on lignocellulose (reed canary grass) compared to less complex substrates, such as cellulose (filter paper), cellobiose, and glucose. AlphaFold was used to predict functions of transcriptionally active yet poorly annotated genes, revealing feruloyl esterases that likely play an important role in lignocellulose degradation by anaerobic fungi. The combination of this strain's genomic and transcriptomic characterization, omics-informed structural prediction, and robustness in microbial co-culture make it a well-suited platform to conduct future investigations into bioprocessing and enzyme discovery.

CAZymes↗
DOE OSTI2022

The tRNA discriminator base defines the mutual orthogonality of two distinct pyrrolysyl-tRNA synthetase/tRNAPyl pairs in the same organism

Abstract Site-specific incorporation of distinct non-canonical amino acids into proteins via genetic code expansion requires mutually orthogonal aminoacyl-tRNA synthetase/tRNA pairs. Pyrrolysyl-tRNA synthetase (PylRS)/tRNAPyl pairs are ideal for genetic code expansion and have been extensively engineered for developing mutually orthogonal pairs. Here, we identify two novel wild-type PylRS/tRNAPyl pairs simultaneously present in the deep-rooted extremely halophilic euryarchaeal methanogen Candidatus Methanohalarchaeum thermophilum HMET1, and show that both pairs are functional in the model halophilic archaeon Haloferax volcanii. These pairs consist of two different PylRS enzymes and two distinct tRNAs with dissimilar discriminator bases. Surprisingly, these two PylRS/tRNAPyl pairs display mutual orthogonality enabled by two unique features, the A73 discriminator base of tRNAPyl2 and a shorter motif 2 loop in PylRS2. In vivo translation experiments show that tRNAPyl2 charging by PylRS2 is defined by the enzyme's shortened motif 2 loop. Finally, we demonstrate that the two HMET1 PylRS/tRNAPyl pairs can simultaneously decode UAG and UAA codons for incorporation of two distinct noncanonical amino acids into protein. This example of a single base change in a tRNA leading to additional coding capacity suggests that the growth of the genetic code is not yet limited by the number of identity elements fitting into the tRNA structure.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

Four Families of Folate-independent Methionine Synthases

Abstract Although most organisms synthesize methionine from homocysteine and methyl folates, some have “core” methionine synthases that lack folate-binding domains and use other methyl donors. In vitro , the characterized core synthases use methylcobalamin as a methyl donor, but in vivo , they probably rely on corrinoid (vitamin B12-binding) proteins. We identified four families of core methionine synthases that are distantly related to each other (under 30% pairwise amino acid identity). From the characterized enzymes, we identified the families MesA, which is found in methanogens, and MesB, which is found in anaerobic bacteria and archaea with the Wood-Ljungdahl pathway. A third uncharacterized family, MesC, is found in anaerobic archaea that have the Wood-Ljungdahl pathway and lack known forms of methionine synthase. We predict that most members of the MesB and MesC families accept methyl groups from the iron-sulfur corrinoid protein of that pathway. The fourth family, MesD, is found only in aerobic bacteria. Using transposon mutants and complementation, we show that MesD does not require 5-methyltetrahydrofolate or cobalamin. Instead, MesD requires an uncharacterized protein family (DUF1852) and oxygen for activity.

Price, Morgan↗
DOE OSTI2020

Anaerobic guilds responsible for mercury methylation in boreal wetlands of varied trophic status serving as either a methylmercury source or sink

Wetlands are common sites of active Hg methylation by anaerobic microbes; however, the amount of methylmercury produced varies greatly, as Hg methylation is dependent upon both the availability of Hg and the composition and activity of the microbial community involved. In this study, we identified the major microbial guilds responsible for Hg methylation along a trophic gradient composed of two sites and three different types of wetlands: a bog–fen peatland gradient and a black alder swamp, serving as net sources and a sink for methylmercury respectively. Iron-reducing bacteria in the Geobacteraceae were important Hg methylators across all wetlands and seasons examined, as evidenced by abundant 16S rRNA and hgcA transcripts clustering with this family. Molybdate inhibited Hg methylation more efficiently in the peatlands than in the swamp, suggesting an increasing role of sulfate-reducing bacteria and/or related syntrophs in the methylation of Hg with decreasing trophic status. Further, sulfate addition failed to increase Hg methylation rates in the peatlands, suggesting that SRBs/syntrophs were instead likely metabolizing alternative substrates such as syntrophic fermentation of organic compounds with methanogens. These results highlight the interconnectivity of anaerobic metabolism and importance of community dynamics on the methylation of Hg in wetlands with different trophic status.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2022

