Photosystem II does not convert nascent oxygen to the poisonous singlet form

In the light, the Mn 4 CaO 5 complex of Photosystem II (PSII) splits water producing O 2 and the triplet state of the primary donor ( 3 P 680 ) of PSII generates reactive singlet oxygen ( 1 O 2 ). We show that nascent O 2 is not converted to 1 O 2 , but originates exclusively from ambient O 2 , indicating that the sensitivity of PSII to oxidative damage is not a consequence of the water-splitting per se , and showing that the suggested oxygen channels function nearly perfectly, conveying nascent O 2 out of the reach of 3 P 680 . This may have been crucial during evolution of oxygenic photosynthesis, as 3 P 680 cannot be quenched by carotenoids that protect non- oxygenic photosystems. In addition, the data indicate that a 1 O 2 -independent mechanism contributes to the light-induced damage of PSII.