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(毫米汞)的生产和(B)汞和毫米汞的光化学汇，在这两个领域中，北极湖泊中汞的循环似乎对控制自然和人为输入的命运具有特殊和极大的影响。为调查北极湖泊内的毫米汞生产提供了资金。严格的物质平衡证实了这样一种假设，即细菌在沉积物中产生的毫米汞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水的系统有很大不同，在这些系统中，硫酸盐还原细菌(SRB)的活动是导致汞甲基化的主要生物地球化学过程。北极湖的特点是生产力很低，硫酸盐含量低，溶解有机物(DOM)中等到高度，铁含量高。这种生物地球化学背景可能会挑战对SRB在控制毫米汞产量和沉积物浓度方面的作用的传统理解。将进行野外和实验室调查，以考察从不同物理化学特征的湖泊中收集的沉积物的生物地球化学性质(SRB的丰度和多样性、孔隙水中汞的形态、温度以及硫酸盐、铁和有机含量)与毫米汞的生产之间的关系。鉴于沉积物中铁的丰度，铁还原细菌将特别令人感兴趣，因为它们可能影响汞的甲基化/去甲基化过程。这项研究的第二部分将是对北极湖泊中大量的汞和汞的光化学汇的机械调查。汞的光还原导致汞的过饱和和挥发性汞的排放，相当于这些湖泊汞输入总量的20%左右，因此降低了可用于甲基化的汞的量。更戏剧性的是毫米汞的大量光降解，它可以消耗几乎所有从沉积物中释放的毫米汞，减少或阻止其生物积累。这些过程大到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可利用性。将通过实验室和现场研究相结合的方式扩大对汞光化学机理的研究，重点是DOM和Fe在促进汞和毫米汞光反应方面的重要性。这项工作特别重要，因为北极自给自足的猎人社区收获的生物体中存在毫米汞的生物积累。

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.