Chromatophores prepared from C. [vinosum] exhibit a light-dependent O2 uptake in the presence of reduced 2,6-dichlorophenolindophenol, the maximum rate observed being 10.8 .mu.mol (mg of Bchl) [bacteriochlorophyll])-1 h-1 (air-saturated condition). The uptake of O2 was markedly inhibited by superoxide dismutase, indicating that O2 is subject to light-dependent monovalent reduction, resulting in the formation of the superoxide anion radical (O2-). By coupling baker''s yeast transketolase with illuminated chromatophore preparations, [U-14C]fructose 6-phosphate (6-P) is oxidatively split to produce glycolate, and the reaction was markedly inhibited by superoxide dismutase and less strongly by catalase. A coupled system containing yeast transketolase and xanthine plus xanthine oxidase showed a similar oxidative formation of glycolate from [U-14C]fructose 6-P. Apparently, photogenerated O2- serves as an oxidant in the transketolase-catalyzed formation of glycolate from the .alpha.,.beta.-dihydroxyethyl (C2) thiamine pyrophosphate complex, but H2O2 is not an efficient oxidant. The rate of glycolate formation in vitro utilizing O2- does not account for the in vivo rate of glycolate photosynthesis in C. vinosum cells exposed to an O2 atmosphere (10 .mu.mol (mg of Bchl)-1 h-1). The enhancement of glycolate formation by the autoxidizable electron acceptor methyl viologen in C. vinosum cells in O2, and the strong suppression by 1,2-dihydroxybenzene-3,5-disulfonic acid (Tiron), an O2- scavenger suggest that O2- is involved in the light-dependent formation of glycolate in vivo.