Effects of Temperature and Substrate Availability on Methanotrophy in Arctic Permafrost Landscapes

Abstract:

Arctic permafrost ecosystems store ~ 50 % of global belowground carbon (C) and are a considerable source of atmospheric methane (CH₄). Current estimates report that nearly 10 – 40 Tg yr^-1 of CH₄ is released from permafrost environments. In particular, topographic depressions on the landscape are predominantly anoxic and conducive to active methanogenesis. At the sediment-water interfaces of the water-saturated polygonal units, namely low- and flat-centered polygons, CH₄ and oxygen gradients overlap and bacterial CH₄ oxidation is an important process contributing to CH₄ consumption. Methanotrophic bacteria represent the major terrestrial sinks for CH₄ and can reduce CH₄ emissions by ~70 %. Therefore, determining how the activity and abundance of methanotrophic communities respond to warming temperature conditions is critical to predicting effects of permafrost thaw and active layer warming on CH₄ emissions. As ground temperature increases in the Arctic landscape, a major impact of permafrost thaw could be draining of the active layer with resultant subsidence leading to the formation of elevated and relatively oxic high-centered polygons. These changes can impact both methanogen and methanotroph communities and affect net CH₄ fluxes.

To understand the controls of temperature and substrate availability on CH₄ oxidation, we examined process rates and temporal dynamics of methanotroph biomass in contrasting landscape gradients. We investigated the active layer and Cryoturbated permafrost organic soilsd from replicate soil cores collected from high-centered and flat-centered polygonal units in the Barrow Environmental Observatory, Barrow, AK. We used quantitative PCR to quantify methanogen (mcrA) and methanotroph (pmoA) population size by functional gene analysis. We present potential methane oxidation activity in response to three incubation temperatures (-2 ^oC, 4 ^oC, and 10 ^oC) that represent thaw-season ground temperatures. Our objectives were to estimate the rates of CH₄ oxidation in response to seasonal fluctuations in temperature and to contrast the potential of methanogenic environments to oxidize CH₄. We further compare the results with methanogenesis rates to understand the temporal dynamics of CH₄ production and oxidation in warming conditions.