Chemical and genetic carotenoid deficiency delays growth in dark-grown Euglena gracilis.

Light-independent functions of carotenoids in photosynthetic organisms are poorly understood. Here we investigated the growth properties of microalga, Euglena gracilis, under altered light and temperature using norflurazon-treated carotenoid-deficient cells and genetically modified strains, including non-photosynthetic SM-ZK and colorless cl4. Norflurazon treatment decreased carotenoid and chlorophyll contents, causing cell bleaching. SM-ZK strain had lower carotenoid content than WT strain, and it was below the detectable level in the cl4 strain. Norflurazon treatment decreased phytoene synthase EgCrtB levels, although EgcrtB was transcriptionally induced. Carotenoid deficiency in norflurazon-treated cells and the cl4 strain caused similar extent of delayed growth under light and dark conditions at 25 °C, indicating that carotenoids promote growth in darkness. Both WT and SM-ZK strains exhibited similar growth rates. Dark conditions at 20 °C enhanced growth delay of norflurazon-treated cells and the cl4 strain. These results indicate that carotenoids impart environmental stress tolerance to E. gracilis in light-dependent and -independent manners.