Abstract

Mercury (Hg) is a global contaminant that presents public health risks through seafood consumption primarily in the form of monomethylmercury (MMHg). Methylation of inorganic Hg in water column has been considered a major source of seawater MMHg, but the known Hg methylation by anaerobes possessing the hgcAB gene cluster in anoxic environments could not directly explain the formation and widespread presence of MMHg in seawater where oxic conditions are usually present. In this review, we synthesized the information on previously reported possible pathways to explain the Hg methylation in oxic marine waters, including Hg methylation by (1) methyl donors like organic compounds and organometallic complexes in seawater via abiotic pathways, (2) anaerobic microbes in anoxic microenvironments within oxic seawater, and (3) aerobic microbes in oxic seawater. We evaluated the potential contributions of respective Hg methylation pathways to MMHg in seawaters and discussed the perspectives on future research needs for an improved understanding of seawater Hg methylation. We inferred that while all proposed Hg methylation pathways remain to be further verified, at least one and maybe all of them are plausible depending on ocean conditions. Development and application of new techniques, e.g., quantifying Hg isotope fractionation, would help differentiate (e.g., abiotic versus biotic) Hg methylation pathways. Comprehensive studies toward bridging the gaps between microbial gene screening and Hg methylating capability, between Hg methylation incubation and field MMHg measurement, and between mechanistic Hg methylation studies and environmental relevance will benefit the elucidation of Hg methylation pathways and MMHg distribution in seawater.

摘要

汞 (Hg) 是一种全球性污染物，主要以单甲基汞 (MMHg) 的形式通过海产品消费给公众健康带来风险。水柱中无机汞的甲基化一直被认为是海水MMHg的主要来源，但已知的具有hgcAB基因簇的厌氧菌在缺氧环境中的汞甲基化并不能直接解释海水中MMHg的形成和广泛存在，而海水通常是有氧条件的。当下。在这篇综述中，我们综合了先前报道的解释有氧海水中汞甲基化的可能途径的信息，包括（1）甲基供体（如海水中的有机化合物和有机金属络合物）通过非生物途径进行的汞甲基化，（2）厌氧微生物在含氧海水中的缺氧微环境，(3) 好氧海水中的好氧微生物。我们评估了各自的 Hg 甲基化途径对海水中 MMHg 的潜在贡献，并讨论了对未来研究需求的展望，以更好地理解海水 Hg 甲基化。我们推断，虽然所有提出的 Hg 甲基化途径仍有待进一步验证，但根据海洋条件，至少有一个甚至可能所有这些途径都是合理的。新技术的开发和应用，例如量化 Hg 同位素分馏，将有助于区分（例如，非生物与生物）Hg 甲基化途径。全面研究弥合微生物基因筛选和 Hg 甲基化能力之间的差距，Hg 甲基化孵化和现场 MMHg 测量之间的差距，