Nitrogen isotope effects can be used to diagnose N transformations in wastewater anammox systems

Anaerobic ammonium oxidation (anammox) plays an important role in aquatic systems as a sink of bioavailable nitrogen (N), and in engineered processes by removing ammonium from wastewater. The isotope effects anammox imparts in the N isotope signatures (N-15/N-14) of ammonium, nitrite, and nitrate can be used to estimate its role in environmental settings, to describe physiological and ecological variations in the anammox process, and possibly to optimize anammox-based wastewater treatment. We measured the stable N-isotope composition of ammonium, nitrite, and nitrate in wastewater cultivations of anammox bacteria. We find that the N isotope enrichment factor (15)epsilon for the reduction of nitrite to N-2 is consistent across all experimental conditions (13.5 parts per thousand +/- 3.7 parts per thousand), suggesting it reflects the composition of the anammox bacteria community. Values of (15)epsilon for the oxidation of nitrite to nitrate (inverse isotope effect, -16 to -43 parts per thousand) and for the reduction of ammonium to N-2 (normal isotope effect, 19-32 parts per thousand) are more variable, and likely controlled by experimental conditions. We argue that the variations in the isotope effects can be tied to the metabolism and physiology of anammox bacteria, and that the broad range of isotope effects observed for anammox introduces complications for analyzing N-isotope mass balances in natural systems.