The work identifies a novel pathway of abiotic photochemical formation of HgS and explains why freshly deposited Hg is more readily methylated than aged Hg in water.
For the first time, we found that photolysis of the complexes between Hg and naturally dissolved organic matter (DOM) results in rapidly decreased Hg reactivity and bioavailability, resulting in up to 80% decreased methylmercury (MeHg) production by a methylating bacterium Geobacter sulfurreducens PCA. Additionally, we identify a new pathway of photolytic formation of HgS as the main cause of the loss of Hg reactivity and bioavailability.
This research not only identifies a novel pathway of abiotic photochemical formation of HgS, but also provides a mechanism explaining why freshly deposited Hg is more readily methylated than aged Hg in natural water and sediments.
Atmospheric deposition of Hg to surface water is the dominant source of Hg in aquatic environments and ultimately drives MeHg toxin accumulation in fish. It is known that freshly deposited Hg is more readily methylated by microorganisms than preexisting Hg; however, the underlying mechanism is unclear. We report that Hg reactivity and bioavailability are both decreased by photochemical reactions between Hg and DOM. Photo-irradiation of Hg-DOM complexes results in HgS precipitation, loss of Hg reactivity, and an up to 80% decrease in MeHg production by the methylating bacterium G. sulfurreducens PCA. Loss of Hg reactivity proceeds at a faster rate with decreasing Hg/DOM ratio. These results not only suggest a novel pathway of abiotic photochemical formation of HgS, but also provide a mechanism whereby freshly deposited Hg is readily methylated. Over time, however, it progressively becomes less available for microbial uptake and methylation.
Luo, H.-W., X. Yin, A.M. Jubb, H. Chen, X. Lu, W. Zhang, H. Lin, H.-Q. Yu, L. Liang, G.-P. Sheng, and B. Gu. 2017. Photochemical Reactions Between Mercury (Hg) and Dissolved Organic Matter Decrease Hg Bioavailability and Methylation. Environmental Pollution, 220:1359–1365. DOI:10.1016/j.envpol.2016.10.099.
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