Biogeochemical Cycling and Fate of Mercury and Methylmercury in the Arctic Alaskan Lakes

先前的工作确定了两个一般领域，（a）一甲基汞（MMHg）生产和（B）汞和MMHg的光化学汇，其中北极湖泊中汞的循环似乎是例外，在控制自然和人为输入的命运方面具有很大的影响力。 为调查北极湖泊内的MMHg生产情况提供了资金。 良好约束的质量平衡证实了这一假设，即细菌在沉积物中产生的MMHg是这些湖泊的主要来源。 然而，这些系统可能与温带湖泊和海洋沃茨的系统有很大的不同，在温带湖泊和海洋水域中，硫酸盐还原菌（SRB）的活动是导致汞甲基化的主要生物地球化学过程。 北极湖泊的特点是生产力非常低，硫酸盐含量低，溶解有机物（DOM）含量中等偏高，铁含量高。 这种生物地球化学环境可能会挑战传统的理解SRB在控制MMHg生产和沉积物浓度的作用。 将进行实地和实验室调查，研究从具有不同物理化学特征的湖泊中收集的MMHg生产与沉积物的地球化学性质（SRB的丰度和多样性、孔隙沃茨中的汞形态、温度以及硫酸盐、铁和有机物含量）之间的关系。 由于沉积物中富含铁，铁还原细菌将特别令人感兴趣，因为它们可能影响汞甲基化/脱甲基化过程。 这项研究的第二个组成部分将是北极湖泊中MMHg和汞的大量光化学汇的机制调查。 汞的光还原导致汞的过饱和和挥发性形式的排放，代表了钙的汇。20%的汞输入到这些湖泊中，因此降低了可用于甲基化的汞量。 更引人注目的是MMHg的大量光降解，几乎可以消耗沉积物释放的所有MMHg，减少或防止其生物累积。 这些过程的规模足以控制这些湖泊中汞的浓度和形态，从而控制这种有毒金属的生物利用度。 汞的光化学机制的研究将扩大与实验室和现场研究相结合，这将集中在DOM和Fe在促进汞和MMHg的光反应的重要性。 这项工作特别重要，因为MMHg会在北极以狩猎为生的生物体中累积。

Prior work has identified two general areas, (a) monomethylmercury (MMHg) production and (b) photochemical sinks of mercury and MMHg, in which the cycling of mercury in arctic lakes appears exceptional and highly influential in controlling the fate of natural and anthropogenic inputs. Funds are provided to investigate the production of MMHg within arctic lakes. Well-constrained mass balances have confirmed the hypothesis that production of MMHg in sediments by bacteria is the dominant source to these lakes. These systems may be quite different, however, from those of temperate lakes and marine waters where the activity of sulfate-reducing bacteria (SRB) is the dominant biogeochemical process resulting in methylation of mercury. The arctic lakes are characterized by very low productivity, low sulfate, moderate to high dissolved organic matter (DOM), and high levels of iron. This biogeochemical setting could challenge conventional understanding of the role of SRB in controlling MMHg production and sediment concentrations. Field and laboratory investigations that examine the relationship between MMHg production and biogeochemical properties of sediments (abundance and diversity of SRB, mercury speciation in pore waters, temperature, and sulfate, iron, and organic contents) collected from lakes with varying physicochemical characteristics will be conducted. Given the abundance of Fe in the sediments, iron-reducing bacteria will be of particular interest as they may affect mercury methylation/demethylation processes. The second component of this research will be a mechanistic investigation of the substantial photochemical sinks for MMHg and mercury in arctic lakes. Photoreduction of mercury, leading to supersaturation and emission of a volatile form of mercury, represents a sink of ca. 20% of total mercury inputs to these lakes, and as a result lowers the amount of mercury available for methylation. Even more dramatic is the substantial photodegradation of MMHg, which can consume nearly all of the MMHg released from sediments, reducing or preventing its bioaccumulation. These processes are large enough to control concentrations and speciation of mercury in these lakes, and thereby control bioavailability of this toxic metal. The study of the mechanisms of mercury photochemistry will be expanded with a combination of laboratory and field studies, which will focus on the importance of DOM and Fe in facilitating mercury and MMHg photoreactions. This work is particularly important because of the bioaccumulation of MMHg in the organisms harvested by the subsistence hunter communities of the Arctic.