Thesis defence: Mathieu Caffin (CYBELE), on December 21 at 9am will present his thesis

On the following topic: "Fate of dinitrogen fixation and carbon export in the western tropical South Pacific Ocean"

Friday, December 21, 2018 at 9:00 am; OCEANOMED Amphitheatre, 26M Building, 163 avenue de Luminy, 13009 Marseille

Thesis Director: Sophie Bonnet, IRD/MIO Research Director
Co-supervisor: Thierry Moutin, Professor AMU/MIO

We have found that the western tropical South Pacific Ocean was a hotspot of N2 fixation, with N2 fixation rates of 570 µmol N m-2 d-1 on average. In the region, diazotrophy would be a source of 16 Tg N yr-1, contributing to more than 10 % of the nitrogen input by diazotrophy in the global ocean. Two distincts sub-regions were identified during the OUTPACE cruise (transect from New Caledonia to French Polynesia). In the western part, the oligotrophic waters of the Melanesian archipelago presented high N2 fixation rates ( 600 µmol N m-2 d-1) and diazotrophes were dominated by Trichodesmium. In the eastern part, the ultra-oligotrophic waters of the South Pacific gyre presented lower N2 fixation rates ( 90 µmol N m-2 d-1), and diazotrophs were dominated by UCYN-B. Along the OUTPACE transect, N2 fixation rates were positively correlated with the primary production, the bacterial production, picoplankton and heterotrophic bacteria abundances.
The nitrogen budgets, performed using a lagrangian strategy at three representative stations of the region, show that N2 fixation contributed to more than 90 % of the of new nitrogen input in the photic layer, then nitrate input by vertical diffusion (1-8 %) and atmospheric deposition (< 1.5 %). N2 fixation sustained almost all of the new production and contributed to 13-18 % of the primary production in the Melanesian archipelago and 3 % in the South Pacific gyre. The e-ratios (particulate carbon export / primary production) measured in the region (up to 9.7 %) are far higher than the one measured in other regions impacted by diazotrophy, revealing the high export efficiency of this region. Diazotrophs in the water column were not effeciently exported, suggesting an indirect export after transfer of fixed nitrogen through non-diazotrophic planktonic organisms, which were subsequently exported.
The study of the transfer pathways of the fixed nitrogen has shown that 7 to 15 % of total N2 fixation was transferred to non-diazotrophs. The main beneficiaries of this nitrogen were autotrophic picoplancton (Synechococcus, Prochlorococcus) and heterotrophic bacteria. In addition, the transfer of fixed nitrogen to zooplankton which has been poorly studied, represented 8±5 % of the total N2 fixation when Trichodesmium was dominant, and 28±7 % when it was UCYN-B.
This PhD thesis indicates that diazotrophy sustains the biological pump in the western tropical South Pacific Ocean, and can have a critical influence in the planktonic community structure and in biogeochemical cycles of carbon and nitrogen in oligotrophic regions.

Keywords : Dinitrogen fixation, release, transfer, particulate export, biological pump, biogeochemical cycles