The peculiar NPQ regulation in the Pinguiphyte Phaeomonas challenge the xanthopyll cycle dogma

Photosynthetic organisms can experience fluctuating light conditions in their environment, and can develop different strategies to maintain a fine balance between light harvesting, photochemistry, and protection from excess photon absorption. One of those strategies is the high energy state quenching (or qE, a component of the Non Photochemical Quenching) which consists in the dissipation of excess light energy in the form of heat in the photosystem II antenna. This mechanism is tightly related to the xanthophyll cycle, which consists in the reversible deepoxidation of some carotenoids: violaxanthin into zeaxanthin or diadinoxanthin into diatoxanthin/dinoxanthin.
In the pinguiophyte Phaeomonas sp whose NPQ is strictly proportional tothe amount of zeaxanthin, the light regulation of this qE mechanism is very peculiar, displaying a very important quenching in the dark, which recovers under low to moderate light. This peculiar feature of the qE in Phaeomonas offers a unique opportunity to study the xanthophyll cycle, and to dissect its different regulators. We could show that, because of the existence of a transthylakoidal proton gradient in the dark, the deepoxydase is already active in the absence of a light-induced proton transport. At the contrary, the epoxidase activity was completely inactive in the dark, and slow under high light, but showed an optimum under low light conditions.