Predominance of Methanomicrobiales and diverse hydrocarbon–degrading taxa in the Appalachian coalbed biosphere revealed through metagenomics and genome–resolved metabolisms

Coalbed deposits are a unique subsurface environment and represent an underutilized resource for methane generation. Microbial communities extant in coalbed deposits are responsible for key subsurface biogeochemical cycling and could be utilized to enhance methane production in areas where existing gas wells have depleted methane stores, or in coalbeds that are unmined, or conversely be utilized for mitigation of methane release. Here we utilize metagenomics and metagenome-assembled genomes to identify extant microbial lineages and genome-resolved microbial metabolisms of coalbed produced water, which has not yet been explored in the Appalachian Basin. Our analyses resulted in the recovery of over 40 metagenome-assembled genomes (MAGs) from eight coalbed methane wells. The most commonly identified taxa among samples were hydrogenotrophic methanogens from the order Methanomicrobiales and these dominant MAGs were highly similar to one another. Conversely, low-abundance coalbed bacterial populations were taxonomically and functionally diverse, mostly belonging to a variety of Proteobacteria classes, and encoding various hydrocarbon solubilization and degradation pathways. Further, the data presented herein provides novel insights into Appalachian Basin coalbed microbial ecology, and our findings provide new perspectives on underrepresented Methanocalculus species and low-relative abundance bacterial assemblages in coalbed environments, and their potential roles in stimulation or mitigation of methane release.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2025

Expanded Diversity of Microbial Groups Capable of Anaerobic Pyrite Reduction and Assimilation of Dissolution Products

Pyrite, the most abundant iron sulfide mineral in the Earth's crust, has traditionally been considered as a sink for iron and sulfur in the absence of oxygen. Recent research, however, has shown that anaerobic methanogenic archaea can reductively dissolve pyrite and assimilate its products as sources of iron and sulfur. This study explores whether other anaerobic bacteria, including fermentative, nitrate-, iron oxide-, fumarate-, and sulfate-respiring bacteria, can also reduce pyrite and use its dissolution products as sources of iron and sulfur. Results indicate that heterotrophic bacteria respiring fumarate or sulfate, or fermenting organic carbon, can reduce pyrite and assimilate released iron and sulfur. In contrast, nitrate- or iron oxide-respiring cells did not reduce pyrite, suggesting that microbial pyrite reduction is metabolism-specific. All strains capable of reducing pyrite could also use mackinawite as an iron and sulfur source. With the exception of fermentative Bacteroides, strains did not require direct contact with pyrite to reduce the mineral, indicating extracellular electron transfer via electron shuttles. These findings expand the known diversity of microbial groups capable of pyrite reduction and highlight the mineral's lability in various anaerobic environments, with potential implications for the biogeochemical cycles of iron, sulfur, carbon, and oxygen.

