O₂ Uptake in the Light in Chlamydomonas

Abstract

The nature of the process responsible for the stationary O₂ uptake occurring in the light under saturating CO₂ concentration in Chlamydomonas reinhardii has been investigated. For this purpose, a mass spectrometer with a membrane inlet system was used to measure O₂ uptake and evolution in the algal suspension. First, we observed that the O₂ uptake rate was constant (about 0.5 micromoles of O₂ per milligram chlorophyll per minute) during a light to dark transition and was not affected by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Salicylhydroxamic acid had no effect on O₂ uptake in the dark or in the light, but was found to have the same inhibitory effect either in the dark or in the light when added to cyanide-treated algae. The stimulation of the O₂ uptake rate due to the uncoupling effect of carbonyl cyanide m-chlorophenylhydrazone was about the same in the dark or in the light. From these results, we conclude that mitochondrial respiration is maintained during illumination and therefore is not inhibited by high ATP levels. Another conclusion is that in conditions where photorespiration is absent, no other light-dependent O₂ uptake process occurs. If Mehler reactions are involved, in Chlamydomonas, under conditions where both photosynthetic carbon oxidation and reduction cycles cannot operate (as in cyanide-treated algae), their occurrence in photosynthesizing algae either under saturating CO₂ concentration or at the CO₂ compensation point appears very unlikely. The comparison with the situation previously reported in Scenedesmus (R. J. Radmer and B. Kok 1976 Plant Physiol 58: 336-340) suggests that different O₂ uptake processes might be present in these two algal species.