Abstract

Nitrogen (N) isotope ratios (N-15/N-14) provide integrative constraints on the N inventory of the modern ocean. Anaerobic ammonium oxidation (anammox), which converts ammonium and nitrite to dinitrogen gas (N-2) and nitrate, is an important fixed N sink in marine ecosystems. We studied the so far unknown N isotope effects of anammox in batch culture experiments. Anammox preferentially removes N-14 from the ammonium pool with an isotope effect of +23.5% to +29.1%, depending on factors controlling reversibility. The N isotope effects during the conversion of nitrite to N-2 and nitrate are (i) inverse kinetic N isotope fractionation associated with the oxidation of nitrite to nitrate (-31.1 +/- 3.9%), (ii) normal kinetic N isotope fractionation during the reduction of nitrite to N-2 (+16.0 +/- 4.5%), and (iii) an equilibrium N isotope effect between nitrate and nitrite (-60.5 +/- 1.0%), induced when anammox is exposed to environmental stress, leading to the superposition of N isotope exchange effects upon kinetic N isotope fractionation. Our findings indicate that anammox may be responsible for the unresolved large N isotope offsets between nitrate and nitrite in oceanic oxygen minimum zones. Irrespective of the extent of N isotope exchange between nitrate and nitrite, N removed from the combined nitrite and nitrate (NOx) pool is depleted in N-15 relative to NOx. This net N isotope effect by anammox is superimposed on the N isotope fractionation by the co-occurring reduction of nitrate to nitrite in suboxic waters, possibly enhancing the overall N isotope effect for N loss from oxygen minimum zones.