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O₂ photoreduction by photosynthetic electron transfer, the Mehler reaction [1], was observed in all groups of oxygenic photosynthetic organisms [2–4], but the electron transport chain mediating this reaction remains unidentified. We provide the first evidence for the involvement of A-type flavoproteins that reduce O₂ directly to water in vitro. Synechocystis sp. strain PCC 6803 mutants defective in flv1 and flv3, encoding A-type flavoproteins, failed to exhibit O₂ photoreduction but performed normal photosynthesis and respiration. We show that the light-enhanced O₂ uptake was not due to respiration or photorespiration. After dark acclimation, photooxidation of P₇₀₀ was severely depressed in mutants Δflv1 and Δflv3 but recovered after light activation of CO₂ fixation, which gives P₇₀₀ an additional electron acceptor. Inhibition of CO₂ fixation prevented recovery but scarcely affected P₇₀₀ oxidation in the wild-type …