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Aphotic N2 fixation along an oligotrophic to ultraoligotrophic transect in the Western Tropical South Pacific Ocean

Mar Benavides; Katyanne M. Shoemaker; Pia H. Moisander; Jutta Niggemann; Thorsten Dittmar; Solange Duhamel; Olivier Grosso; Mireille Pujo-Pay; Sandra Hélias-Nunige; Alain Fumenia; Sophie Bonnet

Title:
Aphotic N2 fixation along an oligotrophic to ultraoligotrophic transect in the Western Tropical South Pacific Ocean
Author(s):
Benavides, Mar
Shoemaker, Katyanne M.
Moisander, Pia H.
Niggemann, Jutta
Dittmar, Thorsten
Duhamel, Solange
Grosso, Olivier
Pujo-Pay, Mireille
Hélias-Nunige, Sandra
Fumenia, Alain
Bonnet, Sophie
Date:
Type:
Articles
Department(s):
Lamont-Doherty Earth Observatory
Volume:
15
Persistent URL:
Book/Journal Title:
Biogeosciences
Geographic Area:
South Pacific Ocean
Abstract:
The western tropical South Pacific (WTSP) Ocean has been recognized as a global hot spot of dinitrogen (N2) fixation. Here, as in other marine environments across the oceans, N2 fixation studies have focused on the sunlit layer. However, studies have confirmed the importance of aphotic N2 fixation activity, although until now only one had been performed in the WTSP. In order to increase our knowledge of aphotic N2 fixation in the WTSP, we measured N2 fixation rates and identified diazotrophic phylotypes in the mesopelagic layer along a transect spanning from New Caledonia to French Polynesia. Because non-cyanobacterial diazotrophs presumably need external dissolved organic matter (DOM) sources for their nutrition, we also identified DOM compounds using Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) with the aim of searching for relationships between the composition of DOM and non-cyanobacterial N2 fixation in the aphotic ocean. N2 fixation rates were low (average 0.63 ± 0.07 nmol N L−1 d−1) but consistently detected across all depths and stations, representing ∼ 6–88 % of photic N2 fixation. N2 fixation rates were not significantly correlated with DOM compounds. The analysis of nifH gene amplicons revealed a wide diversity of non-cyanobacterial diazotrophs, mostly matching clusters 1 and 3. Interestingly, a distinct phylotype from the major nifH subcluster 1G dominated at 650 dbar, coinciding with the oxygenated Subantarctic Mode Water (SAMW). This consistent pattern suggests that the distribution of aphotic diazotroph communities is to some extent controlled by water mass structure. While the data available are still too scarce to elucidate the distribution and controls of mesopelagic non-cyanobacterial diazotrophs in the WTSP, their prevalence in the mesopelagic layer and the consistent detection of active N2 fixation activity at all depths sampled during our study suggest that aphotic N2 fixation may contribute significantly to fixed nitrogen inputs in this area and/or areas downstream of water mass circulation.
Subject(s):
Nitrogen--Fixation
Biogeochemistry
Oceanography
Publisher DOI:
https://doi.org/10.5194/bg-15-3107-2018
Item views
10
Metadata:
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Suggested Citation:
Mar Benavides, Katyanne M. Shoemaker, Pia H. Moisander, Jutta Niggemann, Thorsten Dittmar, Solange Duhamel, Olivier Grosso, Mireille Pujo-Pay, Sandra Hélias-Nunige, Alain Fumenia, Sophie Bonnet, , Aphotic N2 fixation along an oligotrophic to ultraoligotrophic transect in the Western Tropical South Pacific Ocean, Columbia University Academic Commons, .

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