Making waves: Exploring scenarios of abiotic mercury methylation in nature.

The transformation of inorganic mercury (Hg) into neurotoxic methylmercury (MeHg) is critical for Hg's entry into aquatic food chains and its subsequent threats to wildlife and human health. While microbial Hg methylation is well recognized, growing evidence suggests that abiotic Hg methylation could be a significant yet overlooked source of MeHg. However, extrapolating laboratory-derived abiotic Hg methylation data to real-world conditions remains challenging. To address current knowledge gaps and guide future research, we examine the environmental distribution of three critical abiotic Hg methylation determinants-methyl donors (D), Hg species (S), and reaction energy (E) (e.g., light, heat)-and identify potential D-S-E co-occurrence hotspots in aquatic systems and their interconnected systems. By synthesizing theoretical, experimental, and field evidence under the D-S-E framework, we suggest that abiotic Hg methylation exhibits widespread occurrence across diverse environmental compartments and matrices, with pronounced contributions in systems conducive to abiotic pathways or unfavorable for microbial activity. We thus propose a research framework for abiotic Hg methylation based on the D-S-E nexus, requiring a fundamental revision of Hg biogeochemical cycling paradigms to incorporate abiotic methylation processes for improved MeHg risk prediction and management.