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.

The rates of oxygenic and anoxygenic photosynthesis, the microorganisms responsible for these processes, and the hydrochemical characteristics of the sulfide-containing karst lakes, Black Kichier and Big Kichier (Mari El Republic), were investigated. In these lakes, a plate of anoxygenic phototrophic bacteria (APB) is formed at the upper boundary of sulfide occurrence in the water. The phototrophic community of the chemocline zone was analyzed using a combination of high-throughput sequencing of the 16S rRNA gene fragments and light and electron microscopic techniques. Green-colored Chlorobium clathratiforme were absolutely predominant in both lakes. The minor components included green sulfur bacteria (GSB) Chlorobium spp., symbiotic consortia Chlorochromatium magnum and Pelochromatium roseum, purple sulfur bacteria (PSB) Chromatium okenii, and unidentified phylotypes of the family Chromatiaceae, as well as members of the Chloroflexota: Chloronema sp. and Oscillochloris sp. Based on the results of the molecular analysis, the taxonomic status of Ancalochloris perfilievii and other prosthecate GSB, as well as of the PSB Thiopedia rosea, which were visually revealed in the studied freshwater lakes, is discussed.

Saline and highly alkaline soda lakes are often characterized by a persistent nitrogen loss and high sulfide levels. Cyanobacteria are key aerobic diazotrophs in soda lakes, where light-dependent nitrogen fixation (NF) is crucial for sustaining ecosystem functioning. While sulfide is a well-known inhibitor of oxygenic photosynthesis, some cyanobacteria may tolerate it and utilize it via anoxygenic photosynthesis. In this study, we investigated the NF and anoxygenic photosynthesis in the genus Sodalinema, including non-heterocystous cyanobacteria widely distributed in soda and saline environments around the world and possessing an anaerobe-like nitrogenase. Our data suggest that their nif-operon could have been more likely acquired in soda or saline–alkaline lakes from natronophilic sulfate-reducing bacteria of the family Desulfonatronovibrionaceae than in the marine environment. It was shown that Sodalinema sp. P-1104, isolated from a southwestern Siberian soda lake, is capable of NF only in a light/dark switching mode, both in oxic and anoxic conditions. Sulfide did not suppress photosynthesis and stimulated NF up to threefold in oxygenic conditions. Anaerobic NF was obligately sulfide-dependent and supported by anoxygenic photosynthesis. However, removal of photosynthetic oxygen due to the high reducing potential of sulfide stimulated NF to a greater extent than does the use of sulfide through anoxygenic photosynthesis.