Description of a redox-controlled sulfidostat for the growth of sulfide-oxidizing phototrophs.

This paper describes a novel type of continuous culture for the growth of phototrophic sulfur oxidizers under constant concentrations of hydrogen sulfide. The culture maintains a constant concentration of sulfide despite possible variations in external factors likely to affect photosynthetic activity. Variations in biological activity lead to small departures from the steady-state concentration of hydrogen sulfide which result in variations of the redox potential. These changes in redox, monitored through a redox controller, modulate the rate at which the medium is pumped into the culture and therefore govern the dilution rate. As a result, when changes in external factors such as the light supply occur, the dilution rate of the culture adjusts to the new rate of sulfide oxidation, while maintaining a virtually constant concentration of hydrogen sulfide. The system has been successfully tested for an extended period of several weeks and under conditions of shifting illumination (868 to 113, 113 to 23, and 23 to 7 (mu)E(middot)m(sup-2)(middot)s(sup-1)). After changes in illumination, a transition to a new dilution rate started immediately, reaching a new equilibrium in less than 3 h.