Collaborative research: Transformations and mercury isotopic fractionation of methylmercury by marine phytoplankton

海鲜中汞的积累是一个公共卫生问题。海产品中汞的存在在很大程度上取决于汞的海气交换，大气沉积导致汞在海洋中积累。获取海鲜的途径始于浮游植物从海水中吸收汞，人们一直认为汞在海水中积累，被食草动物吃掉，并传递给更大的有机体。该项目挑战了这一假设，初步数据表明，某些浮游植物物种可以将汞转化为挥发性形式(汞蒸气和二甲基汞)，然后损失到大气中，这是一个从海洋中去除汞的过程，而不是简单地将其浓缩到生态系统和海鲜中。这个以前从未被研究过的过程，可能会极大地改变我们对海洋中汞循环的看法。由该项目资助的研究人员将寻找能够挥发汞的特定浮游植物物种，并量化它们挥发汞的速率。他们还将审查相关的硫和硒化合物在这一过程中的可疑作用，以及量化汞同位素值的变化，以潜在地用作生态系统和食物供应中汞来源的化学示踪剂。这些结果应该可以让海洋学家更好地量化和提炼我们对海洋中汞循环的知识。该项目将支持研究生、博士后科学家的参与，并将新信息直接纳入研究人员教授的课程。资金还将支持参与者继续开展活动，传播有关汞及其对公众和环境健康的影响的信息。海洋中汞的生物地球化学循环可能比之前假设的要复杂得多。新的证据挑战了这样一种观点，即甲基汞(MeHg)只在浮游植物中积累，在进入食物链之前几乎不经历任何转化。该项目旨在更全面地阐明甲基汞在细胞内转化为挥发性汞和二甲基汞(Me2Hg)的机制，并利用定向培养工作评估可以进行这种转化的藻类范围。此外，PIS将调查硫醇、有机硒(Se)化合物和硫化物是促进浮游植物内这些反应所需的证据，并将通过这项研究调查和量化具体的途径。稳定的汞同位素数据已被用来追踪海洋系统中的汞来源和路径，并将量化其在这些甲基汞转化过程中的分馏，以用于未来的海洋汞野外研究。研究人员推测，能够进行这些细胞内反应的球藻和其他超植物可能占海洋中挥发性汞产量的很大一部分。如果事实证明是这样，了解和量化这些挥发过程可能会极大地改变我们目前对海洋中汞的整体生物地球化学循环的理解。

The accumulation of mercury (Hg) in seafood is a public health concern. The presence of Hg in seafood depends to a large degree on the air-sea exchange of Hg, with atmospheric deposition leading to accumulation of Hg in the ocean. The pathways to seafood start with the uptake of Hg by phytoplankton from seawater where is has always been assumed to accumulate to be eaten by grazers and passed on to larger organisms. This project challenges this assumption with preliminary data that suggests certain phytoplankton species can transform Hg to volatile forms (mercury vapor & dimethylmercury) that are lost to the atmosphere, a processes that removes Hg from the ocean rather than simply concentrating it into the ecosystem and seafood. This process, which has not been studied before, could dramatically alter our view of the Hg cycle in the ocean. The researchers funded by this project will look for the specific phytoplankton species that are capable of volatilizing Hg and quantify the rates at which they do so. They will also examine the suspected role of associated sulfur and selenium compounds in the process, as well as quantifying the changes in the Hg isotopic values for potential use as chemical tracers of the source of Hg in the ecosystem and food supply. These results should allow oceanographers to better quantify and refine our knowledge of Hg cycling in the ocean. The project will support participation of graduate students, a postdoctoral scientist, and incorporation of new information directly into courses taught by the researchers. Funding will also support continuing activities by the participants in activities that disseminate information on mercury and its effect on public and environmental health. Biogeochemical cycling of mercury (Hg) in the ocean may be more complex than previously assumed. New evidence has challenged the idea that methylmercury (MeHg) merely accumulates in phytoplankton and undergoes little to no transformation before being passed into the food web. This project aims to more fully elucidate the mechanisms behind the intracellular transformation of MeHg to volatile Hg and dimethylmercury (Me2Hg) that can be lost to the atmosphere, as well as to evaluate the range of algal taxa that can perform this transformation using directed culture work. Additionally, the PIs will investigate evidence that thiols, organic selenium (Se) compounds, and sulfides are required to facilitate these reactions within the phytoplankton, and specific pathways will be investigated and quantified through this research. Stable Hg isotopic data has been used to track Hg sources and pathways in marine systems and its fractionation during these MeHg transformations will also be quantified for future field study of marine Hg. The investigators hypothesize that coccolithophorids and other haptophytes capable of these intracellular reactions may account for a significant portion of the production of volatile Hg in the ocean. If this turns out to be the case, understanding and quantifying these volatilization processes may significantly alter our current understanding of the overall biogeochemical cycling of Hg in the ocean.