Factors Regulating Methane Production and Oxidation in Two Shallow Arctic Alaskan Lakes

Lofton, Dendy D.

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March 20, 2019

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Lofton, Dendy D.
- Affiliation: Gillings School of Global Public Health, Department of Environmental Sciences and Engineering

Abstract

Methane (CH4) is second only to CO2 as a greenhouse gas and is produced in the terminal step of organic matter decomposition in anaerobic environments, including lake sediments. Given the widespread distribution of lakes in Arctic Alaska, CH4 emission from these lakes may significantly contribute to the atmospheric CH4 budget. Aerobic methane oxidizing bacteria consume CH4 diffusing from anaerobic zones of production, thereby modulating the flux of CH4 to the atmosphere. Multiple research efforts indicate a significant source strength for arctic environments in the atmospheric CH4 budget. Predicted climate induced alterations to the arctic landscape include increased organic matter loading from the terrestrial environment and increased temperature. These environmental changes can influence both rates of CH4 production and oxidation, possibly altering rates of CH4 exchange between shallow arctic lakes and the atmosphere. I assessed rates and controls on CH4 production and oxidation in two shallow arctic lakes to provide insight into the response of these two microbial groups to projected future climates. Rates of total methanogenesis and the fractional contribution of the acetoclastic pathway decreased with increasing depth below the sediment surface to 5 cm in both lakes. Substrate additions indicated substrate limitation to both methanogenic pathways (acetoclastic and hydrogenotrophic). Rates of total methanogenesis varied spatially in the horizontal dimension in one lake only. However, there was no consistent relationship between rates of methanogenesis and depth of the overlying water as labile organic matter in the shallow sediments are likely resuspended and deposited unevenly by wind action. Under extant conditions, rates of methanogenesis responded positively to increases in temperature, while rates of CH4 oxidation remained unchanged. The former were controlled by substrate x temperature interactions, while the latter were regulated strictly by substrate supply. Analysis of CH4 oxidation kinetics for water samples points to a community of CH4 oxidizing bacteria that is capable of oxidizing CH4 at concentrations far in excess of observed levels. Increases in organic matter supply and temperature under future climates will likely increase rates of methanogenesis, but the impact may be fully mitigated by the excessive capacity of the CH4-oxidizing community to process the added substrate.

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