Activation of diadinoxanthin de-epoxidase due to a chlororespiratory proton gradient in the dark in the diatom Phaeodactylum tricornutum

An exponential dynamic light regime with prolonged dark periods (light/dark cycle 8/40h) was used to simulate deep mixing of algae in natural waters a nd to investigate the adaptation of the diatom Phaeodactylum tricornutum to these extreme light conditions. After prolonged dark periods Phaeodactylum cells showed surprisingly high contents of diatoxanthin, low photosyntheti c efficiency and high non-photochemical quenching (NPQ) of chlorophyll fluo rescence. Diatoxanthin concentrations and NPQ were low at the beginning of the dark period and increased with the duration of the dark incubation. Add ition of the diadinoxanthin de-epoxidase inhibitor, DTT, prevented the form ation of diatoxanthin, thereby excluding de novo synthesis of diatoxanthin during the prolonged dark period. Evidence of chlororespiratory electron fl ow and the establishment of a diadinoxanthin de-epoxidase activating proton gradient in the dark was derived from two observations. First, uncoupling of electron transport with NH4Cl at the beginning of the dark period preven ted the development of non-photochemical quenching of chlorophyll fluoresce nce and the formation of diatoxanthin during the dark period. Second, inhib ition of the electron and proton consuming terminal redox component of chlo rorespiratory electron transport, cytochrome oxidase, by addition of KCN in duced stronger NPQ and a higher de-epoxidation state of the xanthophyll cyc le. These results strongly indicate that the activation of diadinoxanthin d e-epoxidase in the dark is the consequence of a chlororespiratory proton gr adient. We furthermore present evidence that diatoxanthin formed by the chl ororespiratory proton gradient has the same efficiency in the mechanism of enhanced heat dissipation as diatoxanthin induced by a light-driven Delta p H.