Layer-specific microbial community structures in the meromictic lake, Lake Suigetsu, unveiled by 16S rRNA gene amplicon and metagenomic analyses

Meromictic lakes serve as natural laboratories for investigating microbially mediated biogeochemical cycling because their stable redox stratification supports diverse metabolic processes. Lake Suigetsu is a typical meromictic lake in Japan characterized by a permanent oxycline separating the oxic brackish mixolimnion from the anoxic saline sulfidogenic monimolimnion. Recently, Lake Suigetsu's vertical bacterial composition, especially of autotrophs, sulfide oxidizers, and sulfate reducers, has gained significant attention. However, little is known about microbial communities particularly the abundant heterotrophs that contribute to the biogeochemical cycling by degrading organic matter. To characterize biogeochemical cycling in Lake Suigetsu, we investigated the microbial community structures under the amplicon sequencing variants that distinguish species and/or intraspecies populations in each layer of Lake Suigetsu by using 16S rRNA gene amplicon sequencing analysis. We revealed the layer-specific microbial communities and there were abundant heterotrophs in each layer, in addition to the previously reported autotrophs. In the upper of monimolimnion sample, approximately 20% relative abundance was occupied by an uncultured Anaerolineaceae bacterium, that is probably an anaerobic heterotroph. In the mixolimnion, the most abundant one was SAR11 bacterium with approximately 6% relative abundance. In the oxycline, the most abundant, probably heterotrophic prokaryote was an uncultured Ectothiorhodospiraceae bacterium with approximately 5% relative abundance, while the four most abundant prokaryotes were comprised of autotrophic bacteria and occupied more than 43% relative abundance, apparently suggesting the primary production was predominant in the oxycline. This study provides insight into the layer-dependent, prokaryote-mediated biogeochemical cycling in Lake Suigetsu.