Light-induced absorption changes in intact cells of Rhodopseudomonas Sphaeroides. Evidence for interaction between photosynthetic and respiratory electron transfer chains

Absorption changes, following a series of actinic flashes, linked to oxidoreduction states of ubiquinone, cytochrome c_t together with the carotenoid bandshift, have been measured for intact cells of Rhodopseudomonas sphaeroides under aerobic conditions. Binary oscillations are observed for these different contributions: (1) about one molecule of ubisemiquinone and fully reduced quinone are formed on odd and even flashes, respectively; (2) cytochrome c_t re-reduction is faster ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{≈ 50}\text{ms}$) after an even number of flashes than after an odd number; ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{≈ 100}\text{ms}$); (3) a slow-rising phase ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{≈ 5}\text{ms}$, antimycin A-insensitive) of the carotenoid bandshift is observed after each even flash. These results are compared to the respiratory activity of the cells under flash excitation and discussed in relation to a model, in which respiratory and photosynthetic electron chains interact at the level of cytochrome c₂ and where the terminal oxidase is supposed to have electrogenic properties.