Nitrogen isotope effects induced by anammox bacteria
Benjamin Brunner1, Sergio Contreras, Moritz F Lehmann
1Department of Biogeochemistry, Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany.
Summary
Anaerobic ammonium oxidation (anammox) preferentially removes light nitrogen (14N) from ammonium. This process influences nitrogen isotope ratios in marine ecosystems, particularly in oxygen minimum zones.
Area of Science:
- Marine Biogeochemistry
- Isotope Geochemistry
- Microbial Ecology
Background:
- Nitrogen (N) isotope ratios ((15)N/(14)N) are crucial for understanding marine nitrogen inventories.
- Anaerobic ammonium oxidation (anammox) is a key process in marine nitrogen cycling, converting ammonium and nitrite to dinitrogen gas and nitrate.
- The nitrogen isotope effects associated with anammox have remained largely uncharacterized.
Purpose of the Study:
- To investigate the nitrogen isotope effects of anaerobic ammonium oxidation (anammox).
- To determine how anammox influences nitrogen isotope fractionation in marine environments.
- To explore the potential role of anammox in explaining nitrogen isotope offsets in oceanic oxygen minimum zones.
Main Methods:
- Batch culture experiments were conducted to study anammox reactions.
- Nitrogen isotope ratios ((15)N/(14)N) of reactants and products were measured.
- Kinetic and equilibrium isotope effects were quantified under varying conditions.
Main Results:
- Anammox exhibits a significant preference for removing (14)N from ammonium, with isotope effects ranging from +23.5‰ to +29.1‰.
- Distinct nitrogen isotope effects were observed for the conversion of nitrite to N2 (normal kinetic, +16.0 ± 4.5‰) and nitrite to nitrate (inverse kinetic, -31.1 ± 3.9‰).
- An equilibrium isotope effect (-60.5 ± 1.0‰) between nitrate and nitrite was induced under environmental stress, indicating N isotope exchange.
Conclusions:
- Anammox significantly impacts nitrogen isotope signatures in marine systems.
- The study provides evidence that anammox could explain unexplained nitrogen isotope offsets between nitrate and nitrite in oxygen minimum zones.
- The net effect of anammox results in nitrogen removal depleted in (15)N, potentially enhancing overall nitrogen loss in suboxic waters.
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