Toxicity Mechanism of Silver Nanoparticles on Different Algae: Evidence of Ag⁺ Release and Metabolic Responses

The toxicity of silver nanoparticles (AgNPs) on algae has been extensively studied, but the role of the cell wall in algal responses is still unclear. Therefore, two typical algae, Euglena sp. without a cell wall and Navicula sp. with a rigid cell wall, were selected to explore the role of the cell wall and different toxicity mechanisms of AgNPs using conventional toxicity endpoints and metabolomic analysis. Results showed that Navicula sp. was more sensitive to AgNPs than Euglena sp., with a 3 d EC50 of 36.36 μg L–1. Oxidative damage was not observed in the two algae when exposed to 50 μg L–1 AgNPs, but the activities of antioxidant enzymes in Euglena sp. were significantly increased, implying that the algal defense system was activated. Further analysis indicated that the toxicity of AgNPs on Navicula sp. was mainly mediated by released silver ions, which may be closely related to the presence of diatom frustules. Metabolomic analysis revealed that oxidative stress still existed in Navicula sp. exposed to 50 μg L–1 AgNPs, suggesting that some invisible changes were observed. Moreover, metabolites reflecting the level of hydrogen peroxide also decreased, indicating that oxidative stress was primarily caused by the superoxide anion. In addition, the increased metabolite indoleacetic acid may affect the algal shape, explaining why the diatom frustules played an important role in the toxicity of AgNPs. In contrast, the disruption of energy metabolism in Euglena sp. appeared to be the major cause of AgNP toxicity. Perturbations in algal metabolic pathways also suggested that AgNPs may affect RNA biosynthesis in algal cells. These findings provide insight into understanding the risk assessment of AgNPs in the aquatic environments.