The kinetics of carotenoid absorption changes in intact cells of photosynthetic bacteria

The kinetics of carotenoid absorption changes have been measured in intact cells of Rhodopseudomonas capsulata after short flash excitation. The observed changes were consistent with the thesis that they indicate the development and dissipation of membrane potential. In the generation of the absorption changes in anaerobic cells, fast (complete in 0.5 ms) and slow (half-time 3 ms) components can be distinguished. The slow component corresponds kinetically to the rate of cytochrome c re-reduction and is similarly antimycin sensitive. These data are similar to those observed in isolated chromatophores which have been artifically poised with redox mediators. In aerobic intact cells the kinetic profile is altered, mainly because the decay of the carotenoid change is much faster. Inhibition of respiration with KCN leads to flash-induced changes similar to those in anaerobic cells. At least two components can be distinguished in the decay of the carotenoid absorption changes in anaerobic intact cells. Only the faster decay component was inhibited by venturicidin which suggests that it corresponds to H⁺ flux through the F₀F₁-ATPase during ATP synthesis. The contribution of the venturicidin-sensitive decay to the total decay was dependent upon the initial amplitude of the carotenoid absorption change produced by the flash group. This suggests that there is an apparent threshold of membrane potential for ATP synthesis. Supporting evidence was provided by the finding that venturicidin stimulated the steady-state light-induced carotenoid absorption change at high but not at low light intensities. The entire decay of the carotenoid absorption changes was stimulated by carbonyl cyanide p-trifluoromethoxyphenylhydrazone in a manner that can be interpreted as an ionophore catalysing the dissipation of membrane potential.