Activation of Diadinoxanthin De-Epoxidase Due to a Chlororespiratory Proton Gradient in the Dark in the Diatom Phaeodactylum tricornutum

Abstract

An exponential dynamic light regime with prolonged dark periods (light/dark cycle 8/40 h) was used to simulate deep mixing of algae in natural waters and 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 photosynthetic efficiency and high non-photochemical quenching (NPQ) of chlorophyll fluorescence. Diatoxanthin concentrations and NPQ were low at the beginning of the dark period and increased with the duration of the dark incubation. Addition of the diadinoxanthin de-epoxidase inhibitor, DTT, prevented the formation of diatoxanthin, thereby excluding de novo synthesis of diatoxanthin during the prolonged dark period. Evidence of chlororespiratory electron flow 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 prevented the development of non-photochemical quenching of chlorophyll fluorescence and the formation of diatoxanthin during the dark period. Second, inhibition of the electron and proton consuming terminal redox component of chlororespiratory electron transport, cytochrome oxidase, by addition of KCN induced stronger NPQ and a higher de-epoxidation state of the xanthophyll cycle. These results strongly indicate that the activation of diadinoxanthin de-epoxidase in the dark is the consequence of a chlororespiratory proton gradient. We furthermore present evidence that diatoxanthin formed by the chlororespiratory proton gradient has the same efficiency in the mechanism of enhanced heat dissipation as diatoxanthin induced by a light-driven ΔpH.

Key words

Chlororespiration - xanthophyll cycle - prolonged darkness - non-photochemical quenching of chlorophyll fluorescence - dynamic light - Phaeodactylum tricornutum