1987 Volume 33 Issue 5 Pages 391-400
The effect of dissolved oxygen concentrations on the rate of biological oxidation of sulfide and elemental sulfur by the A-type sulfur-turf was examined. The rate of sulfide oxidation markedly increased concomitant with the increase in dissolved oxygen. Accompanying the sulfide oxidation, elemental sulfur was formed only a few minutes after the addition of hydrogen sulfide to the culture medium at a dissolved oxygen concentration of about 6ppm. The catalase test of the A-type sulfur-turf was negative. As hydrogen peroxide is a very reactive intermediate inevitably accompanying the biological reduction of oxygen to water, it is probable that hydrogen sulfide was oxidized by hydrogen peroxide. Consequently, these results could be explained if the production of hydrogen peroxide was more active under oxic than the microoxic conditions. On the other hand, the rate of the elemental sulfur oxidation was suppressed both at low (0.2ppm) and at high (3 to 6ppm) dissolved oxygen concentrations. The optimum was about 1.5ppm (Fig. 5). Therefore, it was concluded
that the elemental sulfur oxidation by the A-type sulfur-turf was a microaerophilic reaction. Due to the continuous supply of anoxic water from a hot spring source, dissolved oxygen concentrations of the effluents where the A-type sulfur-turf occurred were kept under 1ppm (1). Accordingly, at these low concentrations of dissolved oxygen, it seems likely that the oxidation of elemental sulfur was maximized, while the production of toxic hydrogen peroxide was minimized. These assumptions are sufficient to account for the preference of the A-type sulfur-turf for the microoxic habitat.