Response of Arctic soil methanotrophs to climate change and grazing

Methane oxidizing bacteria (methanotrophs) within the genus Methylobacter constitute the biological filter for methane (CH 4 ) in many Arctic soils. Multiple Methylobacter strains have been identified in these environments but we seldom know the ecological significance of the different strains. High-Arctic peatlands in Svalbard are heavily influenced by herbivory, leading to reduced vascular plant and root biomass. Here, we have measured potential CH 4 oxidation rates and identified the active methantrophs in grazed peat and peat protected from grazing by fencing (exclosures) for 18 years. Grazed peat sustained a higher water table, higher CH 4 concentrations and lower oxygen (O 2 ) concentrations than exclosed peat.Correspondingly, the highest CH 4 oxidation potentials were closer to the O 2 rich surface in the grazed than in the protected peat. A comparison of 16S rRNA genes showed that the majority of methanotrophs in both sites belong to the genus Methylobacter. Further analyses of pmoA transcripts revealed that several Methylobacter OTUs were active in the peat but that different OTUs dominated the grazed peat than the exclosed peat. We conclude that grazing influences soil conditions, the active CH 4 filter and that different Methylobacter populations are responsible for CH 4 oxidation depending on the environmental conditions.Rising temperatures in the Arctic affect soil microorganisms, herbivores, and peatland vegetation, thus directly and indirectly influencing microbial CH 4 production. It is not currently known how methanotrophs in Arctic peat respond to combined changes in temperature, CH 4 concentration, and vegetation. We studied methanotroph responses to temperature and CH 4 concentration in peat exposed to herbivory and protected by exclosures. The methanotroph activity was assessed by CH 4 oxidation rate measurements using peat soil microcosms and a pure culture of Methylobacter tundripaludum SV96, qPCR, and sequencing of pmoA transcripts. Elevated CH 4 concentrations led to higher CH 4 oxidation rates both in grazed and exclosed peat soils, but the strongest response was observed in grazed peat soils. Furthermore, the relative transcriptional activities of different methanotroph community members were affected by the CH 4 concentrations. While transcriptional responses to low CH 4 concentrations were more prevalent in grazed peat soils, responses to high CH 4 concentrations were more prevalent in exclosed peat soils. We observed no significant methanotroph responses to increasing temperatures. We conclude that methanotroph communities in these peat soils respond to changes in the CH 4 concentration depending on their previous exposure to grazing.This “conditioning” influences which strains will thrive and, therefore, Determines the function of the methanotroph community.