Shifting Microbial Communities, Soil Organic Matter Composition, and Carbon Gas Emissions Across a Permafrost Thaw Gradient.

Permafrost soils store one third of global soil carbon in a relatively unavailable form. Climate change-induced thaw initiates a cascade of system changes, that can culminate in significant positive feedbacks to change. One endpoint habitat for permafrost thaw is wetlands, which are a major source of the microbially-produced greenhouse gas methane. Across a natural peat permafrost thaw gradient from intact palsas to partially-thawed bogs and fully-thawed fens, we identified radical shifts in microbial community composition, soil organic matter lability, and carbon gas emissions, within the context of changing hydrology and plant communities. Archaeal relative abundances increased, particularly methanogens, with the appearance of acetoclastic methanogens in the final thaw stage. Dissolved organic matter compounds shifted towards lower molecular weight, aromaticity, and organic oxygen content, while peat humification rates increased. The carbon gas emitted from the system becomes increasingly dominated by methane rather than carbon dioxide, and from bog to fen the methane becomes isotopically heavier indicating a shift from hydrogenotrophic to mixed acetoclastic production. Collectively, these investigations describe an interplay between “old” permafrost carbon and “new” plant-derived carbon, hydrology, and microbial activity, that control the magnitude and character of climate feedbacks from this rapidly changing system. Furthermore, the one particular novel methanogenic microbial lineage (that possesses the genes for hydrogenotrophic production, and expresses them highly in the community proteome) seems to be a “first responder” to the thawing conditions, and the dominant driver of the isotopic signature of the emitted methane. We are currently investigating how the broader microbial soil organic matter transformations influence the amount and nature of the carbon lost from the system.