O2 reduction by photosystem I involves phylloquinone under steady-state illumination

•Rate of O2 reduction by PSI increases with increasing light intensity.•Quinone nature at the A1-site affects O2 reduction by PSI.•Phyllosemiquinone at the A1-site reduces O2. O2 reduction was investigated in photosystem I (PSI) complexes isolated from cyanobacteria Synechocystis sp. PCC 6803 wild type (WT) and menB mutant strain, which is unable to synthesize phylloquinone and contains plastoquinone at the quinone-binding site A1. PSI complexes from WT and menB mutant exhibited different dependencies of O2 reduction on light intensity, namely, the values of O2 reduction rate in WT did not reach saturation at high intensities, in contrast to the values in menB mutant. The obtained results suggest the immediate phylloquinone involvement in the light-induced O2 reduction by PSI.