Ammonium and nitrite oxidation at nanomolar oxygen concentrations in oxygen minimum zone waters.
A major percentage of fixed nitrogen (N) loss in the oceans occurs within nitrite-rich oxygen minimum zones (OMZs) via denitrification and anammox. It remains unclear to what extent ammonium and nitrite oxidation co-occur, either supplying or competing for substrates involved in nitrogen loss in the OMZ core. Assessment of the oxygen (O2) sensitivity of these processes down to the O2 concentrations present in the OMZ core (<10 nmol⋅L(-1)) is therefore essential for understanding and modeling nitrogen loss in OMZs. We determined rates of ammonium and nitrite oxidation in the seasonal OMZ off Concepcion, Chile at manipulated O2 levels between 5 nmol⋅L(-1) and 20 μmol⋅L(-1) Rates of both processes were detectable in the low nanomolar range (5-33 nmol⋅L(-1) O2), but demonstrated a strong dependence on O2 concentrations with apparent half-saturation constants (Kms) of 333 ± 130 nmol⋅L(-1) O2 for ammonium oxidation and 778 ± 168 nmol⋅L(-1) O2 for nitrite oxidation assuming one-component Michaelis-Menten kinetics. Nitrite oxidation rates, however, were better described with a two-component Michaelis-Menten model, indicating a high-affinity component with a Km of just a few nanomolar. As the communities of ammonium and nitrite oxidizers were similar to other OMZs, these kinetics should apply across OMZ systems. The high O2 affinities imply that ammonium and nitrite oxidation can occur within the OMZ core whenever O2 is supplied, for example, by episodic intrusions. These processes therefore compete with anammox and denitrification for ammonium and nitrite, thereby exerting an important control over nitrogen loss.
Nordic Center for Earth Evolution, Department of Biology, University of Southern Denmark, 5230 Odense M, Denmark; lbristow@mpi-bremen.de.
Laura A
LA
Bristow
Department of Bioscience, Arctic Research Centre, Aarhus University, 8000 Aarhus C, Denmark;
Tage
T
Dalsgaard
Department of Bioscience, Microbiology, Aarhus University, 8000 Aarhus C, Denmark;
Laura
L
Tiano
Nordic Center for Earth Evolution, Department of Biology, University of Southern Denmark, 5230 Odense M, Denmark;
Daniel B
DB
Mills
Departmento de Oceanografía and Instituto Milenio de Oceanografía, Universidad de Concepción, 4070386 Concepcion, Chile;
Anthony D
AD
Bertagnolli
Department of Microbiology and Immunology, University of British Columbia, Vancouver, BC, Canada V6T 1Z3.
Jody J
JJ
Wright
Department of Microbiology and Immunology, University of British Columbia, Vancouver, BC, Canada V6T 1Z3.
Steven J
SJ
Hallam
Departmento de Oceanografía and Instituto Milenio de Oceanografía, Universidad de Concepción, 4070386 Concepcion, Chile;
Osvaldo
O
Ulloa
Nordic Center for Earth Evolution, Department of Biology, University of Southern Denmark, 5230 Odense M, Denmark;
Donald E
DE
Canfield
Department of Bioscience, Microbiology, Aarhus University, 8000 Aarhus C, Denmark;
Niels Peter
NP
Revsbech
Nordic Center for Earth Evolution, Department of Biology, University of Southern Denmark, 5230 Odense M, Denmark;
Bo
B
Thamdrup
OMZ; ammonium oxidation; kinetics; nitrite oxidation; oxygen