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

A combination of N-15(2) 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-mu m 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-mu m fraction or in association with mucilage, particles, or eukaryotes in the > 10-mu m fraction, leading to a possible overestimation of this fraction to total N-2 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.