Protonmotive force in freshwater sulfate-reducing bacteria, and its role in sulfate accumulation in Desulfobulbus propionicus.

The protonmotive force in several sulfate-reducing bacteria has been determined by means of radiolabelled membrane-permeant probes (tetraphenylphosphonium cation, TPP+, for delta psi, and benzoate for delta pH). In six of ten freshwater strains tested only the pH gradient could be determined, while the membrane potential was not accessible due to nonspecific binding of TPP+. The protonmotive force of the other four strains was between -110 and -155 mV, composed of a membrane potential of -80 to -140 mV and a pH gradient between 0.25 and 0.8 (inside alkaline) at pH(out) = 7. In Desulfobulbus propionicus the pH gradient decreased with rising external pH values. This decrease, however, was compensated by an increasing membrane potential. Sulfate, which can be highly accumulated by the cells, did not affect the protonmotive force, if added in concentrations of up to 4 mM. The highest sulfate accumulation observed (2500-fold), which occurred at external sulfate concentrations below 5 microM, could be explained by a symport of three protons per sulfate, if equilibrium with the protonmotive force was assumed. At higher sulfate concentrations the accumulation decreased and suggested an electroneutral symport of two protons per sulfate. At sulfate concentrations above 500 microM, the cells stopped sulfate uptake before reaching an equilibrium with the protonmotive force.