[Sulfur and iron cycling bacteria in low-sulfate meromictic Lake Kuznechikha].

The hydrochemical characteristics, the composition of species and the localization of bacterial species involved in oxidation of sulfide and ferrous salts were studied in the meromictic Lake Kuznechikha with a low sulfate content in summer and in winter. The rate of bacterial sulfate reduction and the rates of bacterial and algal photosynthesis were determined using a radioisotope technique. The meromictic nature of the lake is due to the accumulation of ferrous salts (up to 212 mg/l Fe2+) in the monimolimnion. Free hydrogen sulfide is absent whereas the concentration of sulfides at the bottom reaches 16.5 mg/l. Hydrogen sulfide is produced mainly by sulfate reducing bacteria which are particularly active (0.35 mg of H2S per day) in the surface layer of reduced ooze. The photosynthetic green bacteria Chloronema giganteum, Chlorochromatium aggregatum, Chloroplana vacuolata and Pelochromatium roseum, the thiobacilli Thiobacillus intermedius and Th. trautweinii, the iron bacteria Ochrobium tectum and Arthrobacter sp. (Siderocapsa sp.) are found in the zone of chemocline at a depth of 6 m in the summer. The primary production of the phytoplankton in the summer is 160 mg C per 1 m2 per day while the production of the phototrophic bacteria is 100 mg C per 1 m2 per day. The fixation of carbon dioxide in the dark is performed mainly by heterotrophic bacteria. In the winter, the frontier of the anaerobic zone rises to the surface and photosynthetic bacteria are absent from it.