A new multicomponent NPQ mechanism in the diatom Cyclotella meneghiniana

In the present study we report that in the diatom Cyclotella meneghiniana the diatoxanthin-dependent non-photochemical quenching of chlorophyll fluorescence (NPQ) is heterogeneous and consists of three different components. (i) A transient NPQ component that generates immediately upon illumination, depends on the transthylakoid proton gradient as well as on the light intensity, and is modulated by the initial diatoxanthin content of the cells. It is located in the antenna complexes of C. meneghiniana and is comparable with the transient NPQ observed in vascular plants. (ii) A steady-state NPQ component is observed during later stages of the high-light illumination and depends on the diatoxanthin content formed by the light-activated diadinoxanthin cycle. (iii) A fast relaxing NPQ component is seen upon a transition of high-light-illuminated cells to complete darkness. This component relaxes within a time frame of tens of seconds and its extent is correlated with the amount of diatoxanthin formed during the phase of actinic illumination. It cannot be observed in dithiothreitol-treated cells where the de-epoxidation of diadinoxanthin to diatoxanthin is suppressed. The fast relaxing component can be interpreted as a relaxation of part of the steady-state NPQ. The different diatoxanthin-dependent components are characterized by different quenching efficiencies of diatoxanthin. Diatoxanthin involved in the transient NPQ exhibits a 2-fold higher quenching efficiency compared with diatoxanthin participating in the steady-state NPQ. It is proposed that the different quenching efficiencies of diatoxanthin are caused by the existence of different diatoxanthin pools within the antenna system of C. meneghiniana.