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A combination of physicochemical and radiotracer analysis, high-throughput sequencing of the 16S rRNA, and particulate methane monooxygenase subunit A (pmoA) genes was used to link a microbial community profile with methane, sulfur, and nitrogen cycling processes. The objects of study were surface sediments sampled at five stations in the northern part of the Barents Sea. The methane content in the upper layers (0–5 cm) ranged from 0.2 to 2.4 µM and increased with depth (16–19 cm) to 9.5 µM. The rate of methane oxidation in the oxic upper layers varied from 2 to 23 nmol CH4 L−1 day−1 and decreased to 0.3 nmol L−1 day−1 in the anoxic zone at a depth of 16–19 cm. Sulfate reduction rates were much higher, from 0.3 to 2.8 µmol L−1 day−1. In the surface sediments, ammonia-oxidizing Nitrosopumilaceae were abundant; the subsequent oxidation of nitrite to nitrate can be carried out by Nitrospira sp. Aerobic methane oxidation could be performed by uncultured deep-sea cluster 3 of gamma-proteobacterial methanotrophs. Undetectable low levels of methanogenesis were consistent with a near complete absence of methanogens. Anaerobic methane oxidation in the deeper sediments was likely performed by ANME-2a-2b and ANME-2c archaea in consortium with sulfate-reducing Desulfobacterota. Sulfide can be oxidized by nitrate-reducing Sulfurovum sp. Thus, the sulfur cycle was linked with the anaerobic oxidation of methane and the nitrogen cycle, which included the oxidation of ammonium to nitrate in the oxic zone and denitrification coupled to the oxidation of sulfide in the deeper sediments. Methane concentrations and rates of microbial biogeochemical processes in sediments in the northern part of the Barents Sea were noticeably higher than in oligotrophic areas of the Arctic Ocean, indicating that an increase in methane concentration significantly activates microbial processes.

Seafloor authigenic carbonate crusts are widespread in various oceanic and marine settings, excluding high-latitude basins that are corrosive to carbonate precipitation. Newly formed carbonate formations are relatively rare in modern Arctic marine sediments. Although the first-order principles of seep carbonate formation are currently quite well constrained, little is known regarding the duration or mode of carbonate formation in the Siberian Arctic shelf. Large (massive slabs or blocks) and small crusts that were micrite cemented have been recently discovered on the seafloor of the Siberian Arctic seas within the area of known seep activity in the outer Laptev Sea shelf. Cold methane seeps were detected in the area due to the presence of an acoustic anomaly in the water column (gas flares). Microbial mats, methane gas bubbles, and carbonate crusts were observed using a towed camera platform. Here, we report new geochemical and mineralogical data on authigenic shallow Siberian Arctic cold-seep carbonate crusts to elucidate its genesis. The Laptev Sea carbonate crusts mainly consist of high-Mg calcite (up to 23 mol % MgCO 3 ). The δ 13 C values in carbonates range significantly (from –40.1 to –25.9‰ VPDB), while the δ 18 O values vary in a narrow range (+4.4 ± 0.2‰ VPDB). The δ 13 C values of C org that was determined from carbonates range from –40.2 to –31.1‰ VPDB. Using the isotope data and taking into account the geological setting, we consider that not only microbial but possibly thermogenic methane participated in the authigenic carbonate precipitation. Carbonate crust formation occurred below the water/sediment interface of the shallow Siberian Arctic shelf as a result of gas hydrate dissociation during Holocene warming events. The studied carbonate crusts were exhumated after precipitation into shallow subsurface shelf sediments.

