The role of respiration in the activation of photosynthesis upon illumination of dark adapted Chlamydomonas reinhardtii

It is reported that O₂ is required for the activation of photosynthesis in dark adapted Chlamydomonas reinhardtii in State 1, under low light intensity. The concentration of dissolved O₂ of ca. 9 µM is sufficient to saturate the requirement. When the concentration of O₂ is 3 μM or below, the activation of photosynthesis is strongly inhibited by myxothiazol, a specific inhibitor of the mitochondrial cytochrome bc₁. The effect of this inhibitor decreases as the O₂ concentration is raised, to disappear completely above 50 μM. Low concentrations of uncouplers delay the activation of photosynthesis, but do not inhibit it when steady state is reached. It is concluded that in State 1 C. reinhardtii mitochondrial respiration is required for the activation of photosynthesis upon illumination of dark adapted cells only when the concentration of O₂ is too low (less than 5 μM) to allow an appreciable activity of the Mehler reaction. The role of respiration does not seem to be due to the synthesis of ATP by oxidative phosphorylation, because photosynthesis activation is not sensitive to oligomycin.