Antibiotic Pollution Elevates Microbial Methylmercury Production

Fostering aquaculture is a promising strategy to meet the food demands of a growing population. Yet, marine aquaculture farms are situated primarily in nearshore regions that coincide with major mercury reservoirs─coastal sediments─wherein microbial methylation is most dynamic. Sustainable aquaculture calls for safe practices that minimize unintended stimulation of microbial production of neurotoxic bioaccumulative methylmercury. Here, we show that antibiotics released from aquaculture farms significantly boost methylmercury production by sulfate-reducing bacteria. Mercury methylation risk was shown to vary with the type of antibiotic; ciprofloxacin and tetracycline promoted the cross-membrane transport of Hg(II), overcoming the rate-limiting constraint of microbial methylation, while tobramycin had negligible effects. Enhanced permeability of the outer cell membrane and transcription of metal transporters on the inner cell membrane by ciprofloxacin or tetracycline, plus the formation of specific Hg(II)-antibiotic complexes that bind to the transmembrane transporters, enable cellular uptake of Hg(II) for subsequent methylation. Our discovery of this overlooked unintended effect emphasizes the need to curtail antibiotic release from aquafarms, a fast-growing sector of global food supply, to contribute to food security without compromising food safety.