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

摘要: Abstract. Here we report N2 fixation rates from a ∼  4000 km transect in the western and central tropical South Pacific, particularly undersampled region in world ocean. Water samples were collected the euphotic layer along west to east transect 160 ∘  E to 160  W that covered contrasting trophic regimes, oligotrophy in the Melanesian archipelago (MA) waters ultra-oligotrophy South Pacific Gyre (GY) waters. was detected at all 17 sampled stations with an average depth-integrated rate of 631  ±  286  µ mol N m - 2 d 1 (range 196–1153  ) MA of 85   79  (range 18–172  GY Two cyanobacteria, the larger colonial filamentous Trichodesmium smaller UCYN-B, dominated enumerated diazotroph community ( >  80 %) gene expression of nifH gene (cDNA   10 5 copies L −1 Single-cell isotopic analyses performed by nanoscale secondary ion mass spectrometry (nanoSIMS) selected revealed that always major contributor fixation waters, accounting for 47.1–83.8 % bulk fixation. The most plausible environmental factors explaining such exceptionally high are discussed in detail, emphasizing role macro- micro-nutrient (e.g., iron) availability, seawater temperature currents.