The Kok effect and the light‐inhibition of chlororespiration in Chlamydomonas reinhardtii

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Abstract
Measurements of O2 exchange have been performed as a function of light intensity in the green alga Chlamydomonas reinhardtii by using 18O2, and mass spectrometry. The Kok effect (i.e. a decrease in the slope of net O2 exchange curve) was observed at a light intensity of about 15 μE·m−2·s−1. This effect was due to the inhibition by light of O2 uptake. In conditions where mitochondrial respiration was inhibited by antimycin A and salicyl hydroxamic acid, illumination was found to reduce O2 uptake to the same extent as in the control experiment. We conclude that the Kok effect in Chlamydomonas is not the consequence of the inhibition of mitochondrial respiration, but is due to the inhibition of chlororespiration induced by photosystem I activity.