Elevated methylmercury production in mercury-contaminated soil and its bioaccumulation in rice: key roles of algal decomposition

● AOM input elevates water-soluble cysteine and labile DOM fractions in soil. ● AOM input fuels potential Hg methylators and non-Hg methylators in soil. ● Decayed algal aggregate is Hg methylating “hotspot” and MeHg source in soil. ● AOM-driven SDOM variations elevate soil MeHg production and bioaccumulation in rice. Algal-derived organic matter (AOM) regulates methylmercury (MeHg) fate in aquatic ecosystems, whereas its role in MeHg production and bioaccumulation in Hg-contaminated paddies is unclear. Pot and microcosm experiments were thus performed to understand the response characteristics of MeHg concentrations in soil and rice in different rice-growing periods to algal decomposition. Compared to the control, algal decomposition significantly increased soil water-soluble cysteine concentrations during the rice-tillering and grain-filling periods (P < 0.05). It also significantly lowered the molecular weight of soil-dissolved organic matter (SDOM) during the rice-tillering period (P < 0.05) and SDOM humification/aromaticity during the grain-filling period. Compared to the control, AOM input increased the abundance of potential Hg and non-Hg methylators in soil. Furthermore, it also greatly increased soil MeHg concentrations by 25.6%–80.2% and 12.6%–66.1% during the rice-tillering and grain-filling periods, with an average of 42.25% and 38.42%, respectively, which were significantly related to the elevated cysteine in soil and the decrease in SDOM molecular weight (P < 0.01). In the early stage (within 10 days of microcosm experiments), the MeHg concentrations in decayed algal particles showed a great decrease (P < 0.01), suggesting a potential MeHg source in soil. Ultimately, algal decomposition greatly increased the MeHg concentrations and bioaccumulation factors in rice grains, by 72.30% and 16.77%, respectively. Overall, algal decomposition in Hg-contaminated paddies is a non-negligible factor promoting MeHg accumulation in soil-rice systems.