Coral reefs are known for being highly productive ecosystems in oligotrophic oceans, which is commonly referred to as the Darwin’s Paradox. Nitrogen is an essential component of organisms, but it limits primary productivity in most euphotic ocean, including the coral reef system. Therefore, understanding nitrogen’s transfer and transformation within the coral holobiont is essential to comprehend the holobiont homeostasis and functioning mechanisms, which may help to explain the Darwin’s Paradox. Previous studies have pointed out the fundamental importance of nitrogen cycling between coral host and symbiotic algae. Recently, increasing researches, particularly in quantitative aspect, have significantly improved our understandings of the various roles of nitrogen pathways in regulating the inter-relationship among coral host and symbiotic algae and the associated microbiome. In this paper, we synthesized knowledge advances of different nitrogen processes in coral holobionts standing on the nitrogen cycle perspective. We extracted consensus and contradictions from published research results regarding nitrogen flows of coral holobiont. This review presented the temporal and spatial variation of nitrogen fixation and analyzed the global nitrogen processes rates in coral holobionts. We also summarized projections of specific nitrogen processes of coral holobionts facing climate change from limited reports. We realized that there are significant gaps in our understanding of nitrogen processes in coral holobionts, which hindering our comprehension of nitrogen balance in coral holobionts and, therefore, the coral reef systems. These gaps include the roles and relative importance of nitrification, denitrification, and DNRA in coral holobionts, as well as the self-regulation mechanisms to maintain nitrogen-homeostasis in short-term and long-term, particularly in the context of environmental changes. At the end, we provide our opinions on research methods regarding quantitative coral research in the future.

中文翻译：

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了解珊瑚全生物中的氮动力学：全面回顾过程、进展、差距和未来方向

众所周知，珊瑚礁是贫营养海洋中高产的生态系统，这通常被称为达尔文悖论。氮是生物体的重要组成部分，但它限制了大多数透光海洋（包括珊瑚礁系统）的初级生产力。因此，了解氮在珊瑚全生物体内的转移和转化对于理解全生物体内平衡和功能机制至关重要，这可能有助于解释达尔文悖论。先前的研究指出了珊瑚宿主和共生藻类之间氮循环的根本重要性。最近，越来越多的研究，特别是在定量方面，显着提高了我们对氮途径在调节珊瑚宿主和共生藻类以及相关微生物群之间的相互关系中的各种作用的理解。在本文中，我们从氮循环的角度综合了珊瑚全生物不同氮过程的知识进展。我们从已发表的关于珊瑚全生物氮流的研究结果中提取了共识和矛盾。本综述介绍了固氮作用的时间和空间变化，并分析了珊瑚全生物的全球氮处理速率。我们还从有限的报告中总结了珊瑚全生物面临气候变化的特定氮过程的预测。我们意识到我们对珊瑚全生物中氮过程的理解存在重大差距，这阻碍了我们对珊瑚全生生物以及珊瑚礁系统中氮平衡的理解。这些差距包括硝化、反硝化和 DNRA 在珊瑚全生物中的作用和相对重要性，以及短期和长期维持氮稳态的自我调节机制，特别是在环境变化的背景下。最后，我们对未来定量珊瑚研究的研究方法提出了自己的看法。特别是在环境变化的背景下。最后，我们对未来定量珊瑚研究的研究方法提出了自己的看法。特别是在环境变化的背景下。最后，我们对未来定量珊瑚研究的研究方法提出了自己的看法。