Nitrous Oxide Production by Organisms Other than Nitrifiers or Denitrifiers

Abstract

Heterotrophic bacteria, yeasts, fungi, plants, and animal breath were investigated as possible sources of N 2 O. Microbes found to produce N 2 O from NO 3 − 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 NO 3 − -reducing obligate anaerobe Clostridium KDHS2 did not produce N 2 O. Production of N 2 O occurred only in stationary phase. The nitrate-respiring bacteria produced much more N 2 O than the other organisms, with yields of N 2 O ranging from 3 to 36% of 3.5 mM NO 3 − . Production of N 2 O was apparently not regulated by ammonium and was not restricted to aerobic or anaerobic conditions. Plants do not appear to produce N 2 O, although N 2 O was found to arise from some damaged plant tops, probably due to microbial growth. Concentrations of N 2 O above the ambient level in the atmosphere were found in human breath and appeared to increase after a meal of high-nitrate food.