Mercury Sulfide Nanoparticles Constitute a Long-Term Bioavailable Pool of Mercury in Soil-Rice Systems

Mercury sulfide nanoparticles (HgS_NPs) are widespread yet poorly understood Hg species in contaminated areas. Here, we present a 2.5-year simulated experiment tracking the persistence, transformation, and bioavailability of HgS_NPs relative to larger HgS forms in soil-rice systems. We demonstrate that HgS_NPs persist in soils with minimal physicochemical change but exhibit high methylation potential and rice uptake. Soil amended with HgS_NPs yielded 13- to 26-fold higher methylmercury (MeHg) concentrations than bulk HgS treatments, resulting in 14- to 92-fold and 5.6- to 68-fold increases in MeHg and inorganic Hg in brown rice, respectively. Mechanistically, this sustained bioavailability arises from the synergistic effects of HgS_NPs translocation within rice tissues, reactive Hg release, and enhanced methylation. Our findings identify HgS_NPs as a persistent and highly bioavailable Hg pool in paddies, fundamentally driving long-term Hg transformation and accumulation in rice. This work redefines Hg biogeochemical cycling in agricultural ecosystems and underscores the critical need to incorporate HgS_NPs into food safety assessments.