[Vertical distribution and the characteristics of the species composition of the microorganisms of some stratified lakes in Estonia].

The total number, vertical distribution, and special composition of microorganisms from three stratified lakes in the South-Eastern Estonia containing hydrogen sulfide and ferrous iron were studied in March, 1976. The number of bacteria was lowest in surface water samples (1.2-1.9 x 10(6) cells/ml). It increased first in the zone where oxygen disappeared (2.4-7.6 x 10(6) cells/ml), then slightly decreased, and again increased near the bottom where it became maximal (3.7-14.0 x 10(6) cells/ml). The vertical distribution of individual microbial species corresponded to ecological niches. The content of Spirillum and Flexibacter, colonies of parallel rods containing gaseous vacuoles, was highest in the zone in which oxygen was still present. Iron bacteria, purple sulfur bacteria, colourless sulfur bacteria, and green bacteria Chloronema appeared deeper, in the microaerophilic conditions. Ancalochloris perfilievil, Chloroplana vacuolata, Chlorochromatium aggregatum, Pelochromatium roseum, Hyalosoris sp., and Peloploca sp. were found in the strictly anaerobic conditions. Morphological forms of microorganisms were most versatile in the region of contact between the aerobic and anaerobic zones. Most of the morphologically peculiar species of microorganisms were detected in all three lakes, but some bacterial forms were typical of some of the lakes.