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Aphotic N2fixation in the eastern tropical South Pacific Ocean

  • Author(s): Bonnet, S
  • Dekaezemacker, J
  • Turk-Kubo, KA
  • Moutin, T
  • Hamersley, RM
  • Grosso, O
  • Zehr, JP
  • Capone, DG
  • et al.

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We examined rates of N2fixation 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 N2fixation takes place both in euphotic and aphotic waters, with rates reaching 155 to 509 μmol N m-2d-1in 2010 and 24±14 to 118±87 μmol N m-2d-1in 2011. In the aphotic layers, volumetric N2fixation rates were relatively low (<1.00 nmol N L-1d-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 N2fixation 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 N2fixation in the ETSP. © 2013 Bonnet et al.

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