Dual nitrogen and oxygen isotope fractionation during anaerobic ammonium oxidation by anammox bacteria

Abstract

Natural abundance of stable nitrogen (N) and oxygen (O) isotopes are invaluable biogeochemical tracers for assessing the N transformations in the environment. To fully exploit these tracers, the N and O isotope effects (15ε and 18ε) associated with the respective nitrogen transformation processes must be known. However, the N and O isotope effects of anaerobic ammonium oxidation (anammox), one of the major fixed N sinks and NO3− producers, are not well known. Here, we report the dual N and O isotope effects associated with anammox by three different anammox bacteria including “Ca. Scalindua japonica”, a putative marine species, which were measured in continuous enrichment culture experiments. All three anammox species yielded similar N isotope effects of NH4+ oxidation to N2 (15εNH4→N2) ranging from 30.9‰ to 32.7‰ and inverse kinetic isotope effects of NO2− oxidation to NO3− (15εNO2→NO3 = −45.3‰ to −30.1‰). In contrast, 15εNO2→N2 (NO2− reduction to N2) were significantly different among three species, which is probably because individual anammox bacteria species might possess different types of nitrite reductase. We also report the combined O isotope effects for NO2− oxidation (18ENO2→NO3) by anammox bacteria. These obtained dual N and O isotopic effects could provide significant insights into the contribution of anammox bacteria to the fixed N loss and NO2− reoxidation (N recycling) in various natural environments.