Permafrost contains a reservoir of frozen C estimated to be twice the size of the current atmospheric C pool. In response to changing climate, permafrost is rapidly warming which could result in widespread seasonal thawing. When permafrost thaws, soils that are rich in ice and C often transform into thermokarst wetlands with anaerobic conditions and significant production of atmospheric CH4 and CO2. While most C flux research in recently thawed permafrost concentrates on the few summer months when seasonal thaw has occurred, there is mounting evidence that sizeable portions of annual CO2 and CH4efflux occurs over winter. A potential mechanism for such efflux patterns is that microbial activities can continue in frozen soils, but little is known about specific microbial activities in subzero soils. To better understand microbial transformation of soil C to greenhouse gas over winter, we combined field flux and laboratory process measurements with stable isotope probing (SIP) targeted metagenomics to reveal activities of microbial communities in subzero soil from an Alaskan thermokarst bog.
Field studies revealed positive CO2 and CH4 flux from frozen soils suggesting that microbial activity persisted throughout the winter at our site. Laboratory incubations designed to simulate in-situ winter conditions (-1.5 °C and anaerobic) revealed differential patterns of CO2 and CH4 release; CO2 production remained constant while CH4 production decreased continuously throughout the incubation. Heavy water SIP targeted metagenomics revealed active microbial growth with growth primarily observed in two bacterial phyla, Firmicutes and Bacteroidetes. Genes associated with fermentation dominated C-cycling genes found in actively growing microbes. Results indicate that winter microbial activities can play an important role in controlling seasonal C flux in recently thawed permafrost, a select few bacterial taxa are likely responsible for a large portion of the observed CO2 flux, and fermentation was likely the major pathway driving these fluxes.