Thylakoid membranes of red algae contain water-soluble membrane-bound complexes - phycobilisomes (PBSs) serving as peripheral antennae for photosystem II (PS II). Strong light absorbed by the PBSs triggers a fast formation of transthylakoid ΔpH that follows the non-photochemical quenching of chlorophyll (Chl) fluorescence. The ΔpH build-up seems to be essential for photoprotecting the photosynthetic apparatus in the absence of xantophyll cycle common to higher plants. However, the mechanisms of this process are yet to be studied in detail. Here we report on study the Chl fluorescence quenching in unicellular red algae Rhodella violacea and its correlation with the ΔpH gradient being formed. The relation of this phenomenon to PS II photoprotection in the normal and high light-acclimated Rhodella cells is also examined. Under the photoinhibitory conditions (white light of 2000-3000 μE/m2s), the ΔpH -dependent Chl fluorescence quenching was found to delay the kinetics of PS II photoinhibition. The uncouplers like nigericin and NH4Cl are known to break down ΔpH gradient and lead to the dissipation of Chl fluorescence quenching followed by enhancing the PS II photoinhibition rate. The same effect showed far-red (FR) light transthylakoid ΔpH consumption. ATPase inhibitor (DCCD) having no impact on ΔpH didn’t influence PS 2 photoinhibition as well This implies the photoprotection to be fulfilled by the proton gradient rather than by ATP synthesis. Light acclimation of Rhodella cells to higher irradiances (500-1000 μE/m2s) results in a partial loss of the periphery phycoerhytrin-containing subunits by PBSs. Here we show that the light-acclimated cultures display a higher resistance to the photoinhibitory light than the non-acclimated ones. This could be explained by diminishing the energy transfer from the reduced PBSs to PS II and light screening by the secondary carotenoids synthesized during light exposure. Low-temperature (77K) fluorescence data allowed to evaluate the molecular mechanisms contributing to suppressing Chl fluorescence in Rhodella cells and its recovery in darkness. Key words: antenna reassembling, photoprotection, Rhodella violacea.
antenna reassembling, photoprotection, Rhodella violacea
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