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

Mercury sulfide nanoparticles (HgSNPs) 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 HgSNPs relative to larger HgS forms in soil-rice systems. We demonstrate that HgSNPs persist in soils with minimal physicochemical change but exhibit high methylation potential and rice uptake. Soil amended with HgSNPs 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 HgSNPs translocation within rice tissues, reactive Hg release, and enhanced methylation. Our findings identify HgSNPs 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 HgSNPs into food safety assessments.