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 sulfa te-reducing bacteria. Electrode measurements with washed cells indicat ed 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 forme d back. Since oxygen could be replaced by nitrate or nitrite as electr on acceptor, the described biphasic reaction was independent of molecu lar oxygen. With Desulfobulbus propionicus and the alkaliphilic strain Z-7935, sulfide back-formation started after oxygen was consumed comp letely. By contrast, with the freshwater strains Desulfovibrio desulfu ricans CSN (DSM 9104) and Essex 6 (DSM 2032) and the marine strain P1B , sulfide back-formation already started before oxygen was consumed. T he addition of hydrogen as electron donor increased simultaneously the rate of aerobic respiration and sulfide back-formation. Both reaction s stopped when the oxygen was consumed, indicating that the electron t ransport to oxygen and sulfur was coupled. Sulfide-oxidizing activity (84 nmol O-2 min(-1) (mg protein)(-1)) was found in the periplasmic fr action prepared by osmotic shock treatment of suspensions of D. desulf uricans CSN. This fraction oxidized sulfide with oxygen to elemental s ulfur. It is concluded that in different sulfate-reducing bacteria sul fide oxidation proceeds via a common pathway with the formation of ele mental sulfur as intermediate and its disproportionation to sulfate an d sulfide. The process is independent of molecular oxygen.