Oxygen isotopic fractionation and exchange during bacterial nitrite oxidation

We elucidate the controls on the δ 18 O values of microbially produced nitrate by tracking the δ 18 O of nitrite and nitrate during bacterial nitrite oxidation, which is the final step of the nitrification process. Aside from the δ 18 O values of the nitrite and water substrates, three factors can affect the δ 18 O value of nitrate produced during nitrite oxidation: (1) a kinetic isotope effect for nitrite oxidation ( 18 ε k ,NO 2 ), (2) a kinetic isotope effect for water incorporation by nitrite oxidoreductase ( 18 ε k ,H 2 O,2), and (3) microbially mediated exchange of oxygen atoms between nitrite and water (× NOB ). 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 18 O‐labeled water showed that less than 3% of the oxygen atoms in nitrite were exchanged with water and that 18 ε k ,H 2 O, 2 ranged from +12.8% to +18.2%. With the use of these parameters and the previously measured values of the δ 18 O of nitrite produced by ammonia‐oxidizing bacteria, the δ 18 O of newly produced nitrate in the ocean was estimated to fall between 28.3%to 20.7%, which is within the range necessary for balancing a deep ocean nitrate δ 18 O budget.