[Microbiologic processes in meromictic Lake Sakovo]

The freshwater meromictic lake Sakovo located in the Vologda Region was investigated. Its maximal depth is 16 m. The lake contained very high sulfate concentrations up to 816 mg/l; however, the rate of sulfate reduction in the water was low (not more than 16 mg H2S/l/day, whereas 4.5 mg of H2S was produced per day in the sediments. Bacterial sulfate reduction was shown to be limited by the deficiency of an organic substrate. Not more than 11 mg of H2S/l was contained in the nonmixing layers of the monimolimnion. The boundary line of the H2S zone was at a depth of 3.5-4.5 m and coincided with the thermocline and chemocline. The water in this region was green due to the growth of two species of green sulfur bacteria: Pelodictyon luteolum (the maximum of 7.35 x 10(6) cells in 1 ml) and Chlorobium limicola in the symbiotic complex of Chlorobium aggregatum (the maximum of 0.42 x 10(6) aggregates in 1 ml). Phototrophic bacterial cells synthesized 320 microgram of C per litre per day at a depth of 4.25 m. The assimilation of carbon dioxide in the dark in the zone of contact between H2S and O2 containing waters constituted 100 mg of C per litre per day, and apparently was due to the chemosynthesis of thiobacteria (the maximum of 1000 cells in 1 ml). It has been calculated that 9 mg of H2S per 1 m2 per day is formed in the water, and 500 mg of H2S per 1 m2 per day is produced in the sediments. Green bacteria oxidize about 268 mg of H2S per 1 m2 per day while thiobacteria oxidize 250 mg of H2S pe2 1 m2 per day. Sulfur bacteria may be possibly supplied with supplied with sulfide by syntrophism with sulfate reducing and sulfur reducing bacteria as well as by diffusion of H2S from the sediments.