Polystyrene nanoplastics at predicted environmental concentrations enhance the toxicity of copper on Caenorhabditis elegans

Excessive nanoplastics not only pose a direct threat to the environment but also have the propensity to adsorb and interact with other pollutants, exacerbating their impact. The coexistence of nanoplastics and heavy metals in soils is a prevalent phenomenon. However, limited research existed about the joint effects of the two contaminants on soil organisms. In this paper, we ascertained the combined toxicity of polystyrene nanoplastics (PS-NPs) and copper (Cu²⁺) on soil organisms (Caenorhabditis elegans) at quantities that were present in the environment, further exploring whether the two toxicants were synergistic or antagonistic. The outcomes manifested that single exposure to low-dose PS-NPs (1 μg/L) would not cause significant damage to nematodes. After treatment with PS-NPs and Cu²⁺, the locomotion ability of nematode was impaired, accompanied by an elevation in reactive oxygen species (ROS) level and a biphasic response in antioxidant enzyme activity. Moreover, combined exposure to PS-NPs and Cu²⁺ induced the mRNA up-regulation of vit-6, cyp-35a2, hsp-16.2, age-1, and cep-1, both of which were stress-related genes. The comparative analysis between groups (with or without PS-NPs) revealed that the combined exposure group resulted in significantly greater toxic effects on nematodes compared with Cu²⁺ exposure alone. Furthermore, the addition of PS-NPs influenced the metabolic profiles of Caenorhabditis elegans under Cu²⁺ stress, with numerous differential metabolites associated with oxidative damage or defense mechanism. Overall, these findings manifested that PS-NPs at the expected environmental concentration elevated Cu²⁺ toxicity on nematodes.