Mercury (Hg) in marine shallow-water hydrothermal systems - an overlooked source of Hg in the global cycle

随着2013年联合国环境规划署《水俣公约》的签署，全球各国政府已经接受汞（Hg）是有毒的。尽管对汞的研究已经进行了几十年，但我们仍然无法回答有关汞在环境中的流动和转化的一些最基本的问题。例如，来自热液喷口的通量被怀疑是对全球汞循环最重要的自然贡献，但通量估计数从20到2000吨/年不等。虽然关于深海热液系统的数据有些相互矛盾，但在海岸附近的浅水环境中的热液系统基本上被忽视，尽管它们对沿海表层沃茨的化学组成和生物多样性的影响得到公认。它们不仅将硫化氢和砷等有毒化合物直接引入生产性地表沃茨，还提供铁和碳等营养物质。今天，海洋浅水热液系统（MSWHS）对全球汞循环的贡献仍然未知。尽管有一些研究已经试图将MSWVS视为潜在的汞来源，但没有一项研究能够提供实际的流体测量结果并调查热液汞排放对全球汞循环的可能贡献。部分原因是难以分析具有挑战性的海洋热液喷口流体基质。此外，不同地质/地球化学背景的MSWHS对全球汞循环的贡献可能不同。汞在喷口流体中的化学形态及其沿喷口海水梯度沿着的行为和转化仍是未知的。其中一个根本性的转变是汞在细菌中甲基化为甲基汞，然后甲基汞在鱼体内生物累积和生物放大，最终进入人类消费者。甲基汞的产生是一种海洋现象，但由于公海中汞的初始浓度较低，因此很难精确研究这一反应。因此，MSWHS中较高的初始汞浓度应使其成为确定转化率及其对环境变量依赖性的理想天然实验室。因此，我们建议调查MSWHS中汞的存在、形态和通量，为全球汞循环研究提供更好的估计。计划的工作包括四个部分：（1）利用水肺潜水设备对选定的MSWHS地点进行绘图和取样，（2）对释放的汞物种进行全面定性（无机汞、甲基汞、气态元素汞），（3）甲基化率的测定和（4）估算MSWHS中总汞和甲基化汞的通量。

With the signing of the UNEP Minamata Convention in 2013, governments have globally accepted that mercury (Hg) is toxic. Despite decades of Hg research, we still lack answers to some of the most fundamental questions concerning Hg flux and transformation in the environment. The flux from hydrothermal vents, for example, is suspected to be the single most important natural contribution to the global Hg cycle, yet flux estimates range several orders of magnitude from 20 to 2000 t y-1. While some contradictory data exists for hydrothermal systems in the deep ocean, hydrothermal systems in shallow, near shore environments have been largely ignored, despite their recognized impact on the chemical composition and biodiversity of coastal surface waters. They not only introduce toxic compounds, such as hydrogen sulfide and arsenic directly into productive surface waters, they also provide nutrients such as iron and carbon. Today, the contribution of marine shallow-water hydrothermal systems (MSWHS) to the global Hg cycle remains unknown. Though, a few studies have already tried to address MSWVS as potential Hg source, none of them could provide actual fluid measurements and investigate the possible contribution hydrothermal Hg emissions to the global Hg cycle. Part of this stems from analytical difficulties with the challenging matrix of marine hydrothermal vent fluids. Moreover MSWHS of different geological/geochemical settings may contribute differently to the global Hg cycle. The chemical speciation of Hg in vent fluids and its behavior and transformation along the vent seawater gradient remains unknown. One of those fundamental transformations would be the bacterial methylation of Hg to methylmercury (MeHg), which then bioaccumulates and biomagnifies in fish, eventually finding its way to the human consumer. MeHg production is an ocean-wide phenomenon, but the low initial Hg concentrations in the open ocean make it difficult to precisely study this reaction. Thus higher initial Hg concentrations in MSWHS should make them ideal natural laboratories to determine transformation rates and their dependence on environmental variables. Thus, we propose to investigate the occurrence, speciation and flux of Hg in MSWHS to provide better estimates for the study of the global Hg cycle.The planned work consists of four parts: (1) Mapping and sampling of selected MSWHS sites utilizing SCUBA diving equipment, (2) complete characterization of the released Hg species (inorganic Hg, MeHg, gaseous elemental Hg), (3) determination of methylation rates and (4) estimating the flux of total and methylated Hg from MSWHS .