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 nonphotosynthetic SM-ZK and colorless cl4. Norflurazon treatment decreased carotenoid and chlorophyll contents, causing cell bleaching. SM-ZK strain had lower carotenoid content than wild-type (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 extents 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 the 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 light-independent manners.