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

In the light, the Mn4CaO5 complex of Photosystem II (PSII) splits water producing O2 and the triplet state of the primary donor (3P680) of PSII generates reactive singlet oxygen (1O2). We show that nascent O2 is not converted to 1O2, but originates exclusively from ambient O2, 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 O2 out of the reach of 3P680. This may have been crucial during evolution of oxygenic photosynthesis, as 3P680 cannot be quenched by carotenoids that protect non- oxygenic photosystems. In addition, the data indicate that a 1O2-independent mechanism contributes to the light-induced damage of PSII.