Influence of sulfur-oxidizing bacteria on the budget of sulfate in Yugama crater lake, Kusatsu-Shirane volcano, Japan

Sulfur-oxidizing bacteria, Thiobacillus thiooxidans, werefound in a highly acidic (pH = 11.5) crater lake, Yugama,seasonally flowing streams and soil in the catchment area of thecrater. Thiobacillus ferrooxidans was also found in some ofthe streams but not in the lake itself. The lake water containsaqueous carbon dioxide, hydrogen sulfide, polythionates andelemental sulfur in suspension which are the substrates for thegrowth of the sulfur-oxidizing bacteria as no organic compoundsexcept for the microorganisms themselves were detected. Thebacteria isolated from the Yugama water preferred polythionatesin the following order: S₄O₆ ²> S₅O₆ ²> S₆O₆ ²On the other hand,H₂S was more rapidly consumed by the bacteria thanpolythionates which were followed by elemental sulfur. In thecase of test-tube incubation, the optimum pH of the solution forgrowth of the bacteria was between 1.0 and 1.5, and forcultivation in growth medium plates between 2.5 and 3.5. Thebacteria hardly proliferated at pH 0.5 or below. In accordancewith these characteristics of the bacteria, numbers of thebacteria in the surface Yugama crater lake water were at minimum(< a few cells/mL) in February and at maximum (10⁶ cell/mL)in August. The bacterial activity changed in accordance with thesurface lake water temperature, but not necessarily with thevariations in H₂S and polythionates concentrations of the lakewater. Based on the variation in sulfur isotope ratios of sulfateand experimentally determined rate of oxidation of H₂S in thelake water, the sulfate production rate by the bacteria in thecatchment area and the lake were estimated to 9.5 and 8.4g/m²/day, respectively, during the period from 1988 to 1990when the volcanic activity at Yugama was at minimum. Also stream,hydrothermal, H₂S-oxidated SO² ₄-inputs and outputs byseepage and precipitation have been calculated as 4.1, 32, 0.56,36, and 1.2 ton/day, respectively.