[Molecular nitrogen fixation in the waters of eutrophic and polyhuic lakes of the Estonian SSR].

A modified acetylene technique was used to assay the rate of molecular nitrogen fixation in Estonian lakes containing methane in the hypolimnion. Methods were elaborated to eliminate ethylene cooxidation by methane oxidizing bacteria. Methane oxidation and nitrogen fixation were found in a narrow microaerobic zone in lakes with the stratification of temperature in the water mass; these biochemical processes occurred when the content of dissolved oxygen varied within the range of 0.1 to 0.8 mg O2 per litre. The rate of these microbiological processes was significantly influenced by trophic relationships with microorganisms from the adjacent ecological niches. In the epilmnion of eutrophic lakes, atmospheric nitrogen was fixed by the blue-green algae Anabaena and Gloeocapsa minima and by the heterotrophic bacterium Azomonas agilis; in the microaerobic zone of metalimnion, atmospheric nitrogen was fixed by the methane oxidizing bacteria Methylosinus trichosporium and M. sporium and by the hydrogen oxidizing bacterium Mycobacterium flavum. In polyhumic lakes, nitrogen fixation was performed by the bactera Az. insignes, Methylosinus trichosporium, M. sporium and Mycobacterium flavum only in water layers near the bottom with microaerobic conditions.