Oxygen-exchange studies in Chlamydomonas mutants deficient in photosynthetic electron transport: Evidence for a Photosystem II-dependent oxygen uptake in vivo

Photosynthetic oxygen exchange has been measured using ¹⁸O₂ and the mass-spectrometric technique in two mutant strains of Chlamydomonas reinhardtii deficient in electron transport. In the F15 mutant, deficient in PS I, O₂ was evolved in the light at a constant rate of about 145 nmol O₂/min per mg chlorophyll. At the same time, O₂ uptake was increased in the light by about 28%. O₂ evolution and the light-stimulation of O₂ uptake were inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Antimycin A and salicylhydroxamic acid, both inhibitors of mitochondrial respiration, when added together, inhibited dark respiration and also the light-dependent O₂ evolution by about 80%. Similar properties were observed in a mutant strain of Chlamydomonas (F18) lacking the cytochrome b₆-f complex. We conclude from these results that in the absence of active Photosystem I, a permanent electron flow can occur in the light from Photosystem II to molecular O₂. This electron transfer pathway would involve the plastoquinone pool and the mitochondrial electron transport chain. Because O₂ evolution measured in the F15 mutant was severely inhibited by the uncoupler cyanide m-chlorophenylhydrazone, we propose that an energy-dependent reverse electron transfer similar to that of Rhodospirillaceae might occur in the chloroplast of Chlamydomonas.