High light acclimation of Chromera velia points to photoprotective NPQ.

It has previously been shown that the long-term treatment of Arabidopsis thaliana with the chloroplast inhibitor lincomycin leads to photosynthetic membranes enriched in antennas, strongly reduced in photosystem II reaction centers (PSII) and with enhanced nonphotochemical quenching (NPQ) (Belgio et al. Biophys J 102:2761-2771, 2012). Here, a similar physiological response was found in the microalga Chromera velia grown under high light (HL). In comparison to cells acclimated to low light, HL cells displayed a severe re-organization of the photosynthetic membrane characterized by (1) a reduction of PSII but similar antenna content; (2) partial uncoupling of antennas from PSII; (3) enhanced NPQ. The decrease in the number of PSII represents a rather unusual acclimation response compared to other phototrophs, where a smaller PSII antenna size is more commonly found under high light. Despite the diminished PSII content, no net damage could be detected on the basis of the Photosynthesis versus irradiance curve and electron transport rates pointing at the excess capacity of PSII. We therefore concluded that the photoinhibition is minimized under high light by a lower PSII content and that cells are protected by NPQ in the antennas.