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

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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.