Light intensity influences carotenoid accumulation and modulates the expression of photosynthetic genes in Euglena sanguinea.

Euglena sanguinea (Ehrenberg 1831) is one of the earliest reported species within the genus Euglena. Its prolific proliferation leading to red algal bloom has garnered significant scientific attention due to its ecological and environmental impacts. 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 carotenoids content of E. sanguinea were observed to vary under different abiotic culture conditions, including varying temperatures, light intensities, potassium iodide and sucrose concentration. Notably, significant rapid accumulation of carotenoids in E. sanguinea was observed under continuous culture at a light intensity of 130 µmol m s . Furthermore, exposure to a strong light intensity resulted in changes in the activity of the antioxidant enzymes and malondialdehyde (MDA) content. Moreover, through de novo transcriptome sequencing and Gene Ontology (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. Among these, eight genes are associated with photosynthesis and chloroplast function, including upregulated genes encoding Photosystem II protein D1, Photosystem II protein K, and Cytochrome b6/f complex subunit V, as well as translation elongation factor EF-Tu; conversely, downregulated genes include those encoding cell wall-associated hydrolases and enzymes involved in carbon fixation in photosynthetic organisms. These findings provide foundational data for investigating the photoprotective mechanisms in E. sanguinea and serve as a reference for the regulatory factors involved in carotenoids biosynthesis within Euglena.