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, 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.

Details

Publication status:
Published
Author(s):
Authors: Bonnet, Sophie, Biegala, Isabelle C., Dutrieux, Pierre, Slemons, Lia O., Capone, Douglas G.

On this site: Pierre Dutrieux
Date:
1 January, 2009
Journal/Source:
Global Biogeochemical Cycles / 23
Page(s):
13pp
Digital Object Identifier (DOI):
https://doi.org/10.1029/2008GB003439