Nitrogen fixation in the western equatorial Pacific: Rates, diazotrophic cyanobacterial size class distribution, and biogeochemical significance

A combination of 15N2 labeling, Tyramide Signal Amplification–Fluorescent in Situ Hybridization (TSA-FISH) assay, and chemical analyses were performed along a trophic gradient (8000 km) in the equatorial Pacific. Nitrogen fixation rates were low (0.06 ± 0.02 to 2.8 ± 2.1 nmol L−1 d−1) in HNLC waters, higher in the warm pool (0.11 ± 0.0 to 18.2 ± 2.8 nmol L−1 d−1), and extremely high close to Papua New Guinea (38 ± 9 to 610 ± 46 nmol L−1 d−1). Rates attributed to the <10-μm fraction accounted for 74% of total activity. Both unicellular and filamentous diazotrophs were detected and reached 17 cells mL−1 and 1.85 trichome mL−1. Unicellular diazotrophs were found to be free-living in <10-μm fraction or in association with mucilage, particles, or eukaryotes in the >10-μm fraction, leading to a possible overestimation of this fraction to total N2 fixation. In oceanic waters, 98% of the unicellular diazotrophs were picoplanktonic. Finally, we found a clear longitudinal pattern of niche partitioning between diazotroph groups: while unicellular diazotrophs were present all along the transect, Trichodesmium spp. were detected only in coastal waters, where nitrogen fixation associated to both size fractions was greatly stimulated.