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Research Paper |
Institut für Chemie und Biologie des Meeres, Universität Oldenburg,D-26111 Oldenburg, Germany1
Author for correspondence: Heribert Cypionka. Tel: +49 441 798 5360. Fax: +49 441 798 3583. e-mail: Cypionka{at}icbm.de
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 H2 as electron donor, the cells had extrapolated growth yields [Ymax, g dry cell mass (mol electron acceptor)-1] of 5·5 with sulfate and 12·8 with thiosulfate. The maintenance energy coefficients were 1·9 and 1·3 mmol (g dry mass)-1 h-1, 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 H2. In the absence of H2, 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+. Plasmolysis experiments showed that sulfate entered the cells via an electroneutral symport with Na+ ions. The presence of an electrogenic Na+–H+ antiporter was demonstrated in experiments applying monensin (an artificial electroneutral Na+–H+ antiporter) and propylbenzylylcholine mustard.HCl (a specific inhibitor of Na+–H+ 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 9·5). Pulses of NaCl (0·52 M) to cells incubated in the absence of Na+ did not result in ATP formation, whereas HCl pulses (shifting the pH from 9·2 to 7·0) 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.
Keywords: growth yield, reversed proton gradient, sulfate transport, sodium–proton antiport, sodium ion translocation
Abbreviations: PrBCM, propylbenzylylcholine mustard.HCl; CCCP, carbonyl cyanide m-chlorophenylhydrazone; TCS, tetrachlorosalicylanilide
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