DEFINING NUTRITIONAL BOTTLENECKS OF REEF CORAL GROWTH AND STRESS TOLERANCE

珊瑚礁是地球上最具魅力和最多样化的生态系统之一，仅占海洋表面面积的不到1%，就维持着所有海洋生物多样性的25%，并维持着5亿人的生计。然而，建造栖息地的珊瑚极易受到人为变化的影响。研究得最好的是海洋变暖对珊瑚礁造成的威胁。对自然营养环境的扰动也代表了一种生存危险，但为什么会这样呢？这个问题由来已久。在查尔斯·达尔文的开创性观察超过175年后，我们仍然缺乏一个令人信服的机制解释，为什么珊瑚礁珊瑚在营养匮乏的海洋地区茁壮成长，保持异常高的生物产量。答案当然植根于细胞内微藻（“虫黄藻”）和珊瑚动物宿主之间的共生关系，但其中的生理过程还远未被理解。最近的实地研究从根本上与现有的共生功能模型相矛盾，这些模型表明，珊瑚的大部分营养物质来自水柱，如溶解的无机氮（DIN）和磷（DIP），但在珊瑚动物中没有已知的这种过程的机制。我们的工作（本文的初步研究结果）表明，珊瑚“放牧”它们的共生体种群，为DIN和DIP向宿主转移提供了一种新的机制。我们建议通过在十个具有代表性的造礁物种中测试放牧假设来改变我们对全球珊瑚营养循环和共生的理解。我们将：(i)在受控的长期实验中，通过（15N）同位素标记和元素分析，记录共生体对DIN和DIP的吸收和利用。ii)量化共生放牧与浮游动物摄食对营养物质吸收的效率，并比较其对珊瑚生长和耐受性的影响。（iii）确定当前和预计的未来海洋温度和pH值如何影响溶解无机营养物的利用，并影响珊瑚生长和共生功能。我们亦会举办一系列促进影响的活动，包括知识转移工作坊。在这次活动中，我们将邀请珊瑚礁管理和政策制定方面的利益相关者，以及珊瑚礁营养生物学、空间生态学和生物地球化学模型方面的领先专家，将我们的研究结果转化为政策制定者和珊瑚礁管理方面的建议。

Among the most charismatic and diverse ecosystems on Earth, coral reefs sustain >25% of all marine biodiversity in only <1% of ocean surface area and the livelihoods of >0.5 billion people. Yet the habitat-building corals are acutely vulnerable to anthropogenic change. Best studied are the threats posed to coral reefs by ocean warming. Perturbations to the natural nutrient environment also represent an existential danger, but why is this the case? This question has old origins. More than 175 years after Charles Darwin's seminal observation, we still lack a convincing mechanistic explanation as to why reef corals thrive, sustaining exceptionally high rates of biological production, in nutrient-poor oceanic regions. The answer is certainly rooted in the symbiotic association between endocellular microalgae ("zooxanthellae") and the coral animal host but the physiological processes involved are far from understood. Recent field studies fundamentally contradict existing models for symbiosis functioning by suggesting that corals source most of their nutrients from the water column as dissolved inorganic nitrogen (DIN) and phosphorus (DIP) but there is no known mechanism for such a process in coral animals. Our work (pilot study results herein) suggests that corals "graze" on their symbiont population, providing a novel mechanism for DIN and DIP transfer to the host. We propose to transform our understanding of nutrient cycling and symbiosis in corals worldwide by testing the grazing hypothesis in ten representative reef-building species.We will: (i) Document DIN and DIP uptake and utilisation by symbiont-grazing through (15N) isotopic labelling and elemental analysis in controlled long-term experiments. ii) Quantify the efficiency of nutrient incorporation by symbiont-grazing versus zooplankton-feeding and compare the resulting effects on coral growth and stress tolerance. iii) Determine how present-day and projected future ocean temperature and pH influence the utilisation of dissolved inorganic nutrients and shape coral growth and symbiosis functioning.We will also undertake a broad range of impact-promoting activities including a knowledge transfer workshop. For this event, we will bring stakeholders in coral reef management and policy development together with leading specialists in coral reef nutrient biology, spatial ecology and biogeochemical modelling to translate our findings into advice for policy makers and coral reef management.