Fluorescence quenching in four unicellular algae with different light-harvesting and xanthophyll-cycle pigments

We examined the relationship between non-photochemical quenching (NPQ) and xanthophyll de-epoxidation in the unicellular algae Euglena gracilis, Ochromonas danica, Phaeodactylum tricornutum, and Dunaliella tertiolecta. Generally, low-light-grown algae had a smaller pool of xanthophyll-cycle pigments per chlorophyll than medium-light-grown grown cells, but they developed more NPQ during exposure to high light. Thus, lumen acidification was apparently lower in medium-light-grown cells in spite of the exposure to a photon flux density (PFD) three times the growth PFD. In darkness Dunaliella maintained a relatively large content of de-epoxidized xanthophylls, and NPQ developed without concomitant de-epoxidation in response to a 5-min exposure to high light. Violaxanthin de-epoxidation that occurred during longer exposures to light did not cause a further rise in NPQ in Dunaliella. In Ochromonas, NPQ and xanthophyll de-epoxidation increased simultaneously during a 15-min exposure to high light. A further rise in NPQ was not accompanied by xanthophyll de-epoxidation. In Phaeodactylum, the rise in NPQ and de-epoxidation were nearly linearly related during a 60-min exposure to high light. NPQ recovered quickly after darkening in these three algae and no significant photodamage occurred. In Euglena no xanthophyll-conversions and no quickly reversible NPQ occured in response to high light, suggesting that photodamage occurred. Dunaliella has similar light-harvesting and xanthophyll-cycle pigments as higher plants but the relationship between NPQ and DPS during the exposure to high light was different from the linear relationship that is commonly observed in plants. Conversely, Phaeodactylum, which has different light-harvesting and xanthophyll-cycle pigments, had a relationship similar to that in plants.