Lifecycle dynamics of mercury sulfide nanoparticles in a soil-rice system

Current understanding of mercury sulfide nanoparticles (HgS NPs) fails to adequately capture their long-term effects in soil-rice systems, particularly the dynamic interplay between NPs, rice plants, and soil microbes under field-relevant conditions. In this study, single-particle inductively coupled plasma mass spectrometry (spICP-MS) delineated a soil-to-grain mercury pathway mediated by HgS NPs at environmentally realistic concentrations (10⁸-10⁹ NPs g^-1). Across the 111-day rice growth cycle, HgS NPs functioned as both vectors for plant uptake and key precursors for methylmercury (MeHg) formation, thereby defining grain mercury risk. Mercury-containing NPs detected in roots can be translocated to grains, with the filling stage as the critical window. Soil methylation displayed marked temporal dynamics (MeHg: 0.1-25.1 μg kg^-1), governed by Hg bioavailability (glutathione-leachable mercury) and methylator abundance (hgcA gene copy number). Notably, exposure produced opposing yield outcomes: grain weight increased by 47 ± 20% at 10⁸ NPs g^-1 but decreased by 23 ± 12% at 10⁹ NPs g^-1, while total mercury accumulated to 27.1-147.9 ng g^-1 in brown rice at both doses, underscoring concurrent threats to yield and food safety. This refined understanding advocates for an integrated mitigation approach: growth-stage management to block filling-phase translocation into grains, combined with targeted soil interventions to inhibit MeHg formation, thereby mitigating dietary mercury exposure from rice.