State transitions in Chlamydomonas reinhardtii.: The role of the Mehler reaction in state 2-to-state 1 transition

The light intensity-dependent transition to state 1 of dark-adapted anaerobic state 2 Chlamydomonas reinhardtii cells is stimulated by oxygen and by other electron acceptors for photosystem 1, such as oxaloacetate and methylviologen. This suggests that the transition to state I requires the oxidation of the intersystem chain by photosystem I photochemistry. On the other hand, the mere oxidation in the dark of the chain-by addition of O-2-leads only to a slow and incomplete transition. The light-driven stimulation by O-2 of the state 1 transition is saturated at an O-2 concentration of 15 to 20 mum, definitely higher than that of respiration. We suggest that this may represent the affinity for oxygen of the Mehler reaction, a conclusion that is confirmed by the observations that mitochondrial respiration is apparently not involved in modulating state 2-to-state 1 transition. The catalysis of the state 2-to-state 1 transition upon illumination of anaerobically adapted algae might represent, therefore, a relevant physiological role of this process in C. reinhardtii.