Microbial Community Structure and Methane Cycling Potential along a Thermokarst Pond-Peatland Continuum

The thawing of ice-rich permafrost soils in northern peatlands leads to the formation of thermokarst ponds, surrounded by organic-rich soils. These aquatic ecosystemsare sites of intense microbial activity, and CO2 and CH4 emissions. Many of the pondsystems in northern landscapes and their surrounding peatlands are hydrologically contiguous, but little is known about the microbial connectivity of concentric habitats around the thermokarst ponds, or the effects of peat accumulation and infillingon the microbial communities. Here we investigated microbial community structureand abundance in a thermokarst pond-peatland system in subarctic Canada. Several lineages were ubiquitous, supporting a prokaryotic continuum from the thermokarst pondto surrounding peatlands. However, the microbial community structureshifted from typical aerobic freshwater microorganisms ( Betaproteobacteriaand Alphaproteobacteria) in the pondtowards acidophilicand anaerobic lineages ( Acidobacteriaand Choroflexi) in the connected peatland waters, likely selected by the acidification of the water by Sphagnummosses. Marked changes in abundance and community composition of methane cycling microorganisms were detected along the thermokarst pond-peatland transects, suggesting fine tuning of C-1 carbon cyclingwithin a highly connected system, and warranting the need for higher spatial resolution across the thermokarstlandscape to accurately predict net greenhouse gas emissions from northern peatlands.