The role of auxiliary oxidants in the maintenance of a balanced redox poise for photosynthesis in bacteria

Carotenoid absorbance changes were used to monitor the development of membrane potential in intact cell suspensions of Rhodopseudomonas capsulata strain N22. Low concentrations of phenazine methosulphate almost completely inhibited the generation of membrane potential in the light by anaerobic cells. The light-dependent reactions were restored by addition of either trimethylamine N-oxide, dimethylsulphoxide, nitrous oxide, or oxygen. In Rhodopseudomonas capsulata strain N22 DNAR⁺ addition of nitrate was also effective. The inhibition by phenazine methosulphate and restoration by auxiliary oxidant were observed in the presence of sufficient rotenone to block NADH dehydrogenase or with low concentrations of uncoupling agent to dissipate the membrane potential under dark, anaerobic conditions. It is suggested that in intact cells of these organisms there are mechanisms which operate to maintain the electron-transport chain at an optimal redox poise for efficient photosynthesis. Phenazine methosulphate perturbs the optimal redox poise by hastening equilibrium of the photosynthetic electron-transport chain with low-potential couples in the cell. The addition of auxiliary oxidants restores the optimal redox poise. This suggests a role in photosynthesis for the pathways of respiratory electron flow to nitrate, nitrous oxide, trimethylamine N-oxide/dimethylsulphoxide and oxygen.