Microbial Community Composition and Rates of the Methane Cycle Microbial Processes in the Upper Sediments of the Yamal Sector of the Southwestern Kara Sea

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Geochemical, biogeochemical, and molecular genetic investigation of the upper (0–5 cm) bottom sediments of the Yamal sector of the Kara Sea was carried out. The Yamal sector is well-protected from the massive inflow of river water. The sediments were oxidized at the surface and weakly reduced in the 3−5-cm layer. C_org content varied from 0.1 to 1.3%, while the level of dissolved СН₄ was 1.9 to 20.3 μmol L^–1. The isotopic composition of organic matter (OM) carbon, δ¹³Corg, varied from–27.5 to–22.2‰ (–25.4‰ on average). The share of terrigenous OM was 13.3 to 72.2% (48.9% on average). The rate of methane production, methane oxidation, and sulfate reduction varied from 0.8 to 9.0 (2.7 on average) nmol СН₄ dm^–3 day^–1, from 9.9 to 103 (31.6 on average) nmol СН₄ dm^–3 day^–1, and from 0.49 to 2.2 (1.1 on average) μmol S dm–3 day–1, respectively. High-throughput sequencing of the amplicons of the 16S rRNA genes was used to reveal the physiological groups of microorganisms responsible for the processes of methane production and oxidation, sulfate reduction, and oxidation of reduced sulfur compounds. Members of the phylum Woesearchaeota were predominant among archaea. Methanogenic archaea belonged to the families Methanobacteriaceae, Methanococcaceae, and Methanosarcinaceae (Euryarchaeota). Methanotrophs of the family Methylococcaceae were revealed among the Gammaproteobacteria, with their share in the sediments ~1%. In the class Deltaproteobacteria (15.4%), three orders of sulfate reducers were predominant: Desulfobacterales, Desulfovibrionales, and Desulfuromonadales. Oxidation of reduced sulfur compounds was carried out by chemolithoautotrophic bacteria of the genera Sulfurovum, Sulfurimonas, and Arcobacter of the class Epsilonproteobacteria (1.1% of the total microbial number).