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

先前的工作已经确定了两个一般领域，(a)一甲基汞（MMHg）的生产和(b)汞和MMHg的光化学汇，其中北极湖泊中的汞循环在控制自然和人为输入的命运方面表现出特殊和高度的影响力。为调查北极湖泊中MMHg的产生提供了资金。良好的质量平衡证实了细菌在沉积物中产生MMHg是这些湖泊的主要来源的假设。然而，这些系统可能与温带湖泊和海水的系统大不相同，在温带湖泊和海水中，硫酸盐还原细菌（SRB）的活性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极低，硫酸盐含量低，溶解有机质（DOM）中高，铁含量高。这种生物地球化学背景可能会挑战SRB在控制MMHg生成和沉积物浓度中的作用的传统认识。将进行现场和实验室调查，以检查从不同物理化学特征的湖泊收集的沉积物的MMHg产生与生物地球化学性质之间的关系（SRB的丰度和多样性，孔隙水中汞的形态，温度，硫酸盐，铁和有机含量）。鉴于沉积物中铁的丰度，铁还原细菌将特别感兴趣，因为它们可能影响汞甲基化/去甲基化过程。本研究的第二个组成部分将是对北极湖泊中MMHg和汞的大量光化学汇进行机制调查。汞的光还原导致一种挥发性汞的过饱和和排放，约占这些湖泊汞输入总量的20%，从而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，它可以消耗几乎所有从沉积物中释放的MMHg，减少或阻止其生物积累。这些过程足够大，可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可利用性。将通过实验室和实地研究相结合的方式扩大汞光化学机理的研究，重点关注DOM和Fe在促进汞和MMHg光化学反应中的重要性。这项工作尤其重要，因为北极生存猎人社区收获的生物体中MMHg的生物积累。

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.