Responses of methanogenic and methanotrophic communities to warming in varying moisture regimes of two boreal fens

Peatlands are one of the major sources of the powerful greenhouse gas methane (CH₄). Our aim was to detect responses of methanogenic archaeal and methane-oxidizing bacterial (MOB) communities that control the methane (CH₄) cycle to climatic warming. This study took place in two boreal fens three years after experimental warming in un-manipulated wet and drier regimes, thus simulating future climate scenarios. We determined active methanogen and MOB communities as transcripts of mcrA and pmoA genes, along with the abundance of these genes, CH₄ production and oxidation potentials, and in situ CH₄ fluxes. Methanogenic community remained similar, although methanogen abundance decreased after warming. In the wet regime, this decrease resulted in a small but significant reduction on the potential CH₄ production in such peat layers where the average production potential was high. Drying alone, however, reduced the potential CH₄ production more than warming, and this impact was strong enough to mask the small warming impact in the drier regime. Warming did not affect the MOB community or the potential CH₄ oxidation in the wet regime; however, type Ib MOB abundance decreased and MOB related to genus Methylocapsa became typical after warming in the drier regime of the southern fen. The in situ measured CH₄ fluxes indicated similar patterns as potential measurements; both warming and drying reduced methane emissions, drying more than warming. These results indicate that methanogens and MOB may have different controlling patterns on CH₄ fluxes when facing global warming. These patterns may further differ not only between moisture regimes, but inside the same habitat type, here boreal fen. Irrespective of this variation, the in situ CH₄ fluxes still seem to respond similarly across sites.