Composition of Methane Cycle Microbial Communities in the Upper Layers of Bottom Sediments of the Kara Sea

A study of the distribution of methane cycle microbial communities in the upper layers of bottom sediments above large hydrocarbon reservoirs in the South Kara petroleum region of the West Siberian Province revealed the presence in these layers of both aerobic methanotrophic bacteria and anaerobic methanogenic archaea, as well as numerous heterotrophic microorganisms of various phylogenetic groups. Research was carried out in the Baydaratskaya Bay and in the east of the Pukhuchan Depression (southern part of the Kara Sea). Aerobic methanotrophic bacteria belonged to the families Methyloligellaceae, Methylophagaceae and Methylomonaceae were detected in the surface oxidized layers (0-2 cm, Eh from 60 to 175 mV) of bottom sediments. Moreover, representatives of Methyloligellaceae were found in quite significant amount (1.52-2.61% of all 16S rRNA gene sequence reads) at all studied stations of the Kara Sea. In the subsurface reduced layers (18-20 cm, Eh from -63 to -246 mV), methanogenic archaea were dominated by representatives of the order Methanomassiliicoccales (up to 3.3% of all 16S rRNA gene sequence reads). Methanogenic archaea of the orders Methanofastidiosales, Methanobacteriales and Methanomicrobiales were also discovered. In addition, aerobic/facultative anaerobic methanotrophic bacteria of the families Methylococcaceae and Methylomonadaceae were found on these reduced layers of the bottom sediments, but the relative abundance (in percentage of sequence reads of their total number) of anaerobic methanotrophic archaea was extremely low there. Apparently, the oxidation of methane in bottom sediments of the Kara Sea, where its concentration at most of the studied stations dramatically increased with sediment depth, occurs predominantly under aerobic conditions in the surface layers, where the relative abundance and phylogenetic diversity of methanotrophs are higher.