An Unrecognized Greenhouse Gas Emissions Feedback Loop: Nitrous Oxide Inhibits Microbial Methane Production

Human activities accelerate the emissions of greenhouse gases including methane (CH₄) and nitrous oxide (N₂O). While the major microbial sources and sinks of these gases have been identified, knowledge about feedback loops and how environmental changes alter the fluxes of these greenhouse gases are largely lacking. Growth experiments and kinetic analyses with Methanosarcina barkeri strain Fusaro (M. barkeri) demonstrated that micromolar concentrations of N₂O affected CH₄ formation. N₂O chemically oxidizes the Cobalt(I) supernucleophile and inactivates the corrinoid prosthetic group, which methanogens require for methyl group transfer that leads to CH₄ formation and associated energy conservation. Kinetic analyses determined inhibitory constants (K_I) of N₂O for CH₄ formation from acetate, H₂, and methanol (MeOH) in M. barkeri cultures. Acetoclastic methanogenesis was most sensitive to N₂O with a K_I value of 25 μM. Hydrogenotrophic CO₂ reduction to CH₄ and methanogenesis from MeOH exhibited K_I values in the range of 120 μM. More pronounced N₂O inhibition was observed in methanogenic enrichment cultures maintained with acetate, H₂/CO₂, or MeOH with determined K_I values for N₂O inhibition of 18, 62 and 110 μM, respectively. N₂O also reduced the growth yields of methanogens catalyzing acetoclastic, hydrogenotrophic and methylotrophic methanogenic pathways. The majority of microbial CH₄ is derived from acetate via the acetoclastic methanogenesis pathway, which was most sensitive to N₂O inhibition. The measured K_I values for N₂O inhibition of acetoclastic methanogenesis were in the range of N₂O concentrations encountered in various anoxic environments. These findings strongly suggest that a heretofore-unrecognized negative feedback loop with potentially large effects on CH₄ emission from critical ecosystems (e.g., thawing permafrost, wetlands) exists. Human activities and the rising demand for agricultural fertilizer will continue to increase environmental N₂O concentrations. We conclude that the effects of N₂O on microbial CH₄ formation must be quantitatively described to develop a predictive framework for greenhouse gas emissions and to advance Earth System Models.