The strain WBS, an anaerobic, psychro- and halotolerant bacterium belonging to the genus Fusibacter, was isolated from the littoral bottom sediments of the White Sea, Arctic, Russia. Fusibacter bizertensis WBS grew at temperatures between 8 and 32 °C (optimum growth at 18–20 °C), pH between 5.2 and 8.3 (optimum growth at pH 7.2), and at NaCl concentrations between 0 and 70 g L−1 (optimum growth at 32 g L−1). It reduced sulfate, thiosulfate, and elemental sulfur into sulfide, and, probably, the strain is able to disproportionate thiosulfate. The strain also utilized a wide range of substrates as it is a chemoorganotrophic bacterium. Analysis of the sequenced genome revealed genes for all enzymes involved in the Embden–Meyerhof glycolytic pathway as well as genes for the non-oxidative stage of the pentose phosphate pathway. The presence of genes encoding aldehyde dehydrogenases and alcohol dehydrogenases also suggests that, in addition to acetate, alcohols can also be the fermentation products. The strain possessed superoxide dismutase and peroxidase activities and the ability to consume O2, which is in full accordance with the presence of corresponding genes of antioxidant defense in the genome. The phylogenetic analysis suggested that the strain WBS is the closest relative of Fusibacter bizertensis LTF Kr01T (16S rRNA gene sequence similarity 98.78%). Based on biochemical and genomic characteristics, the strain WBS is proposed to represent a novel aero-, halo- and psychrotolerant strain from the genus Fusibacter, isolated for the first time among its members from cold oxygenated marine bottom sediments.

Microbiological and biogeochemical investigation of the bottom sediments of semiclosed basins was carried out at the Kislaya Guba tidal power station (Barents Sea) and in Kanda Bay (White Sea). Suppressed tidal water mixing is known to affect the hydrological regime of isolated basins, resulting in the development of oxygen-free sediments. The upper sediments of the studied bays were shown to contain higher concentrations of sulfide and methane, with increased rates of sulfate reduction, methanogenesis, and methane oxidation. The relative abundance of truly marine microorganisms decreased, while microorganisms common in anoxic sediments of meromictic basins developed. The indicator microorganisms with increased relative abundance were archaea of the genera Methanoregula and Methanosaeta. Bacteria of the class Chlorobia, Chloroflexi of the family Anaerolineaceae, and Rhodoferax-related bacteria were indicators of the stagnant seawater. Members of the genus Woeseia were counter-indicators, occurring only in marine water. In our opinion, under reasonably regulated water exchange via the dams, the ecosystems of the Kanda and Kislaya Guba bays may retain the characteristics of marine bays. Otherwise, the studied bays may become stratified basins with anoxic near-bottom water, harboring microbial communities similar to those inhabiting meromictic basins.

Bottom sediments at methane discharge sites of the Laptev Sea shelf were investigated. The rates of microbial methanogenesis and methane oxidation were measured, and the communities responsible for these processes were analyzed. Methane content in the sediments varied from 0.9 to 37 µmol CH4 dm−3. Methane carbon isotopic composition (δ13C-CH4) varied from −98.9 to −77.6‰, indicating its biogenic origin. The rates of hydrogenotrophic methanogenesis were low (0.4–5.0 nmol dm−3 day−1). Methane oxidation rates varied from 0.4 to 1.2 µmol dm−3 day−1 at the seep stations. Four lineages of anaerobic methanotrophic archaea (ANME) (1, 2a–2b, 2c, and 3) were found in the deeper sediments at the seep stations along with sulfate-reducing Desulfobacteriota. The ANME-2a-2b clade was predominant among ANME. Aerobic ammonium-oxidizing Crenarchaeota (family Nitrosopumilaceae) predominated in the upper sediments along with heterotrophic Actinobacteriota and Bacteroidota, and mehtanotrophs of the classes Alphaproteobacteria (Methyloceanibacter) and Gammaproteobacteria (families Methylophilaceae and Methylomonadaceae). Members of the genera Sulfurovum and Sulfurimonas occurred in the sediments of the seep stations. Mehtanotrophs of the classes Alphaproteobacteria (Methyloceanibacter) and Gammaproteobacteria (families Methylophilaceae and Methylomonadaceae) occurred in the sediments of all stations. The microbial community composition was similar to that of methane seep sediments from geographically remote areas of the global ocean.

