Oxygen isotopic fractionation and exchange during bacterial nitrite oxidation

Abstract

We elucidate the controls on the δ18O values of microbially produced nitrate by tracking the δ18O of nitrite and nitrate during bacterial nitrite oxidation, which is the final step of the nitrification process. Aside from the δ18O values of the nitrite and water substrates, three factors can affect the δ18O value of nitrate produced during nitrite oxidation: (1) a kinetic isotope effect for nitrite oxidation (18εkNO2), (2) a kinetic isotope effect for water incorporation by nitrite oxidoreductase (18εkH2O,2), and (3) microbially mediated exchange of oxygen atoms between nitrite and water (XNOB). These parameters were quantified through batch culture experiments with species from three genera of marine nitrite-oxidizing bacteria: Nitrococcus, Nitrobacter, and Nitrospira. Experiments conducted with 18O-labeled water showed that less than 3% of the oxygen atoms in nitrite were exchanged with water and that 18εk,H2O,2 ranged from +12.8% to +18.2%. With the use of these parameters and the previously measured values of the δ18O of nitrite produced by ammonia-oxidizing bacteria, the δ18O of newly produced nitrate in the ocean was estimated to fall between -8.3% to -0.7%, which is within the range necessary for balancing a deep ocean nitrate δ18O budget. © 2010, by the American Society of Limnology and Oceanography, Inc.