Mechanistic Insights into the Impacts of Microplastics on the Fate of Mercury and Methylmercury in a Simplified Estuarine Model System

Abstract Estuaries are co-pollution hotspots for mercury (Hg) and microplastics (MPs), yet the role of MPs in microbial methylmercury (MeHg) production remains unclear. Using Geobacter sulfurreducens PCA as a model methylator, we investigated MP-Hg-microbe interactions in a controlled laboratory system. Both micron- and millimeter-scale MPs exhibited high inorganic Hg(II) adsorption (80%–97%) with limited desorption and a pronounced binding preference for Hg(II) over MeHg. Compared to MP-free controls, micron- and millimeter-scale MPs were associated with 83%–222% and 43%–143% higher MeHg accumulation, respectively, accompanied by size-dependent MeHg phase partitioning. MPs also leached bioavailable dissolved organic carbon (MP-DOC; 0.20–3.85 mg/g), which was preferentially metabolized by G. sulfurreducens PCA, and bacteria were retained on MP surfaces. These results suggest that MPs can function as conditional biogeochemical microreactors integrating localized Hg(II) enrichment, bioavailable carbon supply, and bacterial surface retention, thereby enhancing MeHg formation under the simplified conditions tested. However, the present design cannot distinguish an MP-specific carbon effect from a general labile carbon effect, nor can resolve the relative contributions of these pathways. These findings clarify the multipath mechanisms potentially governing MP-Hg coupling and underscore the need for validation under environmentally realistic conditions to evaluate coastal MeHg risks.