Charge-dependent negative effects of polystyrene nanoplastics on Oryzias melastigma under ocean acidification conditions

Marine nanoplastics (NPs) have attracted increasing global attentions because of their detrimental effects on marine environments. A co-existing major environmental concern is ocean acidification (OA). However, the effects of differentially charged NPs on marine organisms under OA conditions are poorly understood. We therefore investigated the effects of OA on the embryotoxicity of both positively and negatively charged polystyrene (PS) NPs to marine medaka (Oryzias melastigma). Positively charged PS-NH2 exhibited slighter aggregation under normal conditions and more aggregation under OA conditions than negatively charged PS-COOH. According to the integrated biomarker approach, OA reversed the toxicity of positively and negatively charged NPs towards embryos. Importantly, at environmental relevant concentrations, both types of PS-NPs could enter the embryos through chorionic pores and then transfer to the larvae. OA reversed the internalization of PS-NH2 and PS-COOH in O. melastigma. Overall, the reversed toxicity of PS-NH2 and PS-COOH associated with OA could be caused by the reversed bioavailability of NPs to O. melastigma, which was attributed to altered aggregation of the NPs in acidified seawater. This finding demonstrates the charge-dependent toxicity of NPs to marine fish and provides new insights into the potential hazard of NPs to marine environments under OA conditions that could be encountered in the near future.