In vivo redox poising of the cyclic electron transport system of Rhodobacter capsulatus and the effects of the auxiliary oxidants, nitrate, nitrous oxide and trimethylamine N-oxide, as revealed by multiple short flash excitation

Intact cells of Rhodobacter capsulatus in the presence of myxothiazol were exposed to trains of short flashes of saturating light and the pattern of the absorbance changes due to P870, cytochrome (c₁ + c₂) and the carotenoids that report on the membrane potential were monitored. Myxothiazol inhibits cyclic electron transport and therefore the extent to which electron donors and acceptors of the reaction centre are available for photochemistry is revealed. In darkened anaerobic suspensions of cells in the presence of myxothiazol, only the first two flashes in the train led to charge separation in the photosynthetic reaction centres. The results indicated that the quinone pool and quinone bound at the Q_B site in the reaction centre were extensively reduced and quinone bound at Q_A was partly reduced before initiation of flash excitation. Thus under these conditions, and in the absence of myxothiazol, cyclic electron transport would be restricted. In the presence of oxygen or the auxiliary oxidants trimethylamine N-oxide, NO^-₃ or N₂O, the oxidation/reduction reactions and the electrochromic absorbance changes suggested that the pool and reaction centre quinones became more oxidised. Thus, the system was poised at a potential more conducive to optimal rates of photosynthetic electron transport. By reference to experiments on the growth of Rb. capsulatus (Richardson, D.J., King, G.F., Kelly, D.J., McEwan, A.G., Ferguson, S.J. and Jackson, J.B. (1988) Arch. Microbiol. 150, 131-137), it is argued that redox poising by the auxiliary oxidants is physiologically important, especially at low light intensities. Flash train experiments reveal that over-reduction of the quinones is more severe with succinate as a carbon source than with malate and this accounts for the observation that the rate of growth on succinate is decreased more strongly at low light intensities.