METHANE OXIDATION KINETICS PROVIDE INSIGHT TO METHANOTROPHY IN A THAWING PERMAFROST PEATLAND

Rising summer temperatures are accelerating permafrost thaw in the subarctic, increasing methane (CH₄) production and emissions from permafrost peatlands. Methanotrophic bacteria in these peatlands can consume CH₄, mitigating total CH₄ emissions. Under anaerobic conditions, CH₄ concentration controls the rate of bacterial CH₄ consumption. The amount of available belowground CH₄ varies across the permafrost landscape, increasing from near-atmospheric level in palsa mounds, where permafrost still exists within 1m of ground surface, to near-percent level in fully thawed fens. As methane-oxidizing bacteria are the only biological sink of CH₄ it is critical to constrain the kinetic parameters of CH₄ oxidation in thawing permafrost peatlands. Laboratory incubations of peat were employed to measure the uptake rates of CH₄ oxidation in three environments in Stordalen Mire (68°21'N,18°49'E): palsa, semi-wet sphagnum, and open-water sedge. Peat cores were extracted and incubated at 10°C under a range of headspace CH₄ concentrations from 100 to 10,000 parts per million by volume (ppmv). Headspace samples were collected over a 24-hour period and analyzed for CH₄ concentration using flame ionization detection gas chromatography (GC-FID). Net CH₄ uptake was observed under all experimental treatments. Palsa peat had the lowest potential CH₄ oxidation rates across all treatments and the lowest maximum oxidation potential (V_max = 0.00098 umol CH₄/g*hr). Peat from open water sedge locations had the highest potential CH₄ oxidation rates across all treatments as well as the highest V_max (0.0942 umol CH₄/g*hr). Semi-wet sphagnum sites had a larger Michaelis-Menten constant than sedge sites (26148.4 umol CH₄ vs 11156.8 umol CH₄), indicating that sphagnum sites require higher belowground CH₄ concentrations to reach their V_max than sedge sites, despite having a lower V_max. Sphagnum sites also had the highest apparent CH₄ concentration threshold (Th_a) for CH₄ oxidation; palsa and sedge sites had similar Th_a values. These results suggest that communities of low and high affinity methanotrophic bacteria may overlap in permafrost peatlands, allowing CH₄ oxidation at a variety of belowground CH₄ concentrations. Microbial analyses will further elucidate controls on CH₄ oxidation across the thaw gradient.