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. Biological dinitrogen ( N2 ) fixation provides the major source ofnew nitrogen (N) to the open ocean, contributing more than atmosphericdeposition and riverine inputs to the N supply. Yet the fate of thediazotroph-derived 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 releasedto the dissolved pool during N2 fixation and how much istransferred to bacteria, phytoplankton and zooplankton, and (ii) to comparethe DDN release and transfer efficiencies under contrasting N2 fixation activity and diversity in the oligotrophic waters of the westerntropical South Pacific (WTSP) Ocean. We used nanometre-scale secondary ionmass spectrometry (nanoSIMS) coupled with 15 N2 isotopiclabelling and flow cytometry cell sorting to track the DDN transfer toplankton, in regions where the diazotroph community was dominated by eitherTrichodesmium or by UCYN-B. After 48 h, ∼  20–40 % of the N2 fixed during the experiment was released to the dissolved poolwhen Trichodesmium dominated, while the DDN release was notquantifiable when UCYN-B dominated; ∼  7–15 % of the total fixed N(net N2 fixation  +  release) was transferred to non-diazotrophicplankton 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 andProchlorococcus were the primary beneficiaries of the DDNtransferred ( ∼  65–70 %), followed by heterotrophic bacteria( ∼  23–34 %). The DDN transfer in bacteria was higher(34  ±  7 %) in the UCYN-B-dominating experiment compared to theTrichodesmium-dominating experiments (24  ±  5 %). Regardinghigher trophic levels, the DDN transfer to the dominant zooplankton specieswas less efficient when the diazotroph community was dominated byTrichodesmium ( ∼  5–9 % of the DDN transfer) than when itwas dominated by UCYN-B ( ∼  28  ±  13 % of the DDN transfer). Toour knowledge, this study provides the first quantification of DDN releaseand transfer to phytoplankton, bacteria and zooplankton communities in openocean waters. It reveals that despite UCYN-B fix N2 at lower ratescompared to Trichodesmium in the WTSP, the DDN from UCYN-B is muchmore available and efficiently transferred to the planktonic food web thanthe DDN originating from Trichodesmium.

biogeosciences

Transfer of diazotroph-derived nitrogen to the planktonic food web across gradients of N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fixation activity and diversity in the western tropical South Pacific Ocean