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

This article investigates the roles of carotenoids—photosynthetic pigments and antioxidants—in the growth and stress tolerance of the microalga Euglena gracilis under both light and dark conditions. Using chemical inhibition with norflurazon and genetically modified carotenoid-deficient strains (including the nonphotosynthetic SM-ZK and colorless cl4 strains), the study demonstrates that carotenoid deficiency delays growth not only in light but also in darkness, indicating light-independent functions. Additionally, carotenoids contribute to tolerance against mild low-temperature and high-light stresses, effects observed even in chloroplast-deficient strains, suggesting carotenoid roles beyond photosynthesis, potentially involving antioxidant activity and membrane stabilization. The research also reveals a posttranscriptional regulation of the phytoene synthase gene (EgcrtB), where norflurazon treatment increases its transcript levels but decreases protein abundance, highlighting complex regulation of carotenoid biosynthesis in E. gracilis.