High-intensity illumination induces the over-accumulation of carotenoids by modulates the expression of photosynthetic genes in Euglena sanguinea

Euglena sanguinea (Ehrenberg 1831) has recently gained prominence due to its bloom events and associated red tide phenomena. Despite this, research on E. sanguinea remains relatively sparse. In this study, we isolated and purified algal strains collected from the water of the Shanghai Botanical Garden, identifying them as E. sanguinea based on 16S and 23S rDNA sequence alignment. The cellular density and carotenoid content of E. sanguinea were observed to vary under different abiotic culture conditions, including varying temperatures, light intensities, potassium iodide, and sucrose. Notably, the most significant rapid accumulation of carotenoids in E. sanguinea was observed under continuous culture at a light intensity of 6000 lux. Furthermore, exposure to a strong intensity resulted in changes in the activity of the antioxidant enzymes and MDA content. Moreover, through de novo transcriptome sequencing and GO analysis of E. sanguinea cultured under different light intensities, we identified a total of 111 differentially expressed genes (DEGs), comprising 44 upregulated and 67 downregulated genes. The up-regulated DEGs included three genes encoding photosystem II protein D1, photosystem II protein K, and cytochrome b6/f complex subunit V. Therefore, we hypothesize that light stress regulates photosynthesis in E. sanguinea by altering the expression of these DEGs, thereby regulating the process of the photosynthetic electron transport and, consequently, the accumulation of photosynthetic pigments in vivo. These findings provide foundational reference data for investigating the photoprotective mechanisms in E. sanguinea and offer a theoretical basis for exploring carotenoid synthesis pathways in plants.