Bonnet Sophie auteur aut IRD Dekaezemacker J. auteur aut IRD Turk-Kubo K. A. auteur aut IRD Moutin T. auteur aut IRD Hamersley R.M. auteur aut IRD Grosso O. auteur aut IRD Zehr J.P. auteur aut IRD Capone D.G. auteur aut IRD text journalArticle eng text/pdf reformatted digital access We examined rates of N-2 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 N-2 fixation takes place both in euphotic and aphotic waters, with rates reaching 155 to 509 mu mol N m(-2) d(-1) in 2010 and 24 +/- 14 to 118 +/- 87 mu mol N m(-2) d(-1) in 2011. In the aphotic layers, volumetric N-2 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 alpha-proteobacteria), 1G (predominantly comprised of gamma-proteobacteria), and 3 (sulfate reducing genera of the delta-proteobacteria and Clostridium spp., Vibrio spp.). Organic and inorganic nutrient addition bioassays revealed that amino acids significantly stimulated N-2 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 N-2 fixation in the ETSP. specialized MILIEU MARIN EL NINO COUCHE EUPHOTIQUE FIXATION BIOLOGIQUE DE L'AZOTE OXYGENE MICROORGANISME ANALYSE QUANTITATIVE PROFIL VERTICAL PROFONDEUR LA NINA COUCHE APHOTIQUE OMZ.OXYGENE MINIMUM ZONE ZONE TROPICALE 032MILPHY 034MIORG 8 12 e81265 [14 ] 2013 1932-6203 https://www.documentation.ird.fr/hor/fdi:010061449 10.1371/journal.pone.0081265 1932-6203 [F B010061449] https://www.documentation.ird.fr/hor/fdi:010061449 https://horizon.documentation.ird.fr/exl-doc/pleins_textes/divers17-10/010061449.pdf IRD - Base Horizon / Pleins textes 2014-03-03 2019-09-13 fdi:010061449 fre