Smithsonian Environmental Research Center, 647 Contees Wharf Road, Edgewater, Maryland 21037, United States.
Environ Sci Technol. 2012 Mar 6;46(5):2715-23. doi: 10.1021/es203658f. Epub 2012 Feb 16.
Dissolved organic matter (DOM) is generally thought to lower metal bioavailability in aquatic systems due to the formation of metal-DOM complexes that reduce free metal ion concentrations. However, this model may not be pertinent for metal nanoparticles, which are now understood to be ubiquitous, sometimes dominant, metal species in the environment. The influence of DOM on Hg bioavailability to microorganisms was examined under conditions (0.5-5.0 nM Hg and 2-10 μM sulfide) that favor the formation of β-HgS(s) (metacinnabar) nanoparticles. We used the methylation of stable-isotope enriched (201)HgCl(2) by Desulfovibrio desulfuricans ND132 in short-term washed cell assays as a sensitive, environmentally significant proxy for Hg uptake. Suwannee River humic acid (SRHA) and Williams Lake hydrophobic acid (WLHPoA) substantially enhanced (2- to 38-fold) the bioavailability of Hg to ND132 over a wide range of Hg/DOM ratios (9.4 pmol/mg DOM to 9.4 nmol/mg DOM), including environmentally relevant ratios. Methylmercury (MeHg) production by ND132 increased linearly with either SRHA or WLHPoA concentration, but SRHA, a terrestrially derived DOM, was far more effective at enhancing Hg-methylation than WLHPoA, an aquatic DOM dominated by autochthonous sources. No DOM-dependent enhancement in Hg methylation was observed in Hg-DOM-sulfide solutions amended with sufficient l-cysteine to prevent β-HgS(s) formation. We hypothesize that small HgS particles, stabilized against aggregation by DOM, are bioavailable to Hg-methylating bacteria. Our laboratory experiments provide a mechanism for the positive correlations between DOC and MeHg production observed in many aquatic sediments and wetland soils.
溶解有机质（DOM）通常被认为会降低水生系统中金属的生物利用度，这是由于形成了金属-DOM 配合物，降低了游离金属离子的浓度。然而，对于金属纳米粒子来说，这种模型可能并不适用，因为现在已经了解到金属纳米粒子是环境中无处不在的、有时是主要的金属物种。在有利于β-HgS(s)（辰砂）纳米粒子形成的条件下（0.5-5.0 nM Hg 和 2-10 μM 硫化物），研究了 DOM 对微生物中 Hg 生物利用度的影响。我们使用脱硫弧菌 ND132 在短期洗涤细胞测定中稳定同位素富集（201）HgCl2 的甲基化作为 Hg 摄取的敏感、具有环境意义的替代物。苏万尼河腐殖酸（SRHA）和威廉姆斯湖疏水性酸（WLHPoA）大大提高了 ND132 在广泛的 Hg/DOM 比（9.4 pmol/mg DOM 至 9.4 nmol/mg DOM，包括环境相关比）范围内对 Hg 的生物利用度（2 至 38 倍）。ND132 产生的甲基汞（MeHg）与 SRHA 或 WLHPoA 的浓度呈线性增加，但 SRHA 是一种源自陆地的 DOM，比以自生源为主的水生 DOM WLHPoA 更有效地增强 Hg 甲基化。在添加足够的 l-半胱氨酸以防止β-HgS(s)形成的 Hg-DOM-硫化物溶液中，未观察到 DOM 依赖性的 Hg 甲基化增强。我们假设，被 DOM 稳定的小 HgS 颗粒对 Hg 甲基化细菌是可利用的。我们的实验室实验为许多水生生境沉积物和湿地土壤中观察到的 DOC 和 MeHg 产量之间的正相关提供了一种机制。
