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

在高产的热带珊瑚礁中，氮(N)是一种限制性营养物质，尽管它对珊瑚新陈代谢起着关键作用。一方面，这需要珊瑚虫有效地(重新)循环氮，即珊瑚动物及其藻类共生体(虫黄藻)和相关微生物。另一方面，较高的内部N浓度可能会从N限制转向P限制，从而对珊瑚全氮有害，这可能会导致例如增加漂白敏感性。因此，珊瑚相关微生物群通过固定大气氮素(N_2)提供生物有效氮，或通过反硝化作用潜在地缓解珊瑚全氮的过剩，从而发挥重要作用。重氮菌对氮气的固定对环境参数的变化非常敏感，而对反硝化的影响到目前为止还不清楚。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礁中。令人惊讶的是，这两个过程显然是相关的，并且可能共享全息离子内的共同能量来源，即从虫黄藻释放的光合作用产物。根据现有的证据，我推测一个稳定的珊瑚全氮细菌的特征是反硝化细菌、重氮菌和虫黄藻之间的平衡。偏离这一平衡可能会导致虫黄藻对氮的限制中断，这最终可能导致珊瑚全息健康的降低。拟议的项目DECODE(确定与珊瑚相关的反硝化的驱动因素)结合了季节性分辨率的实地研究和操纵性实验室实验。它将评估推动珊瑚礁反硝化作用的生物和非生物关键参数的影响及其对珊瑚微生物群的影响。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应用整体(包括对所有全息生物成员的研究)和跨学科的方法。因此，DECODE将提供珊瑚全息生物在不同环境变化情况下发挥作用的基本知识。

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.