NIitrogen cycling in Coral reef organisms under Environmental change - NICE

氮(N)是高产珊瑚礁环境中的限制性营养物质之一，对珊瑚礁生物的代谢和生态系统的功能起着关键作用。最近的研究表明，微生物介导的二氮（N2）固定在珊瑚礁中是普遍存在的，并且活性的固氮原核生物（重氮营养体）与许多不同的珊瑚礁生物有关。这些重氮营养体可能与硬核珊瑚形成独特的联系，硬核珊瑚是关键的珊瑚礁生态系统工程师。然而，在珊瑚礁中，还没有研究N2固定与海洋N循环的其他关键过程（即硝化和反硝化）的相互作用，以及所有这些过程对关键环境干扰（如变暖和营养物富集）的易感性。这些过程很可能是珊瑚礁生物，特别是硬珊瑚中微生物N循环的重要特征，这可以使它们适应脆弱而重要的生态系统中的环境变化。在少营养珊瑚礁环境中，珊瑚的氮固定似乎是克服氮饥饿所必需的。同时，在人类活动引起的营养物富集存在的情况下，硝化和反硝化都可能是维持共生藻类（Symbiodinium）在珊瑚宿主体内生长受限的重要过程。在这种情况下，全球变暖导致的海洋表面温度的预期升高可能会改变珊瑚和其他珊瑚礁生物中的微生物氮循环。最终，氮的收益和损失之间的不平衡可能会影响调节珊瑚共生的微妙平衡，导致白化的发生。目前的研究表明，海洋变暖和溶解有机碳（DOC）富营养化对微生物氮循环的干扰可能与珊瑚白化有关。拟议的项目NICE（环境变化下珊瑚礁生物中的氮循环）将在红海中部进行一系列相互关联的描述性和实验研究，从而量化所有主要过程，并确定硬珊瑚和其他常见珊瑚礁生物（包括软珊瑚和藻类）中氮循环的相关微生物参与者。结合珊瑚生理学、分子微生物生态学、生物地球化学和珊瑚礁生态学的专业知识，跨学科的方法将允许测试上述假设。重要的是，将模拟一系列全球和局部环境干扰（温度升高，无机和有机富营养化）以了解N循环响应。相比之下，NICE将为珊瑚礁生物的氮循环提供新颖和基础的知识。这个项目产生的成果将有效地促进以科学为基础的珊瑚礁管理。

Nitrogen (N) is one of the limiting nutrients in highly productive coral reef environments and thus plays a key role in the metabolism of reef organisms and the functioning of their ecosystems. Recent research revealed that microbe-mediated dinitrogen (N2) fixation is ubiquitous in coral reefs, and that active N2-fixing prokaryotes (diazotrophs) are associated with many different reef organisms. These diazotrophs may form characteristic associations with scleractinian corals, the key reef ecosystem engineers. However, the interplay of N2 fixation with other key processes of the marine N cycle, i.e. nitrification and denitrification, and the susceptibility of all these processes to key environmental disturbances (e.g. warming and nutrient enrichment) has not yet been investigated in coral reefs. Such processes are very likely important features of microbial N cycling in reef organisms, particularly hard corals, which can allow them to adapt to environmental changes in their fragile and important ecosystems. N2 fixation in corals appears to be essential for overcoming N starvation in oligotrophic reef environments. At the same time, in the presence of nutrient enrichment caused by human activities, both nitrification and denitrification may be important processes for maintaining the symbiotic algae (Symbiodinium) growth-limited in the coral host. In this context, expected increases in sea surface temperature resulting from global warming can potentially alter microbial N cycling in corals and other reef organisms. Ultimately, an imbalance between N gains and losses may impact the delicate equilibrium that regulates coral symbiosis, resulting in the onset of bleaching. The state of knowledge suggests that a disturbance of microbial N cycling via ocean warming and dissolved organic carbon (DOC) eutrophication may be involved in coral bleaching. The proposed project NICE (NItrogen Cycling in Coral Reef organisms under Environmental change), using a series of interconnected descriptive and experimental studies at the central Red Sea, will thus quantify all major processes and identify associated microbial players of the N cycle in hard corals and other common reef organisms including soft corals and algae. An interdisciplinary approach combining expertize from coral physiology, molecular microbial ecology, biogeochemistry, and reef ecology will allow testing the hypotheses mentioned above. Importantly, a range of global and local environmental disturbances (increased temperature, and inorganic as well as organic eutrophication) will be simulated to understand N cycle responses. NICE thereby will provide novel and fundamental knowledge of N cycling in coral reef organisms in comparison. The results generated by this project will effectively contribute to a better science-based management of coral reefs.