Net light-induced oxygen evolution in photosystem i deletion mutants of the cyanobacterium Synechocystis sp. PCC 6803

Oxygenic photosynthesis in cyanobacteria, algae, and plants requires photosystem II (PSII) to extract electrons from H₂O and depends on photosystem I (PSI) to reduce NADP⁺. Here we demonstrate that mixotrophically-grown mutants of the cyanobacterium Synechocystis sp. PCC 6803 that lack PSI (ΔPSI) are capable of net light-induced O₂ evolution in vivo. The net light-induced O₂ evolution requires glucose and can be sustained for more than 30 min. Utilizing electron transport inhibitors and chlorophyll a fluorescence measurements, we show that in these mutants PSII is the source of the light-induced O₂ evolution, and that the plastoquinone pool is reduced by PSII and subsequently oxidized by an unidentified electron acceptor that does not involve the plastoquinol oxidase site of the cytochrome b₆f complex. Moreover, both O₂ evolution and chlorophyll a fluorescence kinetics of the ΔPSI mutants are highly sensitive to KCN, indicating the involvement of a KCN-sensitive enzyme(s). Experiments using ¹⁴C-labeled bicarbonate show that the ΔPSI mutants assimilate more CO₂ in the light compared to the dark. However, the rate of the light-minus-dark CO₂ assimilation accounts for just over half of the net light-induced O₂ evolution rate, indicating the involvement of unidentified terminal electron acceptors. Based on these results we suggest that O₂ evolution in ΔPSI cells can be sustained by an alternative electron transport pathway that results in CO₂ assimilation and that includes PSII, the platoquinone pool, and a KCN-sensitive enzyme.