The PRI is sensitive to changes in carotenoid pigments (e.g. xanthophyll pigments) in live foliage. It is a xanthophyll cycle pigment, an oil-soluble alcohol within the xanthophyll subgroup of carotenoids. Its yellow-orange pigmentation is due to large concentrations of carotenoids, particularly xanthophyll. Also present were minor amounts of xanthophyll, a molecule structurally related to the carotenes. Carotenoids such as xanthophyll astaxanthin allow "Trebouxia" to tolerate high irradiance. The ("R")-enantiomer is to be expected if 3-hydroxyretinal is produced directly from xanthophyll carotenoids. Dinoxanthin is a type of xanthophyll found in dinoflagellates. Diatoxanthin is a type of xanthophyll found in phytoplankton and diatoms. It is important in the xanthophyll cycle. Capsanthin is a natural red dye of the xanthophyll class. Capsorubin is a natural red dye of the xanthophyll class. See xanthophyll cycle for this topic. Light stress can be tolerated by dissipating excess energy as heat through the xanthophyll cycle. Fucoxanthin is a xanthophyll, with formula CHO. Echinenone is a xanthophyll, with formula CHO.