It is essential to study methane in the Arctic environment in order to understand the potential for large-scale greenhouse gas emissions that may result from melting of relict seafloor permafrost due to ocean warming. Very few data on the sources of methane in the Chukchi Sea were available prior to initiation of the Russian-American Long-term Census of the Arctic (RUSALCA) program in 2004. This article documents for the first time the spatial variation of methane concentrations in the sediment and water column in a significant region of the Pacific Arctic and the influence of methane turnover and net transport from organic-rich environments within the western Chukchi Sea. The study combines historical observations, new data obtained during the RUSALCA collaborative program, and modeling results to provide insights into the contemporary methane dynamics of the western Chukchi Sea. We compare methane evolution at two sites with distinct geological settings, depositional patterns, and methane sources: (1) the deeper, fault-bounded Herald Canyon (northern site) where methane flux is controlled by both northward CH4transport via ocean currents and diffusive influx of thermogenic methane (formed under high-temperature conditions) from source rocks at depth in the canyon's seafloor, and (2) the shallow Chukchi shelf (southern site), where sulfate reduction and anaerobic methane oxidation play a significant role in biogenic methane production and its flux within and from the sediments into the water column. Diffusive methane fluxes at the sediment-water interface within the southern and northern sites were estimated to be 14.5 μmol dm⁻² day⁻¹ and 0.7 nmol dm⁻² day⁻¹, respectively. In addition, we suggest that biogenic methane emanating from the organic-rich southern region is transported northward by the Anadyr Current, leading to a mix of both biogenic and thermogenic methane in Herald Canyon surface waters. Study results indicate that the South Chukchi Basin is an important source of atmospheric CH4. Further work is required to accurately quantify this flux.

Although arctic and subarctic lakes are important sources of methane, the emission of which will increase due to the melting of permafrost, the processes related to the methane cycle in such environments are far from being comprehensively understood. Here we studied the microbial communities in the near-bottom water layer and sediments of the meromictic subarctic Lake Svetloe using high-throughput sequencing of the 16S rRNA and methyl coenzyme M reductase subunit A genes. Hydrogenotrophic methanogens of the order Methanomicrobiales were abundant, both in the water column and in sediments, while the share of acetoclastic Methanosaetaceae decreased with the depth of sediments. Members of the Methanomassiliicoccales order were absent in the water but abundant in the deep sediments. Archaea known to perform anaerobic oxidation of methane were not found. The bacterial component of the microbial community in the bottom water layer included oxygenic (Cyanobacteria) and anoxygenic (Chlorobi) phototrophs, aerobic Type I methanotrophs, methylotrophs, syntrophs, and various organotrophs. In deeper sediments the diversity of the microbial community decreased, and it became dominated by methanogenic archaea and the members of the Bathyarchaeota, Chloroflexi and Deltaproteobacteria. This study shows that the sediments of a subarctic meromictic lake contain a taxonomically and metabolically diverse community potentially capable of complete mineralization of organic matter.

Bacterial production and abundance are linked to areas of high biological production in the water column and in the underlying benthos in the Chukchi Sea. Process measurements taken during the Russian American Long-term Census of the Arctic (RUSALCA) program, such as the carbon isotope composition of sinking particulate organic matter and sediment organic matter, are used to put bacterial production and abundance in context. These measurements show that there are vertical gradients in the water column and that the stable carbon isotope composition of organic materials in the sediments is significantly different from sedimenting materials in the overlying water column. Differences within the water column likely reflect late summer declines in productivity that increase discrimination against13C and also provide indications of carbon metabolism in the water column and underlying sediments. Temporal changes in the stable carbon isotope composition of organic matter in surface sediments, as well as C/N ratios in organic matter during the RUSALCA program, are also being observed, specifically higher ratios of13C/12C at some stations near the Chukotka coast, and lower ratios of13C/12C near Point Hope, Alaska. C/N ratios have increased since 2004 at productive sites in the south central Chukchi Sea, suggesting changes in organic material deposition. Other parameters studied on some or all of the decadal series of joint Russia-US cruises include sediment oxygen demand, the nitrogen isotopic composition of organic matter, sediment grain size, chlorophyll content in surface sediments, and elemental ratios of carbon and nitrogen in surface sediments. These process measurements support interpretations that the ecosystem shows strong coupling between bacterial and primary production and the underlying benthos.

