title: Intensive nitrogen loss over the Omani Shelf due to anammox coupled with dissimilatory nitrite reduction to ammonium.
abstract: A combination of stable isotopes ((15)N) and molecular ecological approaches was used to investigate the vertical distribution and mechanisms of biological N(2) production along a transect from the Omani coast to the central-northeastern (NE) Arabian Sea. The Arabian Sea harbors the thickest oxygen minimum zone (OMZ) in the world's oceans, and is considered to be a major site of oceanic nitrogen (N) loss. Short (<48 h) anoxic incubations with (15)N-labeled substrates and functional gene expression analyses showed that the anammox process was highly active, whereas denitrification was hardly detectable in the OMZ over the Omani shelf at least at the time of our sampling. Anammox was coupled with dissimilatory nitrite reduction to ammonium (DNRA), resulting in the production of double-(15)N-labeled N(2) from (15)NO(2)(-), a signal often taken as the lone evidence for denitrification in the past. Although the central-NE Arabian Sea has conventionally been regarded as the primary N-loss region, low potential N-loss rates at sporadic depths were detected at best. N-loss activities in this region likely experience high spatiotemporal variabilities as linked to the availability of organic matter. Our finding of greater N-loss associated with the more productive Omani upwelling region is consistent with results from other major OMZs. The close reliance of anammox on DNRA also highlights the need to take into account the effects of coupling N-transformations on oceanic N-loss and subsequent N-balance estimates.
authors.affiliation: Department of Biogeochemistry, Max Planck Institute for Marine Microbiology, Bremen, Germany. mmj@biology.sdu.dk
authors.forename: Marlene M
authors.initials: MM
authors.lastname: Jensen
authors.forename: Phyllis
authors.initials: P
authors.lastname: Lam
authors.forename: Niels Peter
authors.initials: NP
authors.lastname: Revsbech
authors.forename: Birgit
authors.initials: B
authors.lastname: Nagel
authors.forename: Birgit
authors.initials: B
authors.lastname: Gaye
authors.forename: Mike Sm
authors.initials: MS
authors.lastname: Jetten
authors.forename: Marcel Mm
authors.initials: MM
authors.lastname: Kuypers
mesh_terms: D001419:Bacteria / Q000378:metabolism*; D002244:Carbon / Q000378:metabolism; D009573:Nitrites / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism*; D009587:Nitrogen Isotopes / Q000378:metabolism; D009792:Oceans and Seas; D009850:Oman; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D012623:Seawater / Q000737:chemistry* / Q000382:microbiology
chemical_list: D009573:Nitrites; D009587:Nitrogen Isotopes; D000644:Quaternary Ammonium Compounds; D002244:Carbon; D009584:Nitrogen; D010100:Oxygen