Changes in the Photosystem II complex associated with hydrogen formation in sulfur deprived Chlamydomonas reinhardtii

Redox properties of the acceptor side of Photosystem II were studied during H2 gas production in cells of Chlamydomonas reinhardtii. Flash-induced variable fluorescence changes and thermoluminescence measurements were performed in wild type and Stm6 mutant cells during different stages of sulfur (S)-deprivation. Analysis of the fluorescence decay kinetics indicated that the forward electron transfer on the acceptor side of Photosystem II was dramatically slowed down during the O2 evolution and O2 consumption stages and was completely blocked in the anaerobic stage of S-deprivation, thus, indicating a complete reduction of the PQ-pool. During the H2 formation stage, the forward electron transfer kinetics in the μsec and msec time scale re-appeared indicating partially restored electron flow from QA− to QB and the PQ-pool. Thermoluminescese measurements fully confirmed the fluorescence kinetic analysis. Activation of hydrogenase in the H2 formation stage is responsible for re-oxidation of the PQ pool and reactivation of the electron flow which was found to be faster and more efficient on the Stm6 mutant due to the higher amount of functionally preserved Photosystem II.