title: Aphotic N2 fixation in the Eastern Tropical South Pacific Ocean.
abstract: We examined rates of N2 fixation from the surface to 2000 m depth in the Eastern Tropical South Pacific (ETSP) during El Niño (2010) and La Niña (2011). Replicated vertical profiles performed under oxygen-free conditions show that N2 fixation takes place both in euphotic and aphotic waters, with rates reaching 155 to 509 µmol N m(-2) d(-1) in 2010 and 24±14 to 118±87 µmol N m(-2) d(-1) in 2011. In the aphotic layers, volumetric N2 fixation rates were relatively low (<1.00 nmol N L(-1) d(-1)), but when integrated over the whole aphotic layer, they accounted for 87-90% of total rates (euphotic+aphotic) for the two cruises. Phylogenetic studies performed in microcosms experiments confirm the presence of diazotrophs in the deep waters of the Oxygen Minimum Zone (OMZ), which were comprised of non-cyanobacterial diazotrophs affiliated with nifH clusters 1K (predominantly comprised of α-proteobacteria), 1G (predominantly comprised of γ-proteobacteria), and 3 (sulfate reducing genera of the δ-proteobacteria and Clostridium spp., Vibrio spp.). Organic and inorganic nutrient addition bioassays revealed that amino acids significantly stimulated N2 fixation in the core of the OMZ at all stations tested and as did simple carbohydrates at stations located nearest the coast of Peru/Chile. The episodic supply of these substrates from upper layers are hypothesized to explain the observed variability of N2 fixation in the ETSP.
authors.affiliation: Mediterranean Institute of Oceanography, Institut de Recherche pour le Développement, Aix Marseille Université, Centre National de la Recherche Scientifique Marseille/Noumea, New Caledonia, France.
authors.forename: Sophie
authors.initials: S
authors.lastname: Bonnet
authors.affiliation: Mediterranean Institute of Oceanography, Institut de Recherche pour le Développement, Aix Marseille Université, Centre National de la Recherche Scientifique Marseille/Noumea, New Caledonia, France.
authors.forename: Julien
authors.initials: J
authors.lastname: Dekaezemacker
authors.affiliation: Department of Ocean Sciences, University of California Santa Cruz, Santa Cruz, California, United States of America.
authors.forename: Kendra A
authors.initials: KA
authors.lastname: Turk-Kubo
authors.affiliation: Mediterranean Institute of Oceanography, Institut de Recherche pour le Développement, Aix Marseille Université, Centre National de la Recherche Scientifique Marseille/Noumea, New Caledonia, France.
authors.forename: Thierry
authors.initials: T
authors.lastname: Moutin
authors.affiliation: Environmental Studies, Soka University of America, Aliso Viejo, California, United States of America.
authors.forename: Robert M
authors.initials: RM
authors.lastname: Hamersley
authors.affiliation: Mediterranean Institute of Oceanography, Institut de Recherche pour le Développement, Aix Marseille Université, Centre National de la Recherche Scientifique Marseille/Noumea, New Caledonia, France.
authors.forename: Olivier
authors.initials: O
authors.lastname: Grosso
authors.affiliation: Department of Ocean Sciences, University of California Santa Cruz, Santa Cruz, California, United States of America.
authors.forename: Jonathan P
authors.initials: JP
authors.lastname: Zehr
authors.affiliation: Department of Biological Sciences, University of Southern California, Los Angeles, California, United States of America.
authors.forename: Douglas G
authors.initials: DG
authors.lastname: Capone
mesh_terms: D020561:Alphaproteobacteria / Q000378:metabolism; D003013:Clostridium / Q000378:metabolism; D020564:Deltaproteobacteria / Q000378:metabolism; D020563:Gammaproteobacteria / Q000378:metabolism; D009586:Nitrogen Fixation / Q000502:physiology*; D010140:Pacific Ocean