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

Here we report N₂ 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 oligotrophy in the Melanesian archipelago (MA) waters to ultraoligotrophy in the South Pacific Gyre (GY) waters. N₂ fixation was detected at all 17 sampled stations with an average depth-integrated rate of 631±286 μmolNm^-2 d^-1 (range 196-1153 μmolNm^-2 d^-1) in MA waters and of 85±79 μmolNm^-2 d^-1 (range 18-172 μmolNm^-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.