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

Here we report inline-formulaN₂ fixation rates from a inline-formula∼ 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 160inline-formula^∘ E to 160inline-formula^∘ W that covered contrasting trophic regimes, from oligotrophy in the Melanesian archipelago (MA) waters to ultra-oligotrophy in the South Pacific Gyre (GY) waters. inline-formulaN₂ fixation was detected at all 17 sampled stations with an average depth-integrated rate of 631 inline-formula± 286 inline-formula $M8inlinescrollmathml\mathrm{normal\; \mu }\mathrm{normal\; mol}\mathrm{normal\; N}{\mathrm{normal\; m}}^{-\mathrm{normal\; 2}}{\mathrm{normal\; d}}^{-\mathrm{normal\; 1}}$ 74pt15ptsvg-formulamathimg572d3011fd537fd8ff7a23225cd3a54b bg-15-4215-2018-ie00001.svg74pt15ptbg-15-4215-2018-ie00001.png (range 196–1153 inline-formula $M9inlinescrollmathml\mathrm{normal\; \mu }\mathrm{normal\; mol}\mathrm{normal\; N}{\mathrm{normal\; m}}^{-\mathrm{normal\; 2}}{\mathrm{normal\; d}}^{-\mathrm{normal\; 1}}$ 74pt15ptsvg-formulamathimgd191923f6cada7ac4c0e3e03e93f0ffd bg-15-4215-2018-ie00002.svg74pt15ptbg-15-4215-2018-ie00002.png ) in MA waters and of 85 inline-formula± 79 inline-formula $M11inlinescrollmathml\mathrm{normal\; \mu }\mathrm{normal\; mol}\mathrm{normal\; N}{\mathrm{normal\; m}}^{-\mathrm{normal\; 2}}{\mathrm{normal\; d}}^{-\mathrm{normal\; 1}}$ 74pt15ptsvg-formulamathimga355157fa135e44af8529f9fd9cba135 bg-15-4215-2018-ie00003.svg74pt15ptbg-15-4215-2018-ie00003.png (range 18–172 inline-formula $M12inlinescrollmathml\mathrm{normal\; \mu }\mathrm{normal\; mol}\mathrm{normal\; N}{\mathrm{normal\; m}}^{-\mathrm{normal\; 2}}{\mathrm{normal\; d}}^{-\mathrm{normal\; 1}}$ 74pt15ptsvg-formulamathimga9f2a1d84d00ca3f55f2ba85a77c47d9 bg-15-4215-2018-ie00004.svg74pt15ptbg-15-4215-2018-ie00004.png ) in GY waters. Two cyanobacteria, the larger colonial filamentous Trichodesmium and the smaller UCYN-B, dominated the enumerated diazotroph community (inline-formula> 80 %) and gene expression of the nifH gene (cDNA inline-formula> 10inline-formula⁵nifH copies Linline-formula⁻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 inline-formulaN₂ fixation in MA waters, accounting for 47.1–83.8 % of bulk inline-formulaN₂ fixation. The most plausible environmental factors explaining such exceptionally high rates of inline-formulaN₂ fixation in MA waters are discussed in detail, emphasizing the role of macro- and micro-nutrient (e.g., iron) availability, seawater temperature and currents.