Thermodynamic Analysis of Intermediary Metabolic Steps and Nitrous Oxide Production by Ammonium-Oxidizing Bacteria.

Nitrous oxide (N 2 O) is a greenhouse gas emitted from wastewater treatment, soils, and agriculture largely by ammonium-oxidizing bacteria (AOB). While AOB are characterized by being aerobes that oxidize ammonium (NH 4 + ) to nitrite (NO 2 - ), fundamental studies in microbiology are revealing the importance of metabolic intermediates and reactions that can lead to the production of N 2 O. These findings about the metabolic pathways for AOB were integrated with thermodynamic electron-equivalents modeling (TEEM) to estimate kinetic and stoichiometric parameters for each of the AOB's nitrogen (N)-oxidation and -reduction reactions. The TEEM analysis shows that hydroxylamine (NH 2 OH) oxidation to nitroxyl (HNO) is the most energetically efficient means for the AOB to provide electrons for ammonium monooxygenation, while oxidations of HNO to nitric oxide (NO) and NO to NO 2 - are energetically favorable for respiration and biomass synthesis. The respiratory electron acceptor can be O 2 or NO, and both have similar energetics. The TEEM-predicted value for biomass yield, maximum-specific rate of NH 4 + utilization, and maximum specific growth rate are consistent with empirical observations. NO reduction to N 2 O is thermodynamically favorable for respiration and biomass synthesis, but the need for O 2 as a reactant in ammonium monooxygenation likely precludes NO reduction to N 2 O from becoming the major pathway for respiration.