Inhibition of a respiratory activity by short saturating flashes in Chlamydomonas : Evidence for a chlororespiration

Excitation with short actinic flashes (2 μs) of oxygenated dark-adapted Chlamydomonas cells deposited on a bare O 2 platinum electrode induces an increase of the amperometric signal after the first two flashes. Mass spectrometer experiments performed in the presence of 18 O 2 showed that this signal was not due to the photolysis of water (H 2 16 O). The insensitivity of this signal to 10 μM DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea), its stimulation by acetate or high O 2 concentration as well as its inhibition by cyanide indicate that these flash-induced changes in O 2 concentration were related to the inhibition of a respiratory process. Because this rather fast inhibition of respiration is insensitive to antimycin A and to salicyl hydroxamic acid, inhibitors of mitochondrial respiration, and because it occurs on a single flash illumination, we conclude that the related respiratory activity takes place inside the chloroplast (chlororespiration) and not in the mitochondria. This interpretation is confirmed by the quite high K m(O 2) of this process (about 23 μM) compared to those measured for the mitochondrial reactions (0.2 μM for the cytochrome oxidase pathway and 5.5 μM for the alternative pathway). In a mutant lacking Photosystem I activity, no photoinhibition of respiration was observed. We conclude from the above results that the light-induced inhibition of chlororespiration is due to the oxidation by Photosystem I activity of electron carriers common to both photosynthetic and chlororespiratory chains.