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 O2 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.

Energy metabolism of the alkaliphilic sulfate-reducing bacterium Desulfonatronovibrio hydrogenovorans strain Z-7935 was investigated in continuous culture and in physiological experiments on washed cells. When grown in chemostats with H 2 as electron donor, the cells had extrapolated growth yields [Y max , g dry cell mass (mol electron acceptor) N1 ] of 55 with sulfate and 128 with thiosulfate. The maintenance energy coefficients were 19 and 13 mmol (g dry mass) N1 h N1 , and the minimum doubling times were 27 and 20 h with sulfate and thiosulfate, respectively. Cell suspensions reduced sulfate, thiosulfate, sulfite, elemental sulfur and molecular oxygen in the presence of H 2 . In the absence of H 2 , sulfite, thiosulfate and sulfur were dismutated to sulfide and sulfate. Sulfate and sulfite were only reduced in the presence of sodium ions, whereas sulfur was reduced also in the absence of Na M . Plasmolysis experiments showed that sulfate entered the cells via an electroneutral symport with Na M ions. The presence of an electrogenic Na M -H M antiporter was demonstrated in experiments applying monensin (an artificial electroneutral Na M -H M antiporter) and propylbenzylylcholine mustard.HCl (a specific inhibitor of Na M -H M antiporters). Sulfate reduction was sensitive to uncouplers (protonophores), whereas sulfite reduction was not affected. Changes in pH upon lysis of washed cells with butanol indicated that the intracellular pH was lower than the optimum pH for growth (pH 95). Pulses of NaCl (052 M) to cells incubated in the absence of Na M did not result in ATP formation, whereas HCl pulses (shifting the pH from 92 to 70) did. Small oxygen pulses, which were reduced within a few seconds, caused a transient alkalinization. The results of preliminary experiments with chemiosmotic inhibitors provided further evidence that the alkalinization was caused by sodium-proton antiport following a primary electron-transport-driven sodium ion translocation. It is concluded that energy conservation in D. hydrogenovorans depends on a proton-translocating ATPase, whereas electron transport appears to be coupled to sodium ion translocation.