In depth characterization of diazotroph activity across the Western Tropical South Pacific hot spot of N₂ fixation

Here we report quantification of N2 fixation rates over a ~ 4000 km transect in the western and central tropical South Pacific. Water samples were collected along a west to east transect from 160° E to 160° W, covering contrasting trophic regimes, from oligotrophy in the Melanesian archipelagoes (MA) waters to ultra-oligotrophy in the South Pacific Gyre (GY) waters. N2 fixation was detected at all 17 sampled stations with an average 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. Exceptionally high rates of N2 fixation in MA waters were favored by availability of both iron and phosphate and the observed warm sea surface temperatures (> 28 °C). Trichodesmium and UCYN-B cyanobacteria dominated the diazotroph community (> 80 %) and gene expression of nitrogenase genes (cDNA > 105 nifH copies L−1) in MA waters, and single-cell isotopic analyses performed by nanoscale secondary ion mass spectrometry at selected stations reveal that Trichodesmium was always the major contributor to N2 fixation in MA waters, accounting for 47.1 to 83.8 % of bulk N2 fixation.