Exposure scenarios modulate the combined toxicity of nanoplastics and Cd to Daphnia magna

Nanoplastics (NPs) have extensively accumulated in aquatic environments and coexist with various contaminants including heavy metals. While the adverse effects of NPs and Cd on aquatic organisms have been widely reported, most studies focus on co-exposure scenarios, leaving the effect of different exposure sequences poorly understood. Here, we evaluated the combined toxicity of NPs and Cd to Daphnia magna under different exposure scenarios (single exposure, pre-exposure, and co-exposure). The bioaccumulation of NPs and Cd, intestinal histology, and gut microbiota were further analyzed to elucidate the underlying mechanisms of combined toxicity. Our findings reveal that environmentally relevant concentrations of NPs stimulated D. magna reproduction, increasing total number of offspring (hormesis). Pre-exposure to NPs enhanced tolerance to subsequent stressors, thus reducing the combined toxicity of NPs and Cd. In contrast, slow toxicological recovery of Cd caused persistent residual effects that amplified the toxicity of subsequent combined exposure to NPs and Cd. Specifically, compared to single-exposure groups, Cd pre-exposure led to a 5-fold and 3-fold increase in the bioaccumulation of NPs and Cd, respectively, accompanied by exacerbated intestinal damage and disruption of the gut microbiota. Our results highlighted the importance of evaluating combined toxicity under different exposure scenarios in natural environments, providing new insight into the potential risks of NPs in aquatic environments.