LIGHT-INDUCED FLUORESCENCE QUENCHING AND LOSS OF PHOTOCHEMISTRY IN CHROMATOPHORES OF PHOTOSYNTHETIC PURPLE BACTERIA

Exposure of chromatophores of Rhodobacter capsulatus to a high light intensity (10 000 mu-E m-2 s-1) in the presence of oxygen results in non-photochemical quenching of bacteriochlorophyll fluorescence and a decrease in the quantum yield of charge separation at the reaction centre. Several observations support the hypothesis that this is due to the formation of a quencher in the light-harvesting antennae. The relationship between quenching of the maximum fluorescence yield F(m) and the dark-level fluorescence yield F0 fits theoretical predictions for antenna quenching, while the decrease in photochemical quantum yield is proportional to the decrease in F(v)/F(m) (where F(v) = F(m) - F0. The number of functional reaction centres appears to be unchanged. No change in redox dependence of charge separation can be detected, suggesting that no damage of the reaction centre occurs. It is suggested that the quencher is formed by the oxidation of some species. Consistent with this, quenching is inhibited by the removal of oxygen. Aerobic quenching can be observed in mutants lacking either the outer light-harvesting complex LHII or both the reaction centre and the inner light-harvesting complex LHI indicative of quenching throughout the light-harvesting apparatus. Under anaerobic conditions there is still some light-dependent inhibition of quantum yield of charge separation by a distinct process that does not effect F(v)/F(m). The mechanism of this process is unknown.