ΔpH-dependent fluorescence quenching and its photoprotective role in the unicellular red alga Rhodella violacea

Plants have developed various photoprotective mechanisms to resist irradiation stress. One of the photoprotective mechanisms described in the literature for LHC2-containing organisms involves a down-regulation of photosystem (PS) 2 occurring simultaneously with the build-up of a proton gradient across the thylakoid membrane (Delta pH). It is often correlated with deepoxidation of xanthophylls located in LHC2. In Rhodophyta instead of LHC2, the peripheral antenna of PS2 consists of a large extramembrane complex, the phycobilisome (PBS), which transfers its excitation to the core antennae of PS2 composed of the CP43 and CP47 protein-chlorophyll complexes and there is no xanthophyll cycle. In the red alga Rhodella violacea a Delta pH-dependent chlorophyll (Chl) a fluorescence quenching can be formed. We characterised this quenching, studied the effects of various irradiances and inhibitors. Under photoinhibitory conditions, the Delta pH-dependent Chi fluorescence quenching exerts a photoprotective role and delays the kinetics of photoinhibition. It is the first time that such a photoprotective mechanism is described in PBS-containing organisms.