In vivo carotenoid triplet formation in response to excess light: a supramolecular photoprotection mechanism revisited.

Carotenoids have been known for their photoprotective role for about 50 years. However, despite many advances in laser flash photolysis, no photodynamic studies have been so far performed on whole cells to determine the harmful threshold of light. In the present work, we investigate the coupling between energy conversion and energy deactivation, in isolated complexes of RC-LH1 and LH2 increasingly integrated systems up to intact cells of the purple anaerobic photosynthetic bacterium Rubrivivax gelatinosus. A continuous light similar to the mean daily sun irradiance on the surface of the earth is found to saturate the in vivo electron transfer turnover and to give rise to carotenoid triplet formation. This accounts for the widespread use of carotenoids among phototrophic prokaryotes and emphasizes their essential protective role in the natural environment.