title: Nitrite oxidation in the Namibian oxygen minimum zone.
abstract: Nitrite oxidation is the second step of nitrification. It is the primary source of oceanic nitrate, the predominant form of bioavailable nitrogen in the ocean. Despite its obvious importance, nitrite oxidation has rarely been investigated in marine settings. We determined nitrite oxidation rates directly in (15)N-incubation experiments and compared the rates with those of nitrate reduction to nitrite, ammonia oxidation, anammox, denitrification, as well as dissimilatory nitrate/nitrite reduction to ammonium in the Namibian oxygen minimum zone (OMZ). Nitrite oxidation (≤372 nM NO(2)(-) d(-1)) was detected throughout the OMZ even when in situ oxygen concentrations were low to non-detectable. Nitrite oxidation rates often exceeded ammonia oxidation rates, whereas nitrate reduction served as an alternative and significant source of nitrite. Nitrite oxidation and anammox co-occurred in these oxygen-deficient waters, suggesting that nitrite-oxidizing bacteria (NOB) likely compete with anammox bacteria for nitrite when substrate availability became low. Among all of the known NOB genera targeted via catalyzed reporter deposition fluorescence in situ hybridization, only Nitrospina and Nitrococcus were detectable in the Namibian OMZ samples investigated. These NOB were abundant throughout the OMZ and contributed up to ~9% of total microbial community. Our combined results reveal that a considerable fraction of the recently recycled nitrogen or reduced NO(3)(-) was re-oxidized back to NO(3)(-) via nitrite oxidation, instead of being lost from the system through the anammox or denitrification pathways.
authors.affiliation: Department of Biogeochemistry, Max Planck Institute for Marine Microbiology, Bremen, Germany. jfuessel@mpi-bremen.de
authors.forename: Jessika
authors.initials: J
authors.lastname: Füssel
authors.forename: Phyllis
authors.initials: P
authors.lastname: Lam
authors.forename: Gaute
authors.initials: G
authors.lastname: Lavik
authors.forename: Marlene M
authors.initials: MM
authors.lastname: Jensen
authors.forename: Moritz
authors.initials: M
authors.lastname: Holtappels
authors.forename: Marcel
authors.initials: M
authors.lastname: Günter
authors.forename: Marcel M M
authors.initials: MM
authors.lastname: Kuypers
mesh_terms: D000641:Ammonia / Q000378:metabolism; D001419:Bacteria / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D017404:In Situ Hybridization, Fluorescence; D009276:Namibia; D009566:Nitrates / Q000378:metabolism; D058465:Nitrification; D009573:Nitrites / Q000378:metabolism*; D009584:Nitrogen / Q000378:metabolism*; D009587:Nitrogen Isotopes / Q000378:metabolism; D009792:Oceans and Seas; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism*; D012623:Seawater / Q000737:chemistry; D014871:Water Microbiology
chemical_list: D009566:Nitrates; D009573:Nitrites; D009587:Nitrogen Isotopes; D000641:Ammonia; D009584:Nitrogen; D010100:Oxygen