Decadal adaptation of methanotroph affinity to peatland water table manipulations

Peatland methanotrophs mitigate greenhouse gas emissions through oxidizing methane in shallow peat layers forming a filter that removes methane during diffusive transported towards the peat surface. Beside methanotrophs abundance, the effectiveness of this filter also depends on their affinity towards methane and oxygen, which might be affected by climate-driven changes in hydrology as wel as land management practices like drainage and restoration.

Here, we quantified the long-term effects of water-table depth (WTD) and WTD manipulation on the methanotroph affinity towards methane and oxygen. Samples were collected at the Lakkasuo peatland in central Finland. Within this site, we collected samples (10-20cm depth) from four sites along a hydrological gradient formed by long-running experiments (73 years drainage for forestry, 23 years experimental water table drawdown, undrained control) and natural in-site WTD variation (undrained lower water table). We quantified affinities (k_M) and specific activities (a=v_max/k_M) towards methane and oxygen in laboratory incubations with 50-50 000 ppm methane and 3-21% oxygen. At the same time, we surveyed the of the methanotroph communities at these sites through quantitative PCR of mmoX and pmoA subtypes as well as targeted metagenomics of the same genes.

Methane affinity increased from control (kM = 842 ppm, 90% central posterior distribution 702-1005ppm) to forestry drained (kM = 379 (268-506) ppm) and the low water table controls (527 (434-633) ppm), but decreased in response to experimental water table drawdown (1251 (988-1557) ppm). This indicates the establishment of relatively high affinity methanotrophs in foresty drained peat and under naturally lower WTD, but not in response to experimental water table drawdown. Substrate saturation toward oxygen was evident over 5-10% oxygen, but the precision was insufficient to identify differences along the gradient. Methanotroph composition showed a shift from pmoA II dominance at the control and undrained lower water table sites to pmoA Ia dominance in experimental water table drawdown and drainage for forestry.

Our results demonstrate that significant changes in methanotroph kinetics occur in response to WTD manipulations which may need to be considered in peatland methane models. Parameters derived from pristine peatlands may not be accurate immediately after rewetting when methanotroph communities are still adapted to low methane and high oxygen concentrations.