A Haxo-Blinks rate-measuring oxygen electrode together with a modulated light source gave an average current signal (change in net Oa exchange) and a modulated current signal (photosynthetic 02 evolution). Using this apparatus, net 02 exchange and photosynthetic 02 evolution at low intensities have been studied in the green alga, Chlamydomonas reinhardi. At both 645 nm and 695 nm, the curves of net 02 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 02 evolution, on the other hand, showed no Kok effect; here, the slope was lowest at lowest intensity. The absence of the Kok effect in 02 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 (24) and of Jones and Myers (27) show that preferential activation of photosystem I exaggerates the Kok effect. Activation of system I in excess of activation of system II could depress respiratory 02 uptake in at least two ways. Hoch et al. (24) proposed that cyclic phosphorylation driven by system I could raise the ratio of cellular ATP to ADP sufficiently to restrict respiratory electron transport. Alternately, preferential activation of system I could shift the flow of respiratory reductant away from respiratory electron transport and into system I of photosynthetic electron transport, as suggested by Jones and Myers (27). Recent observations on anaerobic, DCMU3-resistant "4CO2 fixation by Chlamydomonas reinhardi (47), and on photoevolution of H2 by Scenedesmus obliquus (45) and Chlamydomonas moewusii (18) show that the latter pathway may operate in anaerobic cells. The observation of a marked Kok effect in C. moewusii and C. reinhardi led to a study of the phenomenon in the latter alga in an attempt to gain a fuller understanding of its mechanism.
MATERIALS AND METHODS