Redox determines greenhouse gas production kinetics and metabolic traits in water-saturated thawing permafrost peat.

Redox conditions, influenced by the availability of oxygen, are expected to dictate the rate of CO and CH 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 O concentrations to experimentally cover the variability in redox conditions potentially found across thawing landscapes. The three main greenhouse gases, CO, CH and NO, responded differently to O absence. CO production along the O gradient could be modeled by the Michaelis Menten equation revealing a sharp decrease when oxygen dropped under 100 μM. Under anoxic conditions CO yield decreased by 98% and maximum net production rate by 85% when compared to oxic conditions during the 11 days after thaw. NO production was observed under anoxic conditions, while CH yield and CH accumulation rates did not differ across the redox gradient. The latter is due to the release of stored CH 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 NO reducers, fermenters, denitrifiers and sulfur reducers increasing under anoxic conditions. Genomic changes towards less efficient central metabolism further explained the CO production yields and rates limited by O availability as predicted by thermodynamics. Together with the Michaelis Menten models the metabolic reconstruction pinpoint to critical thresholds of CO release at suboxic conditions and thus need to be considered when explaining and modeling highly variable CO emissions across thawing landscapes.