Inhibition of photosynthetic sulfide oxidation by organic cations

Photosynthetic sulfide oxidation by the purple sulfur bacterium Chromatium vinosum is strongly inhibited by the organic cations benzyl viologen ( BV2 +) and tetraphenylphosphonium (TPP+) at micromolar concentrations. Both are much more inhibitory at pH 8 than at pH 7. Inhibition probably results from uptake of benzyl viologen and tetraphenylphosphonium in response to an electrical potential gradient across the plasma membrane which increases in magnitude with increasing pH. Although both compounds appear to have the same mode of action, the biochemical mechanism of their inhibition remains to be determined.