Biogeosciences, 15, 3107-3119, 2018
https://doi.org/10.5194/bg-15-3107-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
22 May 2018
Aphotic N2 fixation along an oligotrophic to ultraoligotrophic transect in the western tropical South Pacific Ocean
Mar Benavides1,2, Katyanne M. Shoemaker3, Pia H. Moisander3, Jutta Niggemann4, Thorsten Dittmar4, Solange Duhamel5, Olivier Grosso6, Mireille Pujo-Pay7, Sandra Hélias-Nunige6, Alain Fumenia1, and Sophie Bonnet1 1Aix Marseille Université, CNRS/INSU, Université de Toulon, IRD, Mediterranean Institute of Oceanography (MIO) UM 110, 98848 Nouméa, New Caledonia
2Marine Biology Section, Department of Biology, University of Copenhagen, 3000 Helsingør, Denmark
3Department of Biology, University of Massachusetts Dartmouth, 285 Old Westport Road, North Dartmouth, MA 02747, USA
4Research Group for Marine Geochemistry (MPI-ICBM Bridging Group), Institute for Chemistry and Biology of the Marine Environment University of Oldenburg, Carl-von-Ossietzky-Strasse 9–11, 26129 Oldenburg, Germany
5Lamont-Doherty Earth Observatory, Division of Biology and Paleo Environment, Columbia University, P.O. Box 1000, 61 Route 9W, Palisades, NJ 10964, USA
6Aix Marseille Université, CNRS/INSU, Université de Toulon, IRD, Mediterranean Institute of Oceanography (MIO) UM 110, 13288 Marseille, France
7Laboratoire d'Océanographie Microbienne – UMR 7321, CNRS – Sorbonne Universités, UPMC Univ Paris 06, Observatoire Océanologique, 66650 Banyuls-sur-mer, France
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.
Citation: Benavides, M., Shoemaker, K. M., Moisander, P. H., Niggemann, J., Dittmar, T., Duhamel, S., Grosso, O., Pujo-Pay, M., Hélias-Nunige, S., Fumenia, A., and Bonnet, S.: Aphotic N2 fixation along an oligotrophic to ultraoligotrophic transect in the western tropical South Pacific Ocean, Biogeosciences, 15, 3107-3119, https://doi.org/10.5194/bg-15-3107-2018, 2018.
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We measured N2 fixation rates and identified diazotrophic phylotypes in the mesopelagic layer along a transect spanning from New Caledonia to French Polynesia. N2 fixation rates were low but consistently detected across all depths and stations. A distinct diazotrophic phylotype dominated at 650 dbar, coinciding with the oxygenated Subantarctic Mode Water (SAMW) and suggesting that the distribution of aphotic diazotroph communities is to some extent controlled by water mass structure.
We measured N2 fixation rates and identified diazotrophic phylotypes in the mesopelagic layer...
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