Redox conditions, influenced by the availability of oxygen, are expected to dictate the rate of CO2 and CH4 production and to shape the composition and metabolism of microbial communities. Here, we use thawing permafrost peat in thermokarst water under a gradient of initial O2 concentrations to experimentally cover the variability in redox conditions potentially found across thawing landscapes. The three main greenhouse gases, CO2, CH4 and N2O, responded differently to O2 absence. CO2 production along the O2 gradient could be modeled by the Michaelis Menten equation revealing a sharp decrease when oxygen dropped under 100 μM. Under anoxic conditions CO2 yield decreased by 98% and maximum net production rate by 85% when compared to oxic conditions during the 11 days after thaw. N2O production was observed under anoxic conditions, while CH4 yield and CH4 accumulation rates did not differ across the redox gradient. The latter is due to the release of stored CH4 due to thawing. Differences between oxic and anoxic conditions were reflected in the microbial genomic composition, with changes in taxonomic and functional groups, such as N2O reducers, fermenters, denitrifiers and sulfur reducers increasing under anoxic conditions. Genomic changes towards less efficient central metabolism further explained the CO2 production yields and rates limited by O2 availability as predicted by thermodynamics. Together with the Michaelis Menten models the metabolic reconstruction pinpoint to critical thresholds of CO2 release at suboxic conditions and thus need to be considered when explaining and modeling highly variable CO2 emissions across thawing landscapes.
Keywords: arctic; emission; lake; microbial metabolism; soil; thermodynamics.
© The Author(s) 2025. Published by Oxford University Press on behalf of the International Society for Microbial Ecology.