ROLE OF SODIUM-IONS FOR SULFATE TRANSPORT AND ENERGY-METABOLISM IN DESULFOVIBRIO SALEXIGENS

The sodium ion gradient and the membrane potential were found to be th e driving forces of sulfate accumulation in the marine sulfate reducer Desulfovibrio salexigens. The protonmotive force of -158 mV, determin ed by means of radiolabelled membrane-permeant probes, consisted of a membrane potential of - 140 mV and a pH gradient (inside alkaline) of 0.3 at neutral pH(out). The sodium ion gradient, as measured with sili cone oil centrifugation and atomic absorption spectroscopy, was eightf old ([Na+](out)/[Na+](in)) at an external Na+ concentration of 320 mM. The resulting sodium ion-motive force was - 194 mV and enabled D. sal exigens to accumulate sulfate 20000-fold at low external sulfate conce ntrations (<0.1 mu M). Under these conditions high sulfate accumulatio n occurred electrogenically in symport with three sodium ions (assumin g equilibrium with the sodium ion-motive force). With increasing exter nal sulfate concentrations sulfate accumulation decreased sharply, and a second, low-accumulating system symported sulfate electroneutrally with two sodium ions. The sodium-ion gradient was built up by electrog enic Na+/H+ antiport. This was demonstrated by (i) measuring proton tr anslocation upon sodium ion pulses, (ii) studying uptake of sodium sal ts in the presence or absence of the electrical membrane potential, an d (iii) the inhibitory effect of the Na+/H+ antiport inhibitor propylb enzilylcholin-mustard HC1 (PrBCM). With resting cells ATP synthesis wa s found after proton pulses (changing the pH by three units), but neit her after pulses of 500 mM sodium ions, nor in the presence of the unc oupler tetrachorosalicylanilide (TCS). It is concluded that the energy metabolism of the marine strain D. salexigens is based primarily on t he protonmotive force and a proton-translocating ATPase.