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 N2 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 N2, a pathway called ‘symbiotic N2 fixation’. Here we used the 15N2 isotopic labelling in a series of incubations to investigate the relative contribution of each of these DDN transfer pathways in three worldwide distributed coral species: Acropora muricata, Pocillopora damicornis and Galaxea fascicularis. We show that N provision via ‘symbiotic N2 fixation’ is negligible compared to that obtained via ‘heterotrophic nutrition on diazotrophs’, with rates about a thousand times lower for P. damicornis and G. fascicularis, or near-zero for A. muricata. Through heterotrophic feeding on planktonic diazotrophs, G. fascicularis and P. damicornis 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-2 h-1 in their Symbiodiniaceae respectively). They also largely consume the picoplankton that likely benefit from this DDN (Prochlorococcus and Synechococcus cells; respectively 2.56 ± 1.57 104 and 2.70 ± 1.66 104 cell h-1 cm-2 for G. fascicularis; 3.02 ± 0.19 105 and 1.14 ± 0.79 104 cell h-1 cm-2 for P. damicornis). The present study confirms the different dependencies of the three tested species regarding heterotrophy, with P. damicornis and G. fascicularis appearing highly efficient at capturing plankton, while A. muricata, considered as mainly autotroph, does not rely on these food resources to meet its N and energy needs.

中文翻译：

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固氮衍生的氮同化策略因石珊瑚物种而异

造礁珊瑚通常在营养贫乏的热带水域茁壮成长，其中除其他元素外，氮 (N) 的可用性通常会限制初级生产力。除了与共生科的内共生鞭毛藻密切相关，能够有效利用和保留溶解的无机氮 (DIN)，石珊瑚还制定了获取新 N 的策略：（1）它们可以摄取富含 N 的沉积物颗粒和猎物（从微型浮游生物到大型浮游动物）通过异养，包括固氮生物（浮游生物固定 N2 并释放部分氮 - 固氮衍生 N (DDN)） - 在海水中，一种称为“固氮异养营养”的途径；(2) 位于珊瑚全生物体中的共生固氮菌具有固定 N2 的分子机制，这种途径称为“共生 N2 固定”。在这里，我们在一系列孵化中使用 15N2 同位素标记来研究这些 DDN 转移途径中的每一个在三种全球分布的珊瑚物种中的相对贡献：Acropora muricata、Pocillopora damicornis 和 Galaxea fascicularis。我们表明，与通过“固氮菌的异养营养”获得的相比，通过“共生 N2 固定”提供的 N 可以忽略不计，P. damicornis 和 G. fascicularis 的比率低约一千倍，而 A. muricata 的比率接近零。通过对浮游固氮菌的异养喂养，G. fascicularis 和 P. damicornis 可以成功获得 N 并满足其大部分 N 需求（DDN 同化率：0.111 ± 0.056 和 0.517 ± 0.070 µg N cm-2 h-1 in their Symbiodini分别）。它们还大量消耗可能受益于这种 DDN 的微型浮游生物（原绿球藻和聚球藻细胞；对于 G. fascicularis，分别为 2.56 ± 1.57 104 和 2.70 ± 1.66 104 细胞 h-1 cm-2；3.02 ± 0.19 105 和 1.105 ± 1.794 h-1 cm-2 为 P. damicornis）。本研究证实了三个测试物种在异养方面的不同依赖性，P. damicornis 和 G. fascicularis 在捕获浮游生物方面表现出很高的效率，而被认为主要是自养的 A. muricata 并不依赖这些食物资源来满足其自身的需求。 N 和能源需求。