Biomass,production and activity of bacteria in the Black Sea,with special reference to chemosynthesis and the sulfur cycle

Abundance, morphological composition, vertical distribution, production and activity of total bacterioplankton and its specific groups in the Black Sea were investigated in August–September 1989. The total bacterioplankton was highest in the upper mixed layer (0.7–1 × 10⁶ cells ml^–1), corresponding to that in mesotrophic basins. In the N-E shallow part of the sea it attained 3 × 10⁶. Below the thermocline (50–100 m) the total number of bacteria decreased to 0.2–0.4 × 10⁶ ml^–1. In the redox gradient zone (zone of O₂-H₂S interface), it increased again. In deep anoxic waters the bacterioplankton, numbering 0.15–0.2 × 10⁶ ml^–1, was functionally inactive. Its biomass was 12–40 mg C m^–3 in the upper mixed layer, 5–10 mg C m^–3 in the intermediate cold layer (40–100 m depth), and 10–20 mg C^–3 in the redox zone. Maximum production rates occurred in the upper mixed layer (8–20 mg C^–3 d^–1) and in the redox-zone, 80–90% of it was due to chemosynthesis of thiobacilli. Below 200 m, microbial production decreased to about zero in the anoxic zone. Maximum activity of heterotrophic bacteria was recorded in the upper mixed layer, while thiobacilli and methaneoxidezing bacteria were most active in the redox-zone. Here, the maximum rates of H₂S and of thiosulfate oxidation, as well as maximum sulfate reduction were recorded. Chemical oxidation of H₂S was dominant. These results are discussed with respect to the present ecological situation of the Black Sea.