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.
Keywords: Freshwater lake; Methanogenesis; Microbial diversity sediments.