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

Abstract

A combination of 15 N 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‐ μ 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‐ μ m fraction or in association with mucilage, particles, or eukaryotes in the >10‐ μ 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.