Decoupling between sulfate reduction and the anaerobic oxidation of methane in the shallow methane seep of the Black sea.

Methane seepages are widespread in the Black Sea. However, microbiological research has been carried out only at the continental shelf seeps. The present work dealt with coastal gas seepages of the Kalamit Bay (Black Sea). High-throughput 16S rRNA gene sequencing and radiotracer analysis (14С and 35S) were used to determine the composition of the microbial community and the rates of microbial sulfate reduction and methane oxidation. The phylum Proteobacteria, represented mainly by sulfate reducers of the class Deltaproteobacteria, was the predominant in sequence dataset. Bacteroidetes and Planctomycetes were other numerous phyla. Among archaea, the phylum Woesearchaeota and Marine Benthic Group B were predominant in the upper horizons. Relative abundance of Euryarchaeota of the families Methanomicrobiaceae and Methanosarcinaceae (including ANME-3 archaea) increased in deeper sediment layers. Sulfate reduction rate (up to 2.9 mmol/L × day) was considerably higher than the rate of anaerobic methane oxidation (up to 43.4 µmol/L × day), which indicated insignificant contribution of anaerobic methane oxidation to the total sulfide production.

[Geochemical characteristics of the carbonate constructions formed during microbial oxidation of methane under anaerobic conditions]. / Geokhimicheskie osobennosti karbonatnykh postroek, obrazuiushchikhsia pri mikrobnom okislenii metana v anaérobnykh usloviiakh.

The aragonite constructions of the Black Sea are formed in a stable anaerobic zone and are a perfect object to study the natural mechanism of anaerobic methane oxidation. The most probable pathway of methane oxidation is its methanogen-mediated reaction with bicarbonates, dissolved in seawater, with the formation of water and acetate, which is then consumed by other components of the anaerobic community. Comparison of the delta 13C values of carbonate minerals and organic matter once more demonstrated that the formation of the organic matter of biomass is accompanied by intense fractionation of carbon isotopes, as a result of which the total organic matter of biomass acquires an extremely light isotopic composition, characterized by delta 13C values as low as -83.8@1000.