Volume 15, issue 13 | Copyright

Special issue: Interactions between planktonic organisms and biogeochemical...

Biogeosciences, 15, 4215-4232, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Jul 2018

Research article | 12 Jul 2018

In-depth characterization of diazotroph activity across the western tropical South Pacific hotspot of N2 fixation (OUTPACE cruise)

Sophie Bonnet1,2, Mathieu Caffin2, Hugo Berthelot2,3, Olivier Grosso2, Mar Benavides4, Sandra Helias-Nunige2, Cécile Guieu5,6, Marcus Stenegren7, and Rachel Ann Foster7 Sophie Bonnet et al.
  • 1Aix Marseille Univ., Université de Toulon, CNRS, IRD, MIO UM 110, 13288, Noumea, New Caledonia
  • 2Aix Marseille Univ., Université de Toulon, CNRS, IRD, MIO UM 110, 13288, Marseille, France
  • 3Laboratoire des sciences de l'énvironnement marin, IUEM, Université de Brest-UMR 6539 CNRS/UBO/IRD/Ifremer, Plouzané, France
  • 4Marine Biology Section, Department of Biology, University of Copenhagen, 3000 Helsingør, Denmark
  • 5Sorbonne Universités, UPMC Université Paris 06, CNRS, Laboratoire d'Océanographie de Villefranche (LOV), 06230 Villefranche-sur-Mer, France
  • 6Center for Prototype Climate Modeling, New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi, United Arab Emirates
  • 7Department of Ecology, Environment, and Plant Sciences, Stockholm University, Stockholm, 10690, Sweden

Abstract. Here we report N2 fixation rates from a  ∼ 4000km transect in the western and central tropical South Pacific, a particularly undersampled region in the world ocean. Water samples were collected in the euphotic layer along a west to east transect from 160°E to 160°W that covered contrasting trophic regimes, from oligotrophy in the Melanesian archipelago (MA) waters to ultra-oligotrophy in the South Pacific Gyre (GY) waters. N2 fixation was detected at all 17 sampled stations with an average depth-integrated rate of 631±286µmol N m−2 d−1 (range 196–1153µmol N m−2 d−1) in MA waters and of 85±79µmol N m−2 d−1 (range 18–172µmol N m−2 d−1) in GY waters. Two cyanobacteria, the larger colonial filamentous Trichodesmium and the smaller UCYN-B, dominated the enumerated diazotroph community ( > 80%) and gene expression of the nifH gene (cDNA > 105 nifH copies L−1) in MA waters. Single-cell isotopic analyses performed by nanoscale secondary ion mass spectrometry (nanoSIMS) at selected stations revealed that Trichodesmium was always the major contributor to N2 fixation in MA waters, accounting for 47.1–83.8% of bulk N2 fixation. The most plausible environmental factors explaining such exceptionally high rates of N2 fixation in MA waters are discussed in detail, emphasizing the role of macro- and micro-nutrient (e.g., iron) availability, seawater temperature and currents.

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