Microbial Community of a Marine Meromictic Trough (Biofilter Bay) in the Kandalaksha Bay, White Sea.

The composition of microbial communities and the rates of microbial processes in the water column of a meromictic trough in the Biofilter Bay, Kandalaksha Bay, White Sea were investigated in September 2020 and March 2021. The chemocline zone was located at the depths from 8 to 9 m. Sulfide was present in the monimolimnion, its concentration at the bottom reaching 25‒38 mg L^–1 (0.7‒1.1 mmol). Conditions of the redox zone favored development of both anaerobic anoxygenic phototrophs and aerobic sulfur-oxidizing chemoautotrophs. A pinkish-brown bacterial plate was formed in this zone. Sequencing of the 16S rRNA gene fragments revealed predominance of green sulfur bacteria (GSB) Chlorobium phaeovibrioides. Aerobic sulfur-oxidizing bacteria Sulfurimonas sp. were the second most abundant group. Plating of chemocline samples on selective media resulted in massive formation of brown-colored Chlorobi colonies, while green GSB colonies were few. Both morphotypes were identified as Chlorobium phaeovibrioides. At the time of our study, purple sulfur bacteria (PSB) did not form a bacterial plate in the redox zone, although few PSB colonies were revealed. The PSB isolate was identified as Thiocapsa rosea. Members of this species are able to switch from photosynthesis to aerobic chemosynthesis and may compete for sulfide with sulfur-oxidizing chemo-autotrophs. We suggest that due to its openness the Biofilter Bay meromictic trough may act as a source for the propagation of anoxygenic phototrophic bacteria in the Kandalaksha Bay. [ABSTRACT FROM AUTHOR]