title: Abundance and distribution of major groups of diazotrophic cyanobacteria and their potential contribution to N₂ fixation in the tropical Atlantic Ocean.
abstract: The abundances of six N₂-fixing cyanobacterial phylotypes were profiled at 22 stations across the tropical Atlantic Ocean during June 2006, and used to model the contribution of the diazotrophs to N₂ fixation. Diazotroph abundances were measured by targeting the nifH gene of Trichodesmium, unicellular groups A, B, C (UCYN-A, UCYN-B and UCYN-C), and diatom-cyanobiont symbioses Hemiaulus-Richelia, Rhizosolenia-Richelia and Chaetoceros-Calothrix. West to east gradients in temperature, salinity and nutrients [NO₃⁻ + NO₂⁻, PO₄³⁻, Si(OH)₄] showed the influence of the Amazon River plume and its effect on the distributions of the diazotrophs. Trichodesmium accounted for more than 93% of all nifH genes detected, dominated the warmer waters of the western Atlantic, and was the only diazotroph detected at the equatorial upwelling station. UCYN-A was the next most abundant (> 5% of all nifH genes) and dominated the cooler waters of the eastern Atlantic near the Cape Verde Islands. UCYN-C was found at a single depth (200 m) of high salinity and low temperature and nutrients, whereas UCYN-B cells were widespread but in very low abundance (6.1 × 10¹ ± 4.6 × 10² gene copies l⁻¹). The diatom-cyanobionts were observed primarily in the western Atlantic within or near the high Si(OH)₄ input of the Amazon River plume. Overall, highest diazotroph abundances were observed at the surface and declined with depth, except for some subsurface peaks in Trichodesmium, UCYN-B and UCYN-A. Modelled contributions of Trichodesmium, UCYN-B and UCYN-A to total N₂ fixation suggested that Trichodesmium had the largest input, except for the potential of UCYN-A at the Cape Verde Islands.
authors.affiliation: University of California at Santa Cruz, Ocean Sciences Department, Santa Cruz, CA 95064, USA. ngoebel@ucsc.edu
authors.forename: Nicole L
authors.initials: NL
authors.lastname: Goebel
authors.forename: Kendra A
authors.initials: KA
authors.lastname: Turk
authors.forename: Katherine M
authors.initials: KM
authors.lastname: Achilles
authors.forename: Ryan
authors.initials: R
authors.lastname: Paerl
authors.forename: Ian
authors.initials: I
authors.lastname: Hewson
authors.forename: Amanda E
authors.initials: AE
authors.lastname: Morrison
authors.forename: Joseph P
authors.initials: JP
authors.lastname: Montoya
authors.forename: Christopher A
authors.initials: CA
authors.lastname: Edwards
authors.forename: Jonathan P
authors.initials: JP
authors.lastname: Zehr
mesh_terms: D001267:Atlantic Ocean; D055726:Cabo Verde; D000458:Cyanobacteria / Q000145:classification / Q000201:enzymology / Q000235:genetics / Q000302:isolation & purification*; D004269:DNA, Bacterial / Q000302:isolation & purification; D005843:Geography; D008954:Models, Biological; D009586:Nitrogen Fixation; D010088:Oxidoreductases / Q000235:genetics; D012623:Seawater / Q000032:analysis / Q000382:microbiology*; D013696:Temperature; D014871:Water Microbiology
chemical_list: D004269:DNA, Bacterial; D010088:Oxidoreductases; C029130:nitrogenase reductase