The Hg methylation rate is 4 to 10 times higher than the desorption rate of Hg, suggesting direct contacts and interactions between bacterial cells and the particulate-bound Hg for rapid exchange or uptake.
Study reveals important roles of mineral-adsorbed or particulate-bound mercury (Hg) as a significant source of Hg for microbial methylation in the environment
In natural freshwater and sediments, mercury (Hg) is largely associated with particulate minerals and organics, but it remains unclear under what conditions particulates may become a sink or a source for Hg and whether particulate-bound Hg is bioavailable for microbial uptake and methylation. We investigate Hg sorption-desorption characteristics on minerals and a Hg-contaminated sediment and evaluate the potential of particulate-bound Hg for microbial methylation. Mercury rapidly adsorbs onto particulates, especially the cysteine-coated hematite and sediment, with little desorption observed (0.1–4%). However, the presence of Hg-binding ligands, such as low-molecular-weight thiols and humic acids, results in up to 60% of Hg(II) desorption from the Hg-laden hematite particulates, but <6% from the sediment. Importantly, the particulate-bound Hg(II) is bioavailable for uptake and methylation by a sulfate-reducing bacterium Desulfovibrio desulfuricans ND132 under anaerobic incubations, and the methylation rate is 4 to 10 times higher than the desorption rate of Hg(II). These observations suggest direct contacts and interactions between bacterial cells and the particulate-bound Hg(II), resulting in rapid exchange or uptake of Hg(II) by the bacteria. Our results highlight the importance of Hg(II) partitioning at particulate-water interfaces and the role of particulates as a significant source of Hg(II) for methylation in the environment.
Our study indicates an alternative pathway in which microbes take up Hg that is more complicated than previously thought: Particulate-bound Hg does not have to be desorbed or dissolved to make it available for microbial uptake and methylation.
Mineral- or particulate-bound Hg is often considered unavailable for microbial uptake and methylation. Our study reveals that particulate-bound Hg(II) is readily available for uptake and methylation by a sulfate-reducing bacterium, D. desulfuricans ND132, under anaerobic incubations and should be considered in predicting methylmercury production in the natural aquatic environment.
Zhang, L., S. Wu, L. Zhao, X. Lu, E. M. Pierce, and B. Gu. 2019. “Mercury sorption and desorption on organomineral particulates as a source for microbial methylation.” Environmental Science & Technology. 53(5):2426–433. DOI: 10.1021/acs.est.8b06020.
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