Assessment of nitric oxide (NO) redox reactions contribution to nitrous oxide (N₂O) formation during nitrification using a multispecies metabolic network model

Abstract

ABSTRACT Over the coming decades nitrous oxide (N 2 O) is expected to become a dominant greenhouse gas and atmospheric ozone depleting substance. In wastewater treatment systems, N 2 O is majorly produced by nitrifying microbes through biochemical reduction of nitrite (NO 2 − ) and nitric oxide (NO). However it is unknown if the amount of N 2 O formed is affected by alternative NO redox reactions catalyzed by oxidative nitrite oxidoreductase (NirK), cytochromes (i.e., P460 [Cyt P460 ] and 554 [Cyt 554 ]) and flavohemoglobins (Hmp) in ammonia‐ and nitrite‐oxidizing bacteria (AOB and NOB, respectively). In this study, a mathematical model is developed to assess how N 2 O formation is affected by such alternative nitrogen redox transformations. The developed multispecies metabolic network model captures the nitrogen respiratory pathways inferred from genomes of eight AOB and NOB species. The performance of model variants, obtained as different combinations of active NO redox reactions, was assessed against nine experimental datasets for nitrifying cultures producing N 2 O at different concentration of electron donor and acceptor. Model predicted metabolic fluxes show that only variants that included NO oxidation to NO 2 − by Cyt P460 and Hmp in AOB gave statistically similar estimates to observed production rates of N 2 O, NO, NO 2 − and nitrate (NO 3 − ), together with fractions of AOB and NOB species in biomass. Simulations showed that NO oxidation to NO 2 − decreased N 2 O formation by 60% without changing culture's NO 2 − production rate. Model variants including NO reduction to N 2 O by Cyt 554 and cNor in NOB did not improve the accuracy of experimental datasets estimates, suggesting null N 2 O production by NOB during nitrification. Finally, the analysis shows that in nitrifying cultures transitioning from dissolved oxygen levels above 3.8 ± 0.38 to <1.5 ± 0.8 mg/L, NOB cells can oxidize the NO produced by AOB through reactions catalyzed by oxidative NirK. Biotechnol. Bioeng. 2016;113: 1124–1136. © 2015 Wiley Periodicals, Inc.