In-depth characterization of diazotroph activity across the western tropical South Pacific hotspot of N<sub>2</sub> fixation (OUTPACE cruise)

Abstract : Here we report N 2 fixation rates from a ∼ 4000 km 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 oligotro-phy in the Melanesian archipelago (MA) waters to ultra-oligotrophy in the South Pacific Gyre (GY) waters. N 2 fixation was detected at all 17 sampled stations with an average depth-integrated rate of 631 ± 286 µmol N m −2 d −1 (range 196-1153 µmol N m −2 d −1) in MA waters and of 85 ± 79 µmol N m −2 d −1 (range 18-172 µmol N m −2 d −1) in GY waters. Two cyanobacteria, the larger colonial filamen-tous Trichodesmium and the smaller UCYN-B, dominated the enumerated diazotroph community (> 80 %) and gene expression of the nifH gene (cDNA > 10 5 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 N 2 fixation in MA waters, accounting for 47.1-83.8 % of bulk N 2 fixation. The most plausible environmental factors explaining such exceptionally high rates of N 2 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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Sophie Bonnet, Mathieu Caffin, Hugo Berthelot, Olivier Grosso, Mar Benavides, et al.. In-depth characterization of diazotroph activity across the western tropical South Pacific hotspot of N<sub>2</sub> fixation (OUTPACE cruise). Biogeosciences, European Geosciences Union, 2018, 15 (13), pp.4215-4232. ⟨10.5194/bg-15-4215-2018⟩. ⟨hal-02024176⟩

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