Reduction of oxygen by the electron transport chain of chloroplasts during assimilation of carbon dioxide

Active Transport of Inorganic Carbon Increases the Rate of O2 Photoreduction by the Cyanobacterium Synechococcus UTEX 625

“…Quenching does not occur in the presence of DCMU which suggests that the quenching is due to oxidation of the acceptor QA of PSII (14). In the presence ofiodoacetamide, CO2 reduction cannot be a means for oxidation of QA (14, 16), but it is well established that 02 can also be reduced by noncycic electron transport (1,4,6 In this communication, we present evidence that an increase in qQ is the major reason for the drop in Chl a fluorescence yield resulting from DIC transport. We also show that DIC transport increases the rate of 02 photoreduction.…”

“…Quenching does not occur in the presence of DCMU which suggests that the quenching is due to oxidation of the acceptor QA of PSII (14). In the presence ofiodoacetamide, CO2 reduction cannot be a means for oxidation of QA (14,16), but it is well established that 02 can also be reduced by noncycic electron transport (1,4,6). Addition of 02 can cause Q-quenching (qQ2) even in the absence of CO2 (7).…”

“…All the available evidence indicates that the 02 uptake by Synechococcus UTEX 625 was due to a direct photoreduction process (4,18,22). The elegant demonstration by Patterson and Myers (18) of H202 production by intact Anacystis cells due to 02 photoreduction is especially relevant.…”

Effects of washing and osmotic shock on catalase activity of intact chloroplast preparations

“…Production of hydrogen peroxide during photosynthesis by intact chloroplasts in the absence of such inhibitors has nevertheless been reported [3], using washed chloroplast preparations apparently free of catalase. Inhibitory concentrations of hydrogen peroxide could be produced as a result of overloading of the peroxidase reactions [ 15,16] which under physiological conditions may ensure that the product of photosynthetic oxygen reduction is water rather than hydrogen peroxide.…”

“…Since catalase causes release of oxygen from hydrogen peroxide, an enhancement of net oxygen evolution on addition of catalase to illuminated chloroplasts indicates that photosynthetic reduction of oxygen has occurred. This effect has been demonstrated both in broken, washed chloroplasts [2] and in intact chloroplasts capable of CO2 fixation [3][4][5]. In the latter case endogenous catalase activity presents a difficulty [4] which cannot satisfactorily be overcome by addition of the catalase inhibitors, cyanide and azide, since these compounds also inhibit photosynthetic CO2 fixation [6].…”