TY - JOUR T1 - Aphotic N2 fixation along an oligotrophic to ultraoligotrophic transect in the Western Tropical South Pacific Ocean A1 - Benavides,Mar A1 - Shoemaker,Katyanne M. A1 - Moisander,Pia H. A1 - Niggemann,Jutta A1 - Dittmar,Thorsten A1 - Duhamel,Solange A1 - Grosso,Olivier A1 - Pujo-Pay,Mireille A1 - Hélias-Nunige,Sandra A1 - Bonnet,Sophie AD - Aix Marseille Université, CNRS/INSU, Université de Toulon, IRD, Mediterranean Institute of Oceanography (MIO) UM 110, 98848 Nouméa, New Caledonia, France AD - Marine Biology Section, Department of Biology, University of Copenhagen, 3000 Helsingør, Denmark AD - Department of Biology, University of Massachusetts Dartmouth, 285 Old Westport Road, North 10 Dartmouth, MA 02747, USA AD - 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 AD - Lamont-Doherty Earth Observatory, Division of Biology and Paleo Environment, Columbia 15 University, PO Box 1000, 61 Route 9W, Palisades, NJ 10964, USA AD - Aix Marseille Université, CNRS/INSU, Université de Toulon, IRD, Mediterranean Institute of Oceanography (MIO) UM 110, 13288 Marseille, France AD - Laboratoire d’Océanographie Microbienne – UMR 7321, CNRS - Sorbonne Universités, UPMC Univ Paris 06, Observatoire Océanologique, 66650 Banyuls-sur-mer, France UR - https://archimer.ifremer.fr/doc/00420/53118/ DO - 10.5194/bg-2017-542 N2 - 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. Y1 - 2018 PB - Copernicus GmbH JF - Biogeosciences SN - 1726-4170 VL - 15 IS - 10 SP - 3107 EP - 3119 ID - 53118 ER -