Nitrous Oxide Production by Organisms Other than Nitrifiers or Denitrifiers

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Abstract
Heterotrophic bacteria, yeasts, fungi, plants, and animal breath were investigated as possible sources of N2O. Microbes found to produce N2O from NO3− but not consume it were: (i) all of the nitrate-respiring bacteria examined, including strains of Escherichia, Serratia, Klebsiella, Enterobacter, Erwinia, and Bacillus; (ii) one of the assimilatory nitrate-reducing bacteria examined, Azotobacter vinelandii, but not Azotobacter macrocytogenes or Acinetobacter sp.; and (iii) some but not all of the assimilatory nitrate-reducing yeasts and fungi, including strains of Hansenula, Rhodotorula, Aspergillus, Alternaria, and Fusarium. The NO3−-reducing obligate anaerobe Clostridium KDHS2 did not produce N2O. Production of N2O occurred only in stationary phase. The nitrate-respiring bacteria produced much more N2O than the other organisms, with yields of N2O ranging from 3 to 36% of 3.5 mM NO3−. Production of N2O was apparently not regulated by ammonium and was not restricted to aerobic or anaerobic conditions. Plants do not appear to produce N2O, although N2O was found to arise from some damaged plant tops, probably due to microbial growth. Concentrations of N2O above the ambient level in the atmosphere were found in human breath and appeared to increase after a meal of high-nitrate food.