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. Here we report N2 fixation rates from a ∼  4000 km transectin the western and central tropical South Pacific, a particularlyundersampled region in the world ocean. Water samples were collected in theeuphotic layer along a west to east transect from 160 ∘  E to160 ∘  W that covered contrasting trophic regimes, from oligotrophy inthe Melanesian archipelago (MA) waters to ultra-oligotrophy in the SouthPacific Gyre (GY) waters. N2 fixation was detected at all 17sampled stations with an average depth-integrated rate of631  ±  286  µ mol N m - 2 d - 1 (range196–1153  µ mol N m - 2 d - 1 ) in MA waters and of85  ±  79  µ mol N m - 2 d - 1 (range18–172  µ mol N m - 2 d - 1 ) in GY waters. Two cyanobacteria,the larger colonial filamentous Trichodesmium and the smallerUCYN-B, dominated the enumerated diazotroph community ( >  80 %) and geneexpression of the nifH gene (cDNA  >  10 5 nifH copiesL −1 ) in MA waters. Single-cell isotopic analyses performed by nanoscalesecondary ion mass spectrometry (nanoSIMS) at selected stations revealedthat 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 suchexceptionally high rates of N2 fixation in MA waters are discussedin detail, emphasizing the role of macro- and micro-nutrient (e.g., iron)availability, seawater temperature and currents.

biogeosciences

In-depth characterization of diazotroph activity across the western tropical South Pacific hotspot of N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fixation (OUTPACE cruise)