Oxygen isotopic exchange and fractionation during bacterial ammonia oxidation

We examined the oxygen isotopic systematics for ammonia oxidation, the first step in the regeneration of nitrate from ammonium. In particular, oxygen isotopic fractionation and exchange with water were evaluated for their roles in determining the d18O of nitrite produced by four species of ammonia-oxidizing bacteria (AOB). Microbially catalyzed oxygen isotopic exchange between nitrite and water was less than 25 % at low cell densities (106 cells mL21) and ammonium concentrations (less than 50 mmol L21). The amount of exchange was relatively constant for a given species of ammonia oxidizer but varied between 1 % and 25 % among the four species tested. The d18O value of nitrite produced at a given water d18O value also varied by nearly 10 % among the different species. Isotopic fractionation, either during oxygen (O2) incorporation by ammonia monooxygenase and/or water incorporation by hydroxylamine oxidoreductase plays an important role in setting the d18O of nitrite produced by AOB. This work provides a detailed characterization of the oxygen isotopic systematics of ammonia oxidation by AOB, which will help us better interpret the oxygen isotopic distributions of nitrite, nitrate, and nitrous oxide in terrestrial and aquatic environments. Nitrogen is an essential macronutrient whose supply can limit primary production and carbon export from th