Mercury methylation network on microplastics: Multipathway coupling, regulatory mechanisms, and environmental risks.

Microplastics (MPs) have emerged as pervasive environmental contaminants and potential regulators of mercury (Hg) biogeochemistry. Beyond serving as physical carriers, MPs can reshape the physicochemical and microbial microenvironments that control Hg transformation, thereby influencing the formation of methylmercury (MeHg). MPs influence Hg methylation through multiple coupled pathways. Surface adsorption and desorption processes determine the environmental distribution and mobility of Hg species, while aging-induced changes in functional groups on MPs further regulate their interactions with Hg. The plastisphere formed on MPs surfaces also provides ecological niches for Hg-methylating microorganisms, potentially stimulating microbial methylation processes. In addition, MPs-DOM mediated abiotic methylation pathways are evaluated. By integrating these processes, this study proposes a conceptual "Microplastics-Mercury Methylation coupling network" that highlights the coupled roles of MPs in physicochemical interactions, organic matter mediation, and microbial regulation. Based on these mechanisms, the potential environmental risks associated with this network are predicted, and possible mitigation strategies and future research directions are proposed.