Lu Xia, Liu Yurong, Johs Alexander, Zhao Linduo, Wang Tieshan, Yang Ziming, Lin Hui, Elias Dwayne A, Pierce Eric M, Liang Liyuan, Barkay Tamar, Gu Baohua
School of Nuclear Science and Technology, Lanzhou University , Lanzhou, China.
Environmental Sciences Division, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, United States.
Environ Sci Technol. 2016 Apr 19;50(8):4366-73. doi: 10.1021/acs.est.6b00401. Epub 2016 Apr 7.
Microbial methylation and demethylation are two competing processes controlling the net production and bioaccumulation of neurotoxic methylmercury (MeHg) in natural ecosystems. Although mercury (Hg) methylation by anaerobic microorganisms and demethylation by aerobic Hg-resistant bacteria have both been extensively studied, little attention has been given to MeHg degradation by anaerobic bacteria, particularly the iron-reducing bacterium Geobacter bemidjiensis Bem. Here we report, for the first time, that the strain G. bemidjiensis Bem can mediate a suite of Hg transformations, including Hg(II) reduction, Hg(0) oxidation, MeHg production and degradation under anoxic conditions. Results suggest that G. bemidjiensis utilizes a reductive demethylation pathway to degrade MeHg, with elemental Hg(0) as the major reaction product, possibly due to the presence of genes encoding homologues of an organomercurial lyase (MerB) and a mercuric reductase (MerA). In addition, the cells can strongly sorb Hg(II) and MeHg, reduce or oxidize Hg, resulting in both time and concentration-dependent Hg species transformations. Moderate concentrations (10-500 μM) of Hg-binding ligands such as cysteine enhance Hg(II) methylation but inhibit MeHg degradation. These findings indicate a cycle of Hg methylation and demethylation among anaerobic bacteria, thereby influencing net MeHg production in anoxic water and sediments.
微生物甲基化和去甲基化是控制天然生态系统中神经毒性甲基汞（MeHg）净产生和生物累积的两个相互竞争的过程。尽管厌氧微生物进行的汞（Hg）甲基化以及抗汞好氧细菌进行的去甲基化都已得到广泛研究，但厌氧细菌，特别是贝氏地杆菌 Bemidjiensis Bem 对 MeHg 的降解却很少受到关注。在此，我们首次报告贝氏地杆菌 Bemidjiensis Bem 菌株能够在缺氧条件下介导一系列汞转化过程，包括 Hg(II) 还原、Hg(0) 氧化、MeHg 产生和降解。结果表明，贝氏地杆菌利用还原性去甲基化途径降解 MeHg，主要反应产物为元素汞 Hg(0)，这可能是由于存在编码有机汞裂解酶（MerB）和汞还原酶（MerA）同源物的基因。此外，该细胞能够强烈吸附 Hg(II) 和 MeHg，还原或氧化汞，导致汞形态随时间和浓度发生变化。中等浓度（10 - 500 μM）的汞结合配体如半胱氨酸会增强 Hg(II) 甲基化，但抑制 MeHg 降解。这些发现表明厌氧细菌之间存在汞甲基化和去甲基化循环，从而影响缺氧水体和沉积物中 MeHg 的净产生。
