Phenol–Quinone Redox Couples of Natural Organic Matter Promote Mercury Methylation in Paddy Soil

Methylmercury in paddy soils poses threats to food security and thus human health. Redox-active phenolic and quinone moieties of natural organic matter (NOM) mediate electron transfer between microbes and mercury during mercury reduction. However, their role in mercury methylation remains elusive. Here, artificial organic matter (AOM), i.e., biochar, wherein the phenol-quinone ratio and associated redox properties varied, was used as a redox-tunable model NOM to investigate the impact of the phenol-quinone redox couples on mercury methylation in Hg-contaminated paddy soils. Our findings confirm that AOM with higher phenol-quinone ratios (i.e., electron donor capacities) stimulated microbial methylation (4.9-fold increase) and dark abiotic methylation (2.2-fold increase). The phenol-quinone ratio had contrasting effects on the abundance of the Hg methylation gene hgcA and metabolic genes corresponding to Hg-methylating and demethylating clades (i.e., dsrA, dsrB, mcrA, and pmoA), especially under anaerobic (simulated flooding) conditions. The key Hg methylators were from Geobacteraceae, including Oryzomonas, Fundidesulfovibrio, and Geomobilimonas. The microbial methylation driven by the phenol-quinone ratio was further validated by NOM such as humic and fulvic acids. Notably, abiotic methylation was observed in aerobic sterilized soil, yet additional evidence is necessary to confirm the potential abiotic pathway, hampered by the difficulty of identifying effective methyl donors in soil. Our results reveal the potential of phenol-quinone redox properties in NOM to drive mercury methylation, offering novel insights into mercury methylation in paddy soils.