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

Photosynthetic organisms have developed pathways to safely dissipate excessive energy as heat. In eukaryotic cells the major pathway of energy dissipation requires acidification of the thylakoid lumen and is termed ‘energy-dependent non-photochemical quenching’ (NPQ). Lumen acidification also causes de-epoxidation of xanthophyll-cycle pigments. In higher plants, the extent of NPQ and the de-epoxidation state (DPS) of the xanthophyll-cycle show a linear correlation (1). Conversely, the inhibition of xanthophyll de-epoxidation by ditiothreithol (DTT) prevents a large part of NPQ (2). Therefore xanthophyll de-epoxidation is thought to be essential to a rise in NPQ although the mechanism of xanthophyll-dependent NPQ is still uncertain (3, 4). In order to further test the hypothesis that energy-dependent quenching is a function of the accumulation of de epoxidized carotenoids, we examined algae with different combinations of light-har-vesting and xanthophyll-cycle pigments (Table 1).