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

Abstract

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 30min. 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.