Transfer of diazotroph-derived nitrogen to the planktonic food web across gradients of N-2 fixation activity and diversity in the western tropical South Pacific Ocean

Biological dinitrogen (N-2) fixation provides the major source of new nitrogen (N) to the open ocean, contributing more than atmospheric deposition and riverine inputs to the N supply. Yet the fate of the diazotrophderived N (DDN) in the planktonic food web is poorly understood. The main goals of this study were (i) to quantify how much of DDN is released to the dissolved pool during N-2 fixation and how much is transferred to bacteria, phytoplankton and zooplankton, and (ii) to compare the DDN release and transfer efficiencies under contrasting N-2 fixation activity and diversity in the oligotrophic waters of the western tropical South Pacific (WTSP) Ocean. We used nanometre-scale secondary ion mass spectrometry (nanoSIMS) coupled with N-15(2) isotopic labelling and flow cytometry cell sorting to track the DDN transfer to plankton, in regions where the diazotroph community was dominated by either Trichodesmium or by UCYN-B. After 48 h, 2 similar to 20-40% of the N-2 fixed during the experiment was released to the dissolved pool when Trichodesmium dominated, while the DDN release was not quantifiable when UCYNB dominated; similar to 7-15% of the total fixed N (net N-2 fixation + release) was transferred to non-diazotrophic plankton within 48 h, with higher transfer efficiencies (15 +/- 3 %) when UCYN-B dominated as compared to when Trichodesmium dominated (9 +/- 3 %). The pico-cyanobacteria Synechococcus and Prochlorococcus were the primary beneficiaries of the DDN transferred (similar to 65-70 %), followed by heterotrophic bacteria (similar to 23-34 %). The DDN transfer in bacteria was higher (34 +/- 7 %) in the UCYN-B-dominating experiment compared to the Trichodesmium-dominating experiments (24 +/- 5 %). Regarding higher trophic levels, the DDN transfer to the dominant zooplankton species was less efficient when the diazotroph community was dominated by Trichodesmium (similar to 5-9% of the DDN transfer) than when it was dominated by UCYN-B (similar to 28 +/- 13% of the DDN transfer). To our knowledge, this study provides the first quantification of DDN release and transfer to phytoplankton, bacteria and zooplankton communities in open ocean waters. It reveals that despite UCYN-B fix N-2 at lower rates compared to Trichodesmium in the WTSP, the DDN from UCYN-B is much more available and efficiently transferred to the planktonic food web than the DDN originating from Trichodesmium.