A common pathway of sulfide oxidation by sulfate-reducing bacteria

The pathway of sulfide oxidation with oxygen as electron acceptor was studied with five strains of freshwater, marine and alkaliphilic sulfate-reducing bacteria. Electrode measurements with washed cells indicated that all strains oxidized sulfide to elemental sulfur. In a second step, the elemental sulfur formed was disproportionated to sulfate and sulfide. During this phase, most of the disappeared sulfide was formed back. Since oxygen could be replaced by nitrate or nitrite as electron acceptor, the described biphasic reaction was independent of molecular oxygen. With Desulfobulbus propionicus and the alkaliphilic strain Z-7935, sulfide back-formation started after oxygen was consumed completely. By contrast, with the freshwater strains Desulfovibrio desulfuricans CSN (DSM 9104) and Essex 6 (DSM 2032) and the marine strain P1B, sulfide back-formation already started before oxygen was consumed. The addition of hydrogen as electron donor increased simultaneously the rate of aerobic respiration and sulfide back-formation. Both reactions stopped when the oxygen was consumed, indicating that the electron transport to oxygen and sulfur was coupled. Sulfide-oxidizing activity (84 nmol O-2 min(-1) (mg protein)(-1)) was found in the periplasmic fraction prepared by osmotic shock treatment of suspensions of D. desulfuricans CSN. This fraction oxidized sulfide with oxygen to elemental sulfur. It is concluded that in different sulfate-reducing bacteria sulfide oxidation proceeds via a common pathway with the formation of elemental sulfur as intermediate and its disproportionation to sulfate and sulfide. The process is independent of molecular oxygen.