In-depth characterization of diazotroph activity across the western tropical South Pacific hotspot of N2 fixation (OUTPACE cruise)

Here we report N2 fixation rates from a ∼ 4000 km transect in the western and central tropical South Pacific, a particularly undersampled region in the world ocean. Water samples were collected in the euphotic layer along a west to east transect from 160∘ E to 160∘ W that covered contrasting trophic...

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Published inBiogeosciences Vol. 15; no. 13; pp. 4215 - 4232
Main Authors Bonnet, Sophie, Caffin, Mathieu, Berthelot, Hugo, Grosso, Olivier, Benavides, Mar, Helias-Nunige, Sandra, Guieu, Cécile, Stenegren, Marcus, Foster, Rachel Ann
Format Journal Article
LanguageEnglish
Published Katlenburg-Lindau Copernicus GmbH 12.07.2018
Copernicus Publications
Subjects
Archipelagoes
Chemical analysis
Complementary DNA
Cruises
Cyanobacteria
DNA
Environmental factors
Fixation
Gene expression
Iron
Mass spectrometry
Mass spectroscopy
Mineral nutrients
NifH gene
Nitrogenation
Nutrient availability
Oligotrophy
Seawater
Secondary ion mass spectrometry
Stations
Tropical climate
Water analysis
Water sampling
South Pacific
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Summary:Here we report N2 fixation rates from a ∼ 4000 km transect in the western and central tropical South Pacific, a particularly undersampled region in the world ocean. Water samples were collected in the euphotic layer along a west to east transect from 160∘ E to 160∘ W that covered contrasting trophic regimes, from oligotrophy in the Melanesian archipelago (MA) waters to ultra-oligotrophy in the South Pacific Gyre (GY) waters. N2 fixation was detected at all 17 sampled stations with an average depth-integrated rate of 631 ± 286 µmolNm-2d-1 (range 196–1153 µmolNm-2d-1) in MA waters and of 85 ± 79 µmolNm-2d-1 (range 18–172 µmolNm-2d-1) in GY waters. Two cyanobacteria, the larger colonial filamentous Trichodesmium and the smaller UCYN-B, dominated the enumerated diazotroph community (> 80 %) and gene expression of the nifH gene (cDNA > 105 nifH copies L-1) in MA waters. Single-cell isotopic analyses performed by nanoscale secondary ion mass spectrometry (nanoSIMS) at selected stations revealed that Trichodesmium was always the major contributor to N2 fixation in MA waters, accounting for 47.1–83.8 % of bulk N2 fixation. The most plausible environmental factors explaining such exceptionally high rates of N2 fixation in MA waters are discussed in detail, emphasizing the role of macro- and micro-nutrient (e.g., iron) availability, seawater temperature and currents.
ISSN:1726-4170
1726-4189
DOI:10.5194/bg-15-4215-2018