article · Environmental Science & Technology
Methylmercury contamination in rice paddies poses a global environmental risk because rice is a dietary staple for more than half the world population. In oxygen-poor soils, microorganisms transform mercury into this neurotoxin. An investigation using a biogeochemical microcosm examined contaminated paddy soil across controlled oxidation-reduction potential shifts from minus 300 to plus 300 millivolts. Strongly reducing conditions between minus 300 and minus 100 millivolts generated the highest concentrations of dissolved total mercury and methylmercury. This was driven by soil acidification, iron mineral dissolution, and high abundances of methylating bacteria such as Desulfitobacterium. As oxidation levels rose toward plus 200 millivolts, methylmercury concentrations declined substantially. This reduction resulted mainly from lower methylation rates, alongside a 50 percent drop in dissolved mercury, a 96 percent decline in methylating bacteria, and mercury binding to newly formed colloids.
Rice feeds more than half of the global population, making the accumulation of neurotoxic methylmercury in paddy fields a critical food safety threat. Understanding how soil oxidation levels govern microbial and chemical mercury transformations provides essential data to help environmental scientists and agricultural planners predict and manage toxic contamination risks under fluctuating water conditions.
This laboratory-based microcosm study represents early-stage research focused on risk prediction. The findings could eventually assist agronomists, soil remediation specialists, and environmental monitoring agencies in designing water-management protocols or predictive risk models to minimise toxin formation in rice crops. However, the abstract does not describe an applied tool or commercial product, meaning substantial field-scale testing is required before real-world agronomic use is feasible.
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Methylmercury (MeHg) contamination in paddy fields is a significant environmental issue globally since over half of the population of our planet consumes rice. MeHg is a neurotoxin produced by microorganisms in oxygen-limited environments. Microbial effect on MeHg production is a hotspot of research; however, it has been largely ignored how the oxidation-reduction potential (<i>E</i><sub>h</sub>) shapes MeHg formation. Here, we elucidated Hg (de)-methylation in a contaminated soil by increasing <i>E</i><sub>h</sub> stepwise from -300 to +300 mV using a sophisticated biogeochemical microcosm. At the <i>E</i><sub>h</sub> range from -300 to -100 mV, high MeHg concentration and dissolved total Hg (THg) concentration were found due to a high relative abundance of Hg-methylation bacteria (e.g., <i>Desulfitobacterium</i> spp.), acidification, and reductive dissolution of Fe(oxyhydr)oxides. At the <i>E</i><sub>h</sub> range from 0 to +200 mV, the formation of colloids leads to adsorption of Hg and as a result colloidal Hg increased. MeHg reduction with <i>E</i><sub>h</sub> (-300 to +200 mV) increase was mainly attributed to a reduced Hg methylation, as dissolved THg and relative abundance of <i>Desulfitobacterium</i> spp. decreased by 50 and 96%, respectively, at <i>E</i><sub>h</sub> of +200 mV as compared to <i>E</i><sub>h</sub> of -300 mV. Mercury demethylation might be less important since the relative abundance of demethylation bacteria (<i>Clostridium</i> spp.) also decreased over 93% at <i>E</i><sub>h</sub> of +200 mV. These new results are crucial for predicting Hg risks in paddy fields.
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DOI: 10.1021/acs.est.0c07321
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