Xanthophyll-related and xanthophyll-unrelated chlorophyll a fluorescence quenching in four unicellular algae with different light-harvesting and xanthophyll cycle pigments

Energy dissipation by Non-Photochemical Quenching (NPQ) and photosynthesis are known to decrease chlorophyll (chl) a fluorescence yield. Analysis of chl a fluorescence yield of PS II is commonly used to estimate NPQ.; An additional type of post-illumination fluorescence decrease, unrelated to NPQ, was observed in Dunaliella and to a much smaller extent in Ochromonas. This fluorescence decrease, termed RFD, occurred after light-induced NPQ had relaxed and was insensitive to the uncoupler nigericin. RFD correlated with a reduced plastoquinone pool and had characteristics similar to a state transition. Experiments with a variety of inhibitors suggested that (2P-) glycolate, a product of photorespiration, provides the electrons for the PQ pool reduction. Because RFD reduced the fluorescence yield by up to 60% in darkness it has important implications for fluorescence analysis.; NPQ in light was analyzed in Dunaliella tertiolecta, Euglena gracilis, Phaeodactylum tricornutum, and Ochromonas danica. NPQ is known to depend on lumen acidification, and it is thought that NPQ also depends on xanthophyll de-epoxidation in higher plants. To test whether there is a linear relationship between NPQ and xanthophyll de-epoxidation in other photosynthetic organisms, pigments were measured in these four algae which have different combinations of light-harvesting- and xanthophyll-pigments.; The light-induced rise in NPQ and xanthophyll de-epoxidation showed a linear correlation in Phaeodactylum and during the first, fast NPQ increase in Ochromonas. In contrast, NPQ developed without a concurrent xanthophyll de-epoxidation in Dunaliella and during the second, slow, quenching phase in Ochromonas. In the latter two algae NPQ may have depended on de-epoxidized xanthophylls that were present before the rise in NPQ. Then lumen acidification alone governed the kinetics of NPQ. There seemed to be a maximal content of de-epoxidized xanthophylls that was effective in NPQ in Dunaliella and Ochromonas. Euglena had little de-epoxidized xanthophylls and little NPQ. Neither the type of light-harvesting pigments nor the type of xanthophyll cycle determined the relationship between xanthophyll de-epoxidation and NPQ. Conversely, the green alga Dunaliella, which has similar pigments to higher plants, was most distinct from higher plants in regard to the relationship between NPQ and xanthophyll de-epoxidation, and in regard to the NPQ-independent RFD.