Vertical distribution of planktonic autotrophic thiobacilli and dark CO2 fixation rates in lakes with oxygen-sulfide interfaces

Vertical distributions of viable (most probable number, MPN) aerobic chemoautotrophic thiobacilli-like sulfur-oxidizing bacteria (ca. 70 samples in triplicate for MPN counts) and dark C-14-bicarbonate incorporation rates were analyzed in a series of sulfide-rich lakes. A special device for sampling sharply stratified populations on the scale of a few centimeters was used. Detailed analyses focused on the oxic-anoxic transition zone where aerobic sulfur-oxidizing bacteria should display positive chemotaxis, and in both fully oxic epilimnia and sulfide-rich anoxic hypolimnia. Kinetics of sulfide and thiosulfate potential oxidations in the presence of oxygen were followed in microcosm enrichments in one of the lakes. The highest MPN counts (>10(4) to 10(5) cells ml(-1)) were observed at the oxic-anoxic interfaces and in the depleted hypolimnia (1.3 +/- 4.4 x 10(4) cells ml(-1)), whereas 1 order of magnitude lower concentrations were detected in the epilimnia (1.0 +/- 2.3x 10(3) cells ml(-1)). Dark C-14-bicarbonate incorporation rates were higher at the oxic-anoxic interface (11.4 +/- 9.5 mu g C l(-1) h(-1)) than in the hypolimnia (6.4 +/- 5.9 mu g C l(-1) h(-1)) and epilimnia (1.0 +/- 2.1 mu g C l(-1) h(-1)). A lack of correspondence between abundance of MPN thiobacilli, location at the sulfide interface, and dark carbon fixation rates was, however, consistently observed in a correlation analysis. Patterns of in situ potential aerobic thiosulfate oxidation did not match dark carbon fixation rates or MPN vertical distributions. The chemoautotrophic guild of these lakes emerged as a metabolically complex, taxonomically diverse group of aerobic, microaerophilic, and anaerobic microorganisms coexisting in the same lake. Thiobacilli may actively fix CO2 at certain depths but the question of which types of bacteria contribute most to dark CO2 fixation in the investigated lakes is still open, and the application of culture-independent molecular tools and single-cell analyses should be used to substantiate and further explore these findings.