AUTHOR=Meunier Valentine , Bonnet Sophie , Benavides Mar , Ravache Andreas , Grosso Olivier , Lambert Christophe , Houlbrèque Fanny 

TITLE=Diazotroph-Derived Nitrogen Assimilation Strategies Differ by Scleractinian Coral Species

JOURNAL=Frontiers in Marine Science

VOLUME=8

YEAR=2021

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2021.692248

DOI=10.3389/fmars.2021.692248

ISSN=2296-7745

ABSTRACT=<p>Reef-building corals generally thrive in nutrient-poor tropical waters, where among other elements, nitrogen (N) availability often limits primary productivity. In addition to their close association with endosymbiotic dinoflagellates of the family Symbiodiniaceae, enabling an effective use and retention of dissolved inorganic nitrogen (DIN), scleractinian corals have developed strategies to acquire new N: (1) They can ingest N-rich sediment particles and preys (from picoplankton to macro-zooplankton) via heterotrophy, including diazotrophs [plankton fixing dinitrogen (N<sub>2</sub>) and releasing part of this nitrogen—Diazotroph-Derived N (DDN)—in seawater], a pathway called “heterotrophic nutrition on diazotrophs”; (2) Symbiotic diazotrophs located in the coral holobiont have the molecular machinery to fix N<sub>2</sub>, a pathway called “symbiotic N<sub>2</sub> fixation”. Here we used the <sup>15</sup>N<sub>2</sub> isotopic labeling in a series of incubations to investigate the relative contribution of each of these DDN transfer pathways in three worldwide distributed coral species: <italic>Acropora muricata</italic>, <italic>Galaxea fascicularis, and Pocillopora damicornis</italic>. We show that N provision via “symbiotic N<sub>2</sub> fixation” is negligible compared to that obtained via “heterotrophic nutrition on diazotrophs,” with DDN assimilation rates about a thousand times lower for <italic>P. damicornis</italic> and <italic>G. fascicularis</italic>, or assimilation rates via “symbiotic N<sub>2</sub> fixation” almost nil for <italic>A. muricata</italic>. Through heterotrophic feeding on planktonic diazotrophs, only <italic>G. fascicularis</italic> and <italic>P. damicornis</italic> can successfully obtain N and fulfill a large part of their N requirements (DDN asimilation rates: 0.111 ± 0.056 and 0.517 ± 0.070 μg N cm<sup>–2</sup> h<sup>–1</sup> in their Symbiodiniaceae, respectively). Whereas this contribution is again negligible for <italic>A. muricata</italic>. They also largely consume the picoplankton that likely benefit from this DDN (<italic>Prochlorococcus</italic> and <italic>Synechococcus</italic> cells; respectively, 2.56 ± 1.57 10<sup>4</sup> and 2.70 ± 1.66 10<sup>4</sup> cell h<sup>–1</sup> cm<sup>–2</sup> for <italic>G. fascicularis</italic>; 3.02 ± 0.19 10<sup>5</sup> and 1.14 ± 0.79 10<sup>4</sup> cell h<sup>–1</sup> cm<sup>–2</sup> for <italic>P. damicornis</italic>). The present study confirms the different dependencies of the three tested species regarding heterotrophy, with <italic>P. damicornis</italic> and <italic>G. fascicularis</italic> appearing highly efficient at capturing plankton, while <italic>A. muricata</italic>, considered as mainly autotroph, does not rely on these food resources to meet its N and energy needs.</p>