The Kok Effect in Chlamydomonas reinhardi

Abstract

A Haxo-Blinks rate-measuring oxygen electrode together with a modulated light source gave an average current signal (change in net O(2) exchange) and a modulated current signal (photosynthetic O(2) evolution). Using this apparatus, net O(2) exchange and photosynthetic O(2) evolution at low intensities have been studied in the green alga, Chlamydomonas reinhardi. At both 645 nm and 695 nm, the curves of net O(2) exchange as a function of light intensity were steeper at lowest intensities than about compensation, indicative of the Kok effect. The effect was greater at 695 nm than at 645 nm. The corresponding curves of photosynthetic O(2) evolution, on the other hand, showed no Kok effect; here, the slope was lowest at lowest intensity. The absence of the Kok effect in O(2) evolution, together with its sensitivity to monofluoroacetic acid, show that it is due to an interaction of photosynthesis and respiration. The effect was exaggerated by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. In the presence of concentrations of this inhibitor sufficient to inhibit O(2) evolution completely, a light-induced change in net O(2) exchange remained. This was interpreted as a system I dependent depression of respiratory O(2) uptake. The Kok effect remained undiminished in concentrations of carbonyl cyanide m-chlorophenylhydrazone and 2,4-dinitrophenol which partially uncoupled either oxidative phosphorylation alone or both oxidative and photosynthetic phosphorylations. The above results can be explained within a model of the Kok effect in which O(2) uptake is depressed by diversion of reductant away from respiratory electron transport and into photosystem I. The same photodepression of O(2) uptake also appears to account for a transient in net O(2) exchange seen in several algae upon turning off the light.