Coral holobiont functioning under environmental change: Determining the drivers of coral-associated denitrification - DECODE

氮（N）是高产热带珊瑚礁的限制性营养素，尽管它对珊瑚代谢起着关键作用。一方面，这需要珊瑚全生物（即珊瑚动物及其藻类共生体（虫黄藻）和相关微生物）对氮进行有效的（再）循环。另一方面，高的内部N浓度可能是有害的珊瑚holobiont从N限制转向P限制，这可能会导致例如增加漂白敏感性。因此，珊瑚相关的微生物组起着重要的作用，通过固定大气中的二氮（N2）提供生物可利用的N或潜在地减轻珊瑚holobiont从过量的N通过反硝化。固氮生物固氮对环境参数的变化非常敏感，而对反硝化作用的影响迄今为止尚不清楚。最近的研究表明，这两个关键的N-循环过程同时发生在珊瑚holobiont。令人惊讶的是，这两个过程显然是相关的，并可能共享一个共同的能源内的holobiont，即从zooxanthropium释放的光合作用。基于现有的证据，我假设，一个稳定的珊瑚holobiont的特点是由一个平衡之间的浮游生物，固氮菌和zooxanthropium。偏离这种平衡可能会导致氮限制的zooxanthropium，这可能最终导致珊瑚holobiont健康减少中断。拟议的项目DECODE（确定珊瑚相关脱氮的驱动因素）结合了季节性分辨率的实地研究和操纵性实验室实验。它将评估驱动珊瑚全生物脱氮的生物和非生物关键参数的影响及其对珊瑚微生物组的影响。因此，我将在动物生理学，微生物生态学和生物地球化学之间的界面上应用整体（涉及所有holobiont成员的研究）和跨学科的方法。因此，DECODE将提供关于珊瑚全生物在不同环境变化情景下运作的基本知识。

Nitrogen (N) is a limiting nutrient in highly productive tropical coral reefs despite its key role for coral metabolism. This on the one hand requires efficient (re)cycling of N by coral holobionts, i.e. the coral animal with its algal symbionts (zooxanthellae) and associated microbes. On the other hand, high internal N concentrations can be detrimental to the coral holobiont by shifting from N limitation towards P limitation which may lead to e.g. increased bleaching susceptibility. Thus, the coral associated microbiome plays an important role by either providing bioavailable N through fixation of atmospheric dinitrogen (N2) or potentially alleviating the coral holobiont from excess N through denitrification. N2 fixation by diazotrophs is highly susceptible to shifts in environmental parameters, while the effects on denitrification are so far unknown. Very recent research suggests that these two key N-cycling processes occur simultaneously in coral holobionts. Surprisingly, both processes are obviously correlated and likely share a common energy source within the holobiont, i.e. photosynthates released from the zooxanthellae. Based on the available evidence, I hypothesize that a stable coral holobiont is characterized by a balance between denitrifiers, diazotrophs and zooxanthellae. A deviation from this balance can lead to an interruption of N limitation for the zooxanthellae, which may ultimately lead to reduced coral holobiont health. The proposed project DECODE (Determining the drivers of coral-associated denitrification), combines field studies in seasonal resolution and manipulative laboratory experiments. It will assess the effects of the biotic and abiotic key parameters that drive denitrification in coral holobionts and their effects on the coral microbiome. Thereby, I will apply a holistic (involving studies of all holobiont members) and interdisciplinary approach at the interface between animal physiology, microbial ecology, and biogeochemistry. Thereby, DECODE will provide fundamental knowledge on coral holobiont functioning under different environmental change scenarios.