Toxicity and tolerance mechanism of binary zinc oxide nanoparticles and tetrabromobisphenol A regulated by humic acid in Chlorella vulgaris†

Recent studies have reported that nanoparticles (NPs) released into the aquatic environment may interact with persistent organic pollutants such as brominated flame retardants, whereas the environmental processes and toxicological impacts induced by such binary NPs require further specification. This study investigated the ultrastructural damage of Chlorella vulgaris triggered by exposure to zinc oxide (ZnO) NPs, tetrabromobisphenol A (TBBPA), ZnO–TBBPA, and ZnO–TBBPA–humic acid (HA), clarified the uptake and distribution of ZnO NPs in cells, and explored the physiological toxicity and tolerance mechanism. The results demonstrated that ZnO NPs induced irregular morphology in algal cells, and the disruption of the cellular ultrastructure by binary ZnO–TBBPA was also extremely severe due to the excessive uptake of ZnO NPs, which resulted in strong oxidative stress responses. In particular, the accumulation of reactive oxygen species further exacerbated the reduction of total chlorophyll content and algal density. Moreover, the cluster heat map and correlation analysis revealed that superoxide dismutase activity played a critical role in alleviating lipid peroxidation damage and enhancing the tolerance of algal cells to the stress of binary ZnO NPs. More notably, the existence of HA intensified the dispersion stability of NP suspensions and significantly mitigated the synergistic toxicity of binary ZnO–TBBPA. This study provides new insights into the environmental behavior and biological impacts of binary NPs in the natural environment.