title: Nitrogen isotope effects induced by anammox bacteria.
abstract: Nitrogen (N) isotope ratios ((15)N/(14)N) provide integrative constraints on the N inventory of the modern ocean. Anaerobic ammonium oxidation (anammox), which converts ammonium and nitrite to dinitrogen gas (N2) 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 (14)N 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 N2 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 N2 (+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 (15)N 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.
authors.affiliation: Department of Biogeochemistry, Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Brunner
authors.forename: Sergio
authors.initials: S
authors.lastname: Contreras
authors.forename: Moritz F
authors.initials: MF
authors.lastname: Lehmann
authors.forename: Olga
authors.initials: O
authors.lastname: Matantseva
authors.forename: Mark
authors.initials: M
authors.lastname: Rollog
authors.forename: Tim
authors.initials: T
authors.lastname: Kalvelage
authors.forename: Gabriele
authors.initials: G
authors.lastname: Klockgether
authors.forename: Gaute
authors.initials: G
authors.lastname: Lavik
authors.forename: Mike S M
authors.initials: MS
authors.lastname: Jetten
authors.forename: Boran
authors.initials: B
authors.lastname: Kartal
authors.forename: Marcel M M
authors.initials: MM
authors.lastname: Kuypers
mesh_terms: D064751:Ammonium Compounds / Q000378:metabolism*; D001421:Bacteria, Anaerobic / Q000378:metabolism*; D007700:Kinetics; D008954:Models, Biological; D009587:Nitrogen Isotopes / Q000378:metabolism*; D009792:Oceans and Seas; D012623:Seawater / Q000032:analysis* / Q000382:microbiology
chemical_list: D064751:Ammonium Compounds; D009587:Nitrogen Isotopes