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

Abstract

Abstract. Biological dinitrogen (N2) 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 diazotroph-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 released to the dissolved pool during N2 fixation and how much is transferred to bacteria, phytoplankton and zooplankton, and (ii) to compare the DDN release and transfer efficiencies under contrasting N2 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 15N2 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, ∼ 20–40 % of the N2 fixed during the experiment was released to the dissolved pool when Trichodesmium dominated, while the DDN release was not quantifiable when UCYN-B dominated; ∼ 7–15 % of the total fixed N (net N2 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 (∼ 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 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 (∼ 5–9 % of the DDN transfer) than when it was dominated by UCYN-B (∼ 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 N2 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.

The Delayed Island Mass Effect: How Islands can Remotely Trigger Blooms in the Oligotrophic Ocean

“…A secondary Phy peak is also observed around the Tri maximum, supported by nutrients released by Tri that, in turn, reenhance the bloom. Similar biological production enhancements due to Trichodesmium nutrient release have been observed previously (Bonnet et al, ; Caffin, Berthelot, et al, ). Once phosphate and/or iron stress become too high, the PCD causes the Tri bloom to collapse.…”

Distinct nitrogen cycling and steep chemical gradients in Trichodesmium colonies

“…About 10% of the newly fixed N was recovered as ammonium in the ambient water. Similar release rates of ammonium have been reported for natural populations in the South Pacific [4,67]. Consistently, concentrations of ammonium (and DON) have been shown to be enriched in surface waters during Trichodesmium blooms compared to pre-bloom conditions at Station ALOHA [68].…”

Meso-zooplankton structure and functioning in the western tropical South Pacific along the 20th parallel south during the OUTPACE survey (February–April 2015)

“…The percentage of estimated zooplankton respiration rates relative to primary production (averaging 29 % and 60 %, respectively, depending on the region; see Table 6), was high but within the range of global depth-integrated values reported by Calbet (2001). The lower rate recorded at the station west of New Caledonia (7 %, SD-1) was comparable to the 8 % measured by McKinnon et al (2015) in the Great Barrier Reef waters, NE Australia.…”