Sharp water column stratification with an extremely dense microbial population in a small meromictic lake,...

Located on the shore of Kandalaksha Bay (the White Sea, Russia) and previously separated from it, Trekhtzvetnoe Lake (average depth 3.5 m) is one of the shallowest meromictic lakes known. Despite its shallowness, it features completely developed water column stratification with high-density microbial chemocline community (bacterial plate) and high rates of major biogeochemical processes. A sharp halocline stabilizes the stratification. Chlorobium phaeovibrioides dominated the bacterial plate, which reached a density of 2 × 10⁸ cell ml^-1 and almost completely intercepts H₂ S diffusion from the anoxic monimolimnion. The resulting anoxygenic photosynthesis rate reached 240 μmol C l^-1 day^-1 , exceeding the oxygenic photosynthesis rate in the mixolimnion. The rates of other processes are also high, reaching 4.5 μmol CH₄ l^-1 day^-1 for methane oxidation and 35 μmol S l^-1 day^-1 for sulfate reduction. Metagenomic analysis demonstrated that the Chl. phaeovibrioides population in the bacterial plate layer had nearly clonal homogeneity, although some fraction of these cells harbour a plasmid. The Chlorobium population was associated with bacteriophages that share homology with CRISPR spacers in the host. These features make the ecosystem of the Trekhtzvetnoe Lake a valuable model for studying regulation and evolution processes in natural high-density microbial systems.