FN Archimer Export Format PT J TI Aphotic N2 fixation along an oligotrophic to ultraoligotrophic transect in the Western Tropical South Pacific Ocean BT AF Benavides, Mar Shoemaker, Katyanne M. Moisander, Pia H. Niggemann, Jutta Dittmar, Thorsten Duhamel, Solange Grosso, Olivier Pujo-Pay, Mireille Hélias-Nunige, Sandra Bonnet, Sophie AS 1:1,2;2:3;3:3;4:4;5:4;6:5;7:6;8:7;9:6;10:1; FF 1:;2:;3:;4:;5:;6:;7:;8:;9:;10:; C1 Aix Marseille Université, CNRS/INSU, Université de Toulon, IRD, Mediterranean Institute of Oceanography (MIO) UM 110, 98848 Nouméa, New Caledonia, France Marine Biology Section, Department of Biology, University of Copenhagen, 3000 Helsingør, Denmark Department of Biology, University of Massachusetts Dartmouth, 285 Old Westport Road, North 10 Dartmouth, MA 02747, USA Research 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, D- 26129 Oldenburg, Germany Lamont-Doherty Earth Observatory, Division of Biology and Paleo Environment, Columbia 15 University, PO Box 1000, 61 Route 9W, Palisades, NJ 10964, USA Aix Marseille Université, CNRS/INSU, Université de Toulon, IRD, Mediterranean Institute of Oceanography (MIO) UM 110, 13288 Marseille, France Laboratoire d’Océanographie Microbienne – UMR 7321, CNRS - Sorbonne Universités, UPMC Univ Paris 06, Observatoire Océanologique, 66650 Banyuls-sur-mer, France C2 UNIV AIX MARSEILLE, FRANCE UNIV COPENHAGEN, DENMARK UNIV MASSACHUSETTS DARTMOUTH, USA UNIV OLDENBURG, GERMANY LAMONT-DOHERTY EARTH OBSERVATORY, USA UNIV AIX MARSEILLE, FRANCE UNIV PARIS 06, FRANCE IN DOAJ IF 3.951 TC 0 UR https://archimer.ifremer.fr/doc/00420/53118/54070.pdf https://archimer.ifremer.fr/doc/00420/53118/54076.pdf https://archimer.ifremer.fr/doc/00420/53118/56814.pdf https://archimer.ifremer.fr/doc/00420/53118/56815.pdf LA English DT Article CR OUTPACE BO L'Atalante AB The western tropical South Pacific (WTSP) Ocean has been recognized as a global hotspot of dinitrogen (N2) fixation. Here, as in other marine environments across the oceans, N2 fixation studies have focused in 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, here we measure N2 fixation rates and identify 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 Mass Spectrometry (FTICRMS). 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 to DOM compounds. The analysis of nifH gene amplicons revealed a wide diversity of non-cyanobacterial diazotrophs, majorly matching clusters 1 and 3. Interestingly, a distinct phylotype from the major nifH subcluster 1G dominated at 650 dbar, coinciding with the oxygenated Sub-Antarctic 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 is 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. PY 2018 SO Biogeosciences SN 1726-4170 PU Copernicus GmbH VL 15 IS 10 BP 3107 EP 3119 DI 10.5194/bg-2017-542 ID 53118 ER EF