Antibiotic contamination stimulates microbial methylmercury synthesis in rice paddies

Abstract Antibiotic residues are now widespread in agricultural soils, yet their influence on microbe-mediated mercury (Hg) transformations and associated risks is poorly understood. Here, we demonstrate that fluoroquinolone (FQ) antibiotics enhance the production of neurotoxic methylmercury (MeHg) in paddy soils by selectively altering microbial metabolic activity. In 40 paddy soils from China’s major rice-growing regions, FQ addition at 10 mg kg −1 increased MeHg content by 2.9%–19.7% after 21 days of incubation. Metabolic inhibition and microcosm experiments revealed that this increase stemmed primarily from stimulated microbial Hg methylation, rather than inhibited MeHg demethylation. Sulfate-reducing bacteria (SRB) dominated this process since inhibiting sulfate reduction decreased FQ-induced Hg methylation by 31%–75%. Integrated microbiome and functional prediction based on soil microbial communities and metagenomic analyses of enriched SRB consortia further identified hgcA -carrying SRB as the key methylators. FQ exposure selectively suppressed competing microbial groups while promoting metabolically active, antibiotic-tolerant SRB, particularly those using the reductive acetyl-CoA pathway, thereby elevating Hg methylation activity without significantly changing methylator abundance or community structure. Together, these findings reveal that widespread antibiotic contamination amplifies Hg risks in soils via tolerant microbial communities, potentially representing a hidden yet critical link between emerging contaminants, heavy metals, and food safety.