Microbial processes of the carbon and sulfur cycles in Lake Mogil'noe

In the beginning of the summer of 1999, complex microbiological and biogeoc hemical investigations of meromictic Lake Mogil'noe (Kil'din Island, Barent s Sea) were carried out. The analysis of the results shows a clearly pronou nced vertical zonality of the microbial processes occurring in the water co lumn of the lake. To a depth of 8 m, the total number and activity of micro organisms was limited by the relatively low content of organic matter (OM). In the upper part of the hydrogen-sulfide zone of the take (beginning at a depth of 8.25 m), the content of particulate OM and the microbial number s harply increased. In this zone, the daily production of OM during anaerobic photosynthesis at the expense of massive development of colored sulfur bac teria reached 620 mg C/m(2), which was twofold greater than the daily produ ction of phytoplankton photosynthesis) thesis and led to a considerable cha nge in the isotopic composition (delta C-13) of the particulate OM. In the same intermediate layer, the highest rates of sulfate reduction were record ed, and fractionation of stable sulfur isotopes occurred. Below 10 m, was t he third hydrochemical zone, characterized by maximum concentrations of H2S and CH4 and by a relatively high rate of autotrophic methanogenesis. The c omparison of the results obtained with the results of investigations of pre vious years, performed in the end of summer, shows a decrease in the intens ity of all microbial processes inspected. An exception was anoxygenic photo synthesis, which can utilize not only the de novo formed H2S but also the H 2S accumulated in the lake during the winter period.