Characterization of the reversible state of photoinhibition occurring in vitro under anaerobic conditions.

Thylakoid membranes were subjected to photoinhibitory illumination. The use of oxygen-consuming enzymes to obtain strictly anaerobic conditions showed that while the absence of oxygen is a prerequisite for the formation of a reversibly photoinhibited state, the presence of oxygen is required for the recovery in the dark. The formation of the reversibly photoinhibited state did not protect the thylakoid membranes against irreversible damage. The effects of both bicarbonate and formate were found to be qualitatively different for photoinhibition under strictly anaerobic conditions compared to the effects observed under normal aerobic photoinhibition. It is suggested that there are two different mechanisms of photoinhibition, occurring to different extent under aerobic and anaerobic conditions, respectively, involving QA in both cases, but the QB-site in the former only.