[Fixation of molecular nitrogen and activity of the microflora in the bottoms of certain lakes in the Estonian SSR and the Rybinsk reservoir].

The rate of molecular nitrogen fixation was determined in bottom grounds of three Estonian lakes and the Rybinsk water reservoir in the summer of 1977--1978. Certain species of nitrogen fixing bacteria were found to be confined to lakes of certain trophic type. Ecological niches with the mass growth of Clostridium pasteurianum, Azomonas agilis and Clostridium butyricum were detected in the sediments of eutrophic lakes. Ecological niches of Az. insignis and Cl. acetobutylicum occur in polyhumic lakes. Ecological niches for the nitrogen fixing microaerophilic organisms Methylosinus trichosporium, M. sporium and Mycobacterium flavum can be found in the sediments of water reservoirs of any trophic type. The important ecological factors which favour the growth of microflora with a high rate of molecular nitrogen fixation in bottom grounds are as follows: a sufficiently high temperature, a weakly alkaline reaction, the presence of oxygen in the water near the bottom, and the ratio of organic carbon to total nitrogen about 10--11. The destruction of organic substances in such sediments involves both aerobic and anaerobic microflora, and Cl. butyricum is abundant among nitrogen fixing microorganisms.