Supplementary ultraviolet-B radiation induces a rapid reversal of the diadinoxanthin cycle in the strong light-exposed diatom Phaeodactylum tricornutum

A treatment of the diatom Phaeodactylum tricornutum with high light (HL) in the visible range led to the conversion of diadinoxanthin (Dd) to diatoxanthin (Dt). In a following treatment with HL plus supplementary ultraviolet (UV)-B, the Dt was rapidly epoxidized to Dd. Photosynthesis of the cells was inhibited under HL + UV-B. This is accounted for by direct damage by UV-B and damage because of the UV-B-induced reversal of the Dd cycle and the associated loss of photoprotection. The reversal of the Dd cycle by UV-B was faster in the presence of dithiothreitol, an inhibitor of the Dd de-epoxidase. Our results imply that the reversal of the Dd cycle by HL + UV-B was caused by an increase in the rate of gross Dt epoxidation, whereas the de-epoxidation of Dd was unaffected by UV-B. This is further supported by our finding that the in vitro de-epoxidation activity and the affinity toward the cosubstrate ascorbic acid of the Dd de-epoxidase were both unaffected by UV-B pretreatment of intact cells. We provide evidence that Dt epoxidation is normally down-regulated by a high pH gradient under HL. It is proposed that supplementary UV-B affected the pH gradient across the thylakoid membrane, which disrupted the down-regulation of Dt epoxidation and led to the observed increase in the rate of Dt epoxidation.