Co-exposure to polystyrene nanoplastics and mercury synergistically exacerbates toxicity in rare minnow (Gobiocypris rarus) compared to individual exposures.

Nanoplastics (NPs) and mercury (Hg) are ubiquitous pollutants that co-occur in aquatic ecosystems. However, the interaction between NPs and Hg, particularly whether NPs affect the accumulation and in vivo biotransformation of Hg in aquatic organisms, remains unclear. The toxicity of NPs and mercuric chloride (HgCl), both individually and in combination at environmentally relevant concentrations, on rare minnow (Gobiocypris rarus) were investigated in this study. The results demonstrated that NPs increased total Hg accumulation by 33.33 % but had limited effects on methylmercury (MeHg) content and its proportional distribution in muscle tissue compared to single Hg exposure. Both NPs and Hg induced significant growth inhibition, intestinal damage, oxidative stress, and inflammatory responses in rare minnow, with endpoint-specific effect patterns. Moreover, NPs and Hg dramatically altered gut microbiota composition and co-occurrence networks, with NPs inducing more metabolic pathway changes than Hg. Notably, combined exposure exacerbated almost all toxic effects in rare minnow compared to individual exposures, indicating synergistic interactions between NPs and Hg. These findings highlight the need to consider co-existing contaminants when evaluating NP toxicity.