Investigating the Importance of the Coastal Zone as a Source of Methylmercury to the Ocean

汞（Hg）从自然和人为来源释放到环境中，本世纪汞的输入量大幅增加，并使汞沉积到海洋（汞的主要输入）增加了三倍。然而，有证据表明，海洋鱼类体内汞（主要是甲基汞（CH3Hg））的浓度一致增加，这是相互矛盾的。事实上，在迄今为止研究的大多数生态系统中，汞输入和CH3Hg产生之间的关系很弱，这表明有许多因素影响汞甲基化的速度。切萨皮克生物实验室和本尼迪克特河口研究实验室的研究人员一直在研究淡水和河口系统中的这些相互作用和关系，在这个项目中，他们将根据这个提案将调查扩展到沿海海洋。虽然以前的海洋研究主要集中在空气-水交换、海洋分布和生物地球化学上，但沿海地区的研究相对较少，尽管它在全球汞循环中起着不可或缺的作用。研究人员推测，这是甲基化汞进入海洋的潜在来源。对海洋中CH3Hg的初步估算表明，净原位生产是最重要的来源。人们普遍认为硫酸盐还原菌是环境中主要的汞甲基化菌，因此硫酸盐还原是主要矿化机制的高产沿海水域沉积物应该是汞甲基化的重要场所。研究人员推测，由于这些沉积物中铁和硫之间的相互作用，汞对甲基化细菌的生物利用度不会受到高浓度硫化物的影响，高浓度硫化物已被证明可以抑制汞的甲基化。因此，主要的工作假设是，海岸带和陆海边缘是汞甲基化的重要场所，是开放海洋CH3Hg的净来源，并贡献了全球海洋CH3Hg预算的重要部分。此外，研究人员提出，大部分CH3Hg来自于海岸和陆架沉积物中的原位细菌甲基化。因此，主要目标是测量从切萨皮克湾上游到中大西洋湾陆架外围收集的沉积物中溶解相和固相中总汞和CH3Hg的浓度和形态，以及随深度变化的辅助参数（有机碳、铁、锰、硫）。此外，研究小组将确定这些沉积物中甲基化和去甲基化的相对速率，空间和时间，并研究这些测量与汞物种形成、沉积物生物地球化学和微生物活动之间的关系。汞在固相中的分布（形态）将使用顺序萃取方法进行检验。汞在溶解相和固相中的形态对汞甲基化的影响也将被研究。

ABSTRACTOCE-0351050Mercury (Hg) is released to the environment from natural and anthropogenic sources and inputs have increased substantially this century, and have increased Hg deposition to the ocean, the dominant Hg input, by a factor of three. However, the evidence that the concentration of Hg, mostly as methylmercury (CH3Hg), in ocean fish has increased in concert is contradictory. Indeed, in most ecosystems studied to date, the relationship between Hg input and CH3Hg production is weak, suggesting that many factors influence the rate of Hg methylation. Researchers at the Chesapeake Biological Laboratory and the Benedict Estuarine Research Laboratory have been studying these interactions and relationships in freshwater and estuarine systems, and in this project they will extend investigations to the coastal ocean under this proposal.While prior ocean studies have focused on the air-water exchange, and on ocean distributions and biogeochemistry, the coastal zone has been relatively little studied even though it plays an integral role in the global Hg cycle. The researchers hypothesize that it is a potential source of methylated Hg to the ocean. A preliminary budget for CH3Hg in the ocean indicates that net in situ production is the most important source. It is generally accepted that sulfate- reducing bacteria are the primary Hg methylators in the environment and thus sediments under highly productive coastal waters, where sulfate-reduction is the main mineralization mechanism, should be important locations for Hg methylation. The researchers surmise that because of the interaction between Fe and S in these sediments, Hg bioavailability to methylating bacteria will not be effected by high sulfide concentration, which has been shown to inhibit Hg methylation. The primary working hypothesis is, therefore, that the coastal zone and the land-ocean margin are important sites of Hg methylation, are net sources of CH3Hg to the open ocean, and contribute an important fraction of the global ocean CH3Hg budget. Furthermore, the researchers propose that most of this CH3Hg is derived from in situ bacterial methylation within coastal and shelf sediments. The primary objectives are therefore to measure the concentration and speciation of total Hg and CH3Hg, and ancillary parameters (organic carbon, Fe, Mn, S) in the dissolved and solid phase, and with depth, in sediments collected from sites ranging from the upper Chesapeake Bay to the outer reaches of the shelf of the Mid-Atlantic Bight. In addition, researcher team will determine the relative rate of methylation and demethylation in these sediments, spatially and temporally, and investigate relationships between these measures and Hg speciation, sediment biogeochemistry and microbial activity. The distribution (speciation) of Hg in the solid phase will be examined using sequential extraction approaches. The effect of Hg speciation in the dissolved and solid phase on Hg methylation will also be examined.