Oxygen isotopic fractionation and exchange during bacterial nitrite oxidation

We elucidate the controls on the delta O-18 values of microbially produced nitrate by tracking the delta O-18 of nitrite and nitrate during bacterial nitrite oxidation, which is the final step of the nitrification process. Aside from the delta O-18 values of the nitrite and water substrates, three factors can affect the delta O-18 value of nitrate produced during nitrite oxidation: (1) a kinetic isotope effect for nitrite oxidation ((18)epsilon(k,NO2)), (2) a kinetic isotope effect for water incorporation by nitrite oxidoreductase ((18)epsilon(k,H2O,2)), and (3) microbially mediated exchange of oxygen atoms between nitrite and water (x(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 O-18-labeled water showed that less than 3% of the oxygen atoms in nitrite were exchanged with water and that (18)epsilon(k),(H2O),(2) ranged from +12.8 parts per thousand to +18.2 parts per thousand. With the use of these parameters and the previously measured values of the delta O-18 of nitrite produced by ammonia-oxidizing bacteria, the delta O-18 of newly produced nitrate in the ocean was estimated to fall between -8.3 parts per thousand to -0.7 parts per thousand, which is within the range necessary for balancing a deep ocean nitrate delta O-18 budget.