The strain WBS, an anaerobic, psychro- and halotolerant bacterium belonging to the genus Fusibacter, was isolated from the littoral bottom sediments of the White Sea, Arctic, Russia. Fusibacter bizertensis WBS grew at temperatures between 8 and 32 °C (optimum growth at 18–20 °C), pH between 5.2 and 8.3 (optimum growth at pH 7.2), and at NaCl concentrations between 0 and 70 g L[sup.−1] (optimum growth at 32 g L[sup.−1] ). It reduced sulfate, thiosulfate, and elemental sulfur into sulfide, and, probably, the strain is able to disproportionate thiosulfate. The strain also utilized a wide range of substrates as it is a chemoorganotrophic bacterium. Analysis of the sequenced genome revealed genes for all enzymes involved in the Embden–Meyerhof glycolytic pathway as well as genes for the non-oxidative stage of the pentose phosphate pathway. The presence of genes encoding aldehyde dehydrogenases and alcohol dehydrogenases also suggests that, in addition to acetate, alcohols can also be the fermentation products. The strain possessed superoxide dismutase and peroxidase activities and the ability to consume O[sub.2] , which is in full accordance with the presence of corresponding genes of antioxidant defense in the genome. The phylogenetic analysis suggested that the strain WBS is the closest relative of Fusibacter bizertensis LTF Kr01[sup.T] (16S rRNA gene sequence similarity 98.78%). Based on biochemical and genomic characteristics, the strain WBS is proposed to represent a novel aero-, halo- and psychrotolerant strain from the genus Fusibacter, isolated for the first time among its members from cold oxygenated marine bottom sediments.

Microbiological, molecular ecological, biogeochemical, and isotope geochemical research was carried out in four lakes of the central part of the Yamal Peninsula in the area of continuous permafrost. Two of them were large (73.6 and 118.6 ha) and deep (up to 10.6 and 12.3 m) mature lakes embedded into all geomorphological levels of the peninsula, and two others were smaller (3.2 and 4.2 ha) shallow (2.3 and 1.8 m) lakes which were formed as a result of thermokarst on constitutional (segregated) ground ice. Samples were collected in August 2019. The Yamal tundra lakes were found to exhibit high phytoplankton production (340–1200 mg C m−2 d−1) during the short summer season. Allochthonous and autochthonous, particulate and dissolved organic matter was deposited onto the bottom sediments, where methane was the main product of anaerobic degradation, and its content was 33–990 µmol CH4 dm−3. The rates of hydrogenotrophic methanogenesis appeared to be higher in the sediments of deep lakes than in those of the shallow ones. In the sediments of all lakes, Methanoregula and Methanosaeta were predominant components of the archaeal methanogenic community. Methane oxidation (1.4–9.9 µmol dm−3 d−1) occurred in the upper sediment layers simultaneously with methanogenesis. Methylobacter tundripaludum (family Methylococcaceae) predominated in the methanotrophic community of the sediments and the water column. The activity of methanotrophic bacteria in deep mature lakes resulted in a decrease in the dissolved methane concentration in lake water from 0.8–4.1 to 0.4 µmol CH4 L−1 d−1, while in shallow thermokarst lakes the geochemical effect of methanotrophs was much less pronounced. Thus, only small, shallow Yamal lakes may contribute significantly to the overall diffusive methane emissions from the water surface during the warm summer season. The water column of large, deep lakes on Yamal acts, however, as a microbial filter preventing methane emission into the atmosphere. It can be assumed that climate warming will lead to an increase in the total area of thermokarst lakes, which will enhance the effect of methane release into the atmosphere.