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

[摘要] 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 ultra-oligotrophy 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 µ mol N m - 2 d - 1 (range 196–1153 µ mol N m - 2 d - 1 ) in MA waters and of 85 ± 79 µ mol N m - 2 d - 1 (range 18–172 µ mol N m - 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 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.

[学科分类] 地球化学与岩石