In-depth characterization of diazotroph activity across the western tropical South Pacific hotspot of N<sub>2</sub> fixation (OUTPACE cruise)
<p>Here we report N<sub>2</sub> 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 tran...
| Main Authors: | , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-07-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/15/4215/2018/bg-15-4215-2018.pdf |
| Summary: | <p>Here we report N<sub>2</sub> 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. N<sub>2</sub> fixation was detected at all 17
sampled stations with an average depth-integrated rate of
631 ± 286 µmol N m<sup>−2</sup> d<sup>−1</sup> (range
196–1153 µmol N m<sup>−2</sup> d<sup>−1</sup>) in MA waters and of
85 ± 79 µmol N m<sup>−2</sup> d<sup>−1</sup> (range
18–172 µmol N m<sup>−2</sup> d<sup>−1</sup>) in GY waters. Two cyanobacteria,
the larger colonial filamentous <i>Trichodesmium</i> and the smaller
UCYN-B, dominated the enumerated diazotroph community ( >  80 %) and gene
expression of the <i>nifH</i> gene (cDNA  >  10<sup>5</sup> <i>nifH</i> copies
L<sup>−1</sup>) in MA waters. Single-cell isotopic analyses performed by nanoscale
secondary ion mass spectrometry (nanoSIMS) at selected stations revealed
that <i>Trichodesmium</i> was always the major contributor to N<sub>2</sub>
fixation in MA waters, accounting for 47.1–83.8 % of bulk N<sub>2</sub>
fixation. The most plausible environmental factors explaining such
exceptionally high rates of N<sub>2</sub> fixation in MA waters are discussed
in detail, emphasizing the role of macro- and micro-nutrient (e.g., iron)
availability, seawater temperature and currents.</p> |
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| ISSN: | 1726-4170 1726-4189 |