AME 59:217-228 (2010)  -  DOI: https://doi.org/10.3354/ame01399

ABSTRACT: 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 ¹⁴C-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 (>10⁴ to 10⁵ cells ml^–1) were observed at the oxic-anoxic interfaces and in the depleted hypolimnia (1.3 ± 4.4 × 10⁴ cells ml^–1), whereas 1 order of magnitude lower concentrations were detected in the epilimnia (1.0 ± 2.3× 10³ cells ml^–1). Dark ¹⁴C-bicarbonate incorporation rates were higher at the oxic-anoxic interface (11.4 ± 9.5 µg C l^–1 h^–1) than in the hypolimnia (6.4 ± 5.9 µg C l^–1 h^–1) and epilimnia (1.0 ± 2.1 µg 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.

KEY WORDS: Thiobacilli · Karstic lake · Sulfide · Oxic-anoxic interface · Most probable number · Dark CO₂ fixation