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

Here we report N-2 fixation rates from a similar to 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 degrees E to 160 degrees W that covered contrasting trophic regimes, from oligotrophy in the Melanesian archipelago (MA) waters to ultraoligotrophy 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 mu mol Nm(-2) d(-1) (range 196-1153 mu mol Nm(-2) d(-1)) in MA waters and of 85 +/- 79 mu mol Nm(-2) d(-1) (range 18-172 mu mol Nm(-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 > 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.