In Phaeodactylum tricornutum Photosystem II is unusually resistant to damage by exposure to high light intensities. Not only is the capacity to
dissipate excess excitations in the antenna much larger and induced more rapidly than in other organisms, but in addition an electron transfer cycle
in the reaction center appears to prevent oxidative damage when secondary electron transport cannot keep up with the rate of charge separations.
Such cyclic electron transfer had been inferred from oxygen measurements suggesting that some of its intermediates can be reduced in the dark
and can subsequently compete with water as an electron donor to Photosystem II upon illumination. Here, the proposed activation of cyclic
electron transfer by illumination is confirmed and shown to require only a second. On the other hand the dark reduction of its intermediates,
specifically of tyrosine YD, the only Photosystem II component known to compete with water oxidation, is ruled out. It appears that the cyclic
electron transfer pathway can be fully opened by reduction of the plastoquinone pool in the dark. Oxygen evolution reappears after partial
oxidation of the pool by Photosystem I, but the pool itself is not involved in cyclic electron transfer.
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