Repeated release of CeO2 nanoparticles altered algal responses: Growth, photosynthesis, and photosynthetic gene expression

The expanding production of engineered nanomaterials (ENMs) can eventually cause their increased release into and presence in aquatic ecosystems, potentially threatening the health of aquatic organisms and the stability of the ecological environment. Generally, ENMs are repeatedly released into real-world aquatic environments in relatively low concentrations, potentially affecting photosynthesis in primary producers such as algae. However, knowledge regarding the effects of repeated exposure to ENMs on algal photosynthesis is still lacking. Herein, the physiological responses of the freshwater algae Chlorella vulgaris following single and repeated exposures to cerium oxide nanoparticles (CeO2 NPs) were investigated at 10 mg/L, with a focus on photosynthesis. The results showed that repeated exposures triggered increased photosynthetic pigment contents, oxidative stress levels, decreased photosynthetic performance, and lower biomass in C. vulgaris, compared to a single exposure. Photosynthesis-related genes (i.e., petA, petB, psaA, atpB, and rbcL) were found to be upregulated following repeated exposures. Particularly for petB, repeated rather than single exposure treatment significantly upregulated its expression levels by 2.92 to 10.24-fold compared to unexposed controls. Furthermore, increased exposure times could aggravate the interaction between CeO2 NPs and algae, elevating 8.13%, 12.13%, and 20.51% Ce distribution on the algal cell surface or intracellularly, compared to a single exposure. This study is the first to investigate the effects of ENM exposure times on algal photosynthesis, providing new insights into the assessment of the risks these materials pose to real-world aquatic environments.