Thylakoid membrane energization and swelling in photoinhibited Chlamydomonas cells is prevented in mutants unable to perform cyclic electron flow

Photoinhibition of Photosystem II in unicellular algae in vivo is accompanied by thylakoid membrane energization and generation of a relatively high pH as demonstrated by ¹⁴C-methylamine uptake in intact cells. Presence of ammonium ions in the medium causes extensive swelling of the thylakoid membranes in photoinhibited Chlamydomonas reinhardtii but not in Scenedesmus obliquus wild type and LF-1 mutant cells. The rise in pH and the related thylakoid swelling do not occur at light intensities which do not induce photoinhibition. The rise in pH and membrane energization are not induced by photoinhibitory light in C. reinhardtii mutant cells possessing an active Photosystem II but lacking cytochrome b₆/f, plastocyanin or Photosystem I activity and thus being unable to perform cyclic electron flow around Photosystem I. In these mutants the light-induced turnover of the D1 protein of Reaction Center II is considerably reduced. The high light-dependent rise in pH is induced in the LF-1 mutant of Scenedesmus which can not oxidize water but otherwise possesses an active Reaction Center II indicating that PS II-linear electron flow activity and reduction of plastoquinone are not required for this process. Based on these results we conclude that photoinhibition of Photosystem II activates cyclic electron flow around Photosystem I which is responsible for the high membrane energization and pH rise in cells exposed to excessive light intensities.Abbreviations cyt b₆/f cytochrome b₆/f - Diuron 3-(3,4-dichlorophenyl)-1 dimethyl urea - Q_B the secondary quinone acceptor of reaction center II - DNP 2,4,Dinitrophenol - FCCP carbonyl cyanide trifluoromethoxy phenylhydrazone - SDS-PAGE sodium dodecylsulfate polyacrylamide gel electrophoresis