Environmental Sciences Division, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, United States.
Biosciences Division, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, United States.
Environ Sci Technol. 2017 Sep 19;51(18):10468-10475. doi: 10.1021/acs.est.7b02518. Epub 2017 Aug 29.
Natural dissolved organic matter (DOM) affects mercury (Hg) redox reactions and anaerobic microbial methylation in the environment. Several studies have shown that DOM can enhance Hg methylation, especially under sulfidic conditions, whereas others show that DOM inhibits Hg methylation due to strong Hg-DOM complexation. In this study, we investigated and compared the effects of DOM on Hg methylation by an iron-reducing bacterium Geobacter sulfurreducens PCA and a sulfate-reducing bacterium Desulfovibrio desulfuricans ND132 under nonsulfidic conditions. The methylation experiment was performed with washed cells either in the absence or presence of DOM or glutathione, both of which form strong complexes with Hg via thiol-functional groups. DOM was found to greatly inhibit Hg methylation by G. Sulfurreducens PCA but enhance Hg methylation by D. desulfuricans ND132 cells with increasing DOM concentration. These strain-dependent opposing effects of DOM were also observed with glutathione, suggesting that thiols in DOM likely played an essential role in affecting microbial Hg uptake and methylation. Additionally, DOM and glutathione greatly decreased Hg sorption by G. sulfurreducens PCA but showed little effect on D. desulfuricans ND132 cells, demonstrating that ND132 has a higher affinity to sorb or take up Hg than the PCA strain. These observations indicate that DOM effects on Hg methylation are bacterial strain specific, depend on the DOM:Hg ratio or site-specific conditions, and may thus offer new insights into the role of DOM in methylmercury production in the environment.
天然溶解有机质 (DOM) 会影响环境中汞 (Hg) 的氧化还原反应和厌氧微生物甲基化作用。多项研究表明，DOM 可以增强 Hg 的甲基化作用，特别是在硫化物条件下，而其他研究则表明，由于 DOM 与 Hg 形成强络合物，DOM 会抑制 Hg 的甲基化作用。在这项研究中，我们研究并比较了 DOM 对非硫化条件下异化铁还原菌 Geobacter sulfurreducens PCA 和异化硫酸盐还原菌 Desulfovibrio desulfuricans ND132 汞甲基化作用的影响。通过在不存在或存在 DOM 或谷胱甘肽的情况下用洗涤细胞进行甲基化实验，谷胱甘肽通过巯基官能团与 Hg 形成强络合物。结果发现，DOM 极大地抑制了 G. Sulfurreducens PCA 对 Hg 的甲基化作用，但随着 DOM 浓度的增加，增强了 D. desulfuricans ND132 细胞对 Hg 的甲基化作用。在谷胱甘肽中也观察到了这种与菌株相关的 DOM 相反作用，这表明 DOM 中的巯基可能在影响微生物 Hg 摄取和甲基化方面发挥了重要作用。此外，DOM 和谷胱甘肽大大降低了 G. sulfurreducens PCA 对 Hg 的吸附作用，但对 D. desulfuricans ND132 细胞几乎没有影响，这表明 ND132 对 Hg 的吸附或摄取比 PCA 菌株具有更高的亲和力。这些观察结果表明，DOM 对 Hg 甲基化的影响是菌株特异性的，取决于 DOM:Hg 比或特定位置的条件，因此可能为 DOM 在环境中生成甲基汞的作用提供新的见解。