58 GEOSCIENCES↗
DOE OSTI2021

The role of oxygen in stimulating methane production in wetlands

Methane (CH 4 ), a potent greenhouse gas, is the second most important greenhouse gas contributor to climate change after carbon dioxide (CO 2 ). The biological emissions of CH 4 from wetlands are a major uncertainty in CH 4 budgets. Microbial methanogenesis by Archaea is an anaerobic process accounting for most biological CH 4 production in nature, yet recent observations indicate that large emissions can originate from oxygenated or frequently oxygenated wetland soil layers. To determine how oxygen (O 2 ) can stimulate CH 4 emissions, we used incubations of Sphagnum peat to demonstrate that the temporary exposure of peat to O 2 can increase CH 4 yields up to 2000-fold during subsequent anoxic conditions relative to peat without O 2 exposure. Geochemical (including ion cyclotron resonance mass spectrometry, X-ray absorbance spectroscopy) and microbiome (16S rDNA amplicons, metagenomics) analyses of peat showed that higher CH 4 yields of redox-oscillated peat were due to functional shifts in the peat microbiome arising during redox oscillation that enhanced peat carbon (C) degradation. Novosphingobium species with O 2 -dependent aromatic oxygenase genes increased greatly in relative abundance during the oxygenation period in redox-oscillated peat compared to anoxic controls. Acidobacteria species were particularly important for anaerobic processing of peat C, including in the production of methanogenic substrates H 2 and CO 2 . Higher CO 2 production during the anoxic phase of redox-oscillated peat stimulated hydrogenotrophic CH 4 production by Methanobacterium species. The persistence of reduced iron (Fe(II)) during prolonged oxygenation in redox-oscillated peat may further enhance C degradation through abiotic mechanisms (e.g., Fenton reactions). The results indicate that specific functional shifts in the peat microbiome underlie O 2 enhancement of CH 4 production in acidic, Sphagnum-rich wetland soils. They also imply that understanding microbial dynamics spanning temporal and spatial redox transitions in peatlands is critical for constraining CH 4 budgets; predicting feedbacks between climate change, hydrologic variability, and wetland CH 4 emissions; and guiding wetland C management strategies.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2024

Composition and in situ structure of the Methanospirillum hungatei cell envelope and surface layer

Archaea share genomic similarities with Eukarya and cellular architectural similarities with Bacteria, though archaeal and bacterial surface layers (S-layers) differ. Using cellular cryo–electron tomography, we visualized the S-layer lattice surroundingMethanospirillum hungatei, a methanogenic archaeon. Though more compact than known structures,M. hungatei’s S-layer is a flexible hexagonal lattice of dome-shaped tiles, uniformly spaced from both the overlying cell sheath and the underlying cell membrane. Subtomogram averaging resolved the S-layer hexamer tile at 6.4-angstrom resolution. By fitting an AlphaFold model into hexamer tiles in flat and curved conformations, we uncover intra- and intertile interactions that contribute to the S-layer’s cylindrical and flexible architecture, along with a spacer extension for cell membrane attachment.M. hungateicell’s end plug structure, likely composed of S-layer isoforms, further highlights the uniqueness of this archaeal cell. These structural features offer advantages for methane release and reflect divergent evolutionary adaptations to environmental pressures during early microbial emergence.

Science & Technology - Other Topics↗
DOE OSTI2021

Impact of river channel lateral migration on microbial communities across a discontinuous permafrost floodplain

Permafrost soils store approximately twice the amount of carbon currently present in Earth’s atmosphere and are acutely impacted by climate change due to the polar amplification of increasing global temperature. Many organic-rich permafrost sediments are located on large river floodplains, where river channel migration periodically erodes and re-deposits the upper tens of meters of sediment. Channel migration exerts a first-order control on the geographic distribution of permafrost and floodplain stratigraphy and thus may affect microbial habitats. Here, to examine how river channel migration in discontinuous permafrost environments affects microbial community composition, we used amplicon sequencing of the 16S rRNA gene on sediment samples from floodplain cores and exposed riverbanks along the Koyukuk River, a large tributary of the Yukon River in west-central Alaska. Microbial communities are sensitive to permafrost thaw: communities found in deep samples thawed by the river closely resembled near-surface active layer communities in non-metric multidimensional scaling analyses but did not resemble floodplain permafrost communities at the same depth. Microbial communities also displayed lower diversity and evenness in permafrost than in both the active layer and permafrost-free point bars recently deposited by river channel migration. Taxonomic assignments based on 16S and quantitative PCR for the methyl-coenzyme M reductase functional gene demonstrated that methanogens and methanotrophs are abundant in older permafrost-bearing deposits, but not in younger, non-permafrost point bar deposits. The results suggested that river migration, which regulates the distribution of permafrost, also modulates the distribution of microbes potentially capable of producing and consuming methane on the Koyukuk River floodplain.

59 BASIC BIOLOGICAL SCIENCES↗