EAGER: Linking the Chlorine and Carbon Cycles in the Arctic Coastal Plain

氯元素通常不会在自然环境中进行研究，除非是在被杀虫剂、工业溶剂或放射性36Cl等有毒化合物污染的地区。通常假设氯主要以氯化物的形式进入未受污染的生态系统（与食盐中的负离子相同），并且氯化物不与植物或土壤微生物相互作用。然而，越来越多的证据表明，氯化物被植物和土壤微生物吸收并转化为复杂的含氯有机化合物。在氧气匮乏的环境中，一些细菌可以利用这些氯化有机化合物代替氧气进行一种称为有机卤化物呼吸（OHR）的厌氧呼吸。通过这种方式，这些细菌可以迅速消耗掉原本用来产生甲烷的能源。这意味着一个活跃的生物氯循环可以减少释放到大气中的甲烷量。甲烷是一种强温室气体，每个分子吸收的热量是二氧化碳的30倍。北极地区变暖的速度比地球上其他地方都要快，北极土壤中储存着大量的有机碳。因此，了解土壤中有多少碳会以二氧化碳或甲烷的形式流失到大气中是很重要的，因为这两种气体对气候和气候系统有不同的影响。该项目测量了阿拉斯加北部北极沿海平原地区生物氯循环的速率，并测试了有机卤化物呼吸是否确实显著减少了这些地区的甲烷产量。该项目可以为温室气体排放模型提供信息，提高对北极土壤中氯化污染物命运的了解，并进一步推进生物氯循环的基础科学。该项目将涉及一所少数族裔服务机构（圣地亚哥州立大学）的学生和一名高中教师，后者将领导更广泛的推广和教育工作。提出的研究解决了以下两个问题：(1)OHR是否通过H2竞争抑制甲烷生成？(2)北极沿海平原Cl循环的相对强度及其与CH4通量的关系如何沿海岸-内陆梯度变化？实验方法包括实地调查，比较从巴罗到布鲁克斯山脉山麓沿海岸影响梯度的CH4通量和Cl循环指标，以及实验室培养实验，研究OHR、甲烷生成、其他终端电子受体过程和H2有效性之间的关系。Cl循环的指标包括土壤Cl库的大小和转化速率，描述与Cl循环和其他厌氧过程相关的基因和微生物类群的相对丰度的宏基因组，以及用于推断主要Cl循环过程和限制长期循环速率的37Cl和36Cl同位素分析。实验室培养将在不同clg和H2浓度的微观环境中跟踪厌氧过程（OHR、铁还原、甲烷生成、丙酮生成和硫酸盐还原），以建立这些过程之间的热力学等级，以及H2的增加是否会减轻竞争。

The element, chlorine, is not normally studied in natural environments, except in areas that have been contaminated with toxic compounds like pesticides, industrial solvents or radioactive 36Cl. It is often assumed that chlorine enters non-contaminated ecosystems mostly in the form of chloride (the same negative ion in table salt), and that chloride does not interact with plants or soil microorganisms. However, there is growing evidence that chloride is taken up and transformed by plants and soil microorganisms into complex chlorine-containing organic compounds. In environments where oxygen is scarce, some bacteria can use these chlorinated organic compounds instead of oxygen in a form of anaerobic respiration called organohalide respiration (OHR). In this way, these bacteria can quickly use up energy sources that would otherwise be used to produce methane. This means that an active biological chlorine cycle could reduce the amount of methane that is released into the atmosphere. Methane is a strong greenhouse gas, trapping about 30 times as much heat per molecule as carbon dioxide. The Arctic region has been warming faster than the rest of the planet, and large amounts of organic carbon are stored in Arctic soils. It is thus important to understand how much soil carbon will be lost to the atmosphere in the form of carbon dioxide or methane, since these two gases have different effects on the climate the climate system. This project measures rates of biological chlorine cycling in locations across the Arctic Coastal Plain of northern Alaska, and tests whether organohalide respiration does in fact significantly reduces methane production in these areas. This project could inform models of greenhouse gas emissions, improve understanding of the fate of chlorinated contaminants in Arctic soils, and further the basic science of biological chlorine cycling. The project will involve students at a minority-serving institution (San Diego State University) and a high school teacher, who will lead broader outreach and education efforts.The proposed research addresses the following two questions: (1) Does OHR inhibit methanogenesis via competition for H2? (2) How does the relative magnitude of Cl cycling and its relationship to CH4 flux change along a coastal-inland gradient in the Arctic Coastal Plain? The experimental approach consists of a field survey that compares CH4 fluxes and indicators of Cl cycling along a gradient of coastal influence from Barrow to the foothills of the Brooks Range, and a laboratory incubation experiment to study the relationships among OHR, methanogenesis, other terminal electron acceptor processes, and H2 availability. Indicators of Cl cycling include sizes and transformation rates of soil Cl pools, metagenomes describing the relative abundance of genes and microbial taxa associated with Cl cycling and other anaerobic processes, and 37Cl and 36Cl isotopic analysis to infer the dominant Cl cycling processes and to constrain long-term cycling rates. The laboratory incubation will follow anaerobic processes (OHR, iron reduction, methanogenesis, acetogenesis, and sulfate reduction) in microcosms varying in Clorg and H2 concentration to establish the thermodynamic hierarchy among these processes and whether competition is alleviated by increased H2.