Methylmercury accumulation patterns in Coix lacryma-jobi L. and its associated human exposure risks

Mercury (Hg) pollution endangers the environment and human health. Coix (Coix lacryma-jobi L.), an important medicinal and edible crop, requires study of its Hg, particularly methylmercury (MeHg) accumulation characteristics and exposure risks. Paired coix plants and rhizosphere soils were collected from cultivation areas in Southwest China to assess inorganic Hg (IHg) and MeHg distribution and transfer in root, stem, leaf and grain. Combined with partial least squares path modeling (PLS-PM), we quantified the effects of soil properties, bioavailable Hg fractions, and dissolved organic carbon on Hg transportation and accumulation in the soil-coix system. Across tissues, MeHg exhibited stronger bioconcentration than IHg. Approximately 55% of coix grain samples had MeHg bioconcentration factors (BCFs) exceeding 1 (mean = 3.44, comparable to MeHg-sensitive rice); coix grain accumulated MeHg, yet its MeHg concentration (0.281 ± 0.215 µg/kg) remained lower than rice from heavily Hg-contaminated sites. The positive correlations of MeHg concentrations between coix grain and leaf (p < 0.05), and particularly between leaf and rhizosphere soil (p < 0.001), suggest that rhizosphere soil is the primary source of MeHg accumulated in coix grain. In contrast, leaf IHg levels (61.3 ± 46.7 µg/kg) were significantly higher than those in stem and root tissues, and the IHg translocation factors (TFs) from root to aboveground tissues were low. Dietary coix intake was the primary Hg exposure pathway for residents. Although the non-carcinogenic hazard quotients for residents were below the safety threshold (HQ < 1), children faced greater IHg and MeHg health risks. This study is the first to clarify the differential accumulation mechanisms of IHg and MeHg in coix, and informs safe coix production, soil Hg monitoring, and dietary Hg risk management.