Re-evaluation of the emergence of reef coral diseases based on the importance of ciliate infections

全球二氧化碳排放量主要由温带的西方经济体产生，但气候变化的影响主要出现在极地和热带地区的极端气候。虽然极地人口稀少，但珊瑚礁在直接支持全球至少5亿人方面发挥着至关重要的作用，尽管珊瑚礁只占世界海洋面积的0.1%。仅东南亚的珊瑚三角区就有1亿多人，他们几乎完全依赖沿海资源。珊瑚疾病是全球珊瑚礁减少的主要原因（一些科学家认为，在过去40年中，珊瑚礁减少了约40%），发展中国家的沿海居民几乎没有选择。许多科学家将珊瑚疾病的出现与气候变化联系起来，气候变化影响宿主的整体健康和抗病能力，并促进某些病原体的活动。然而，我们对珊瑚疾病的理解主要是基于这样一种假设，即大多数疾病是由细菌引起的。在使用传统的基于培养的方法识别致病因子方面普遍缺乏成功之后，我们的研究小组在十多年前开始使用基于分析环境样品中细菌DNA的现代非培养方法。虽然这项工作成功地促进了对几种常见珊瑚疾病的微生物生态学的理解，但在确定疾病的病因方面几乎没有突破。最近，在一个由NERC资助的项目中，研究了温度应力对珊瑚对疾病易感性的影响，我们发现几种最重要的珊瑚疾病与原生生物病原体以及细菌的大规模感染有关。原生生物（类似草履虫的纤毛虫）作为病原体通过侵入组织杀死珊瑚。正在进行的工作将解决疾病过程中纤毛虫和细菌病原体种群的相对变化，但毫无疑问，纤毛虫是疾病传播和病理学的重要媒介，可能是主要病原体。我们还表明，这些疾病具有高度的温度依赖性，这可能解释了过去20-30年全球疾病流行率的增加。因此，拟议的研究专门针对纤毛虫疾病，并将测试它们是否作为疾病的主要病原体（致病因子）或在另一种主要（可能是细菌）病原体之后侵入组织的次要机会病原体。要做到这一点，我们将应用传统的科赫假设，隔离潜在的病原体在文化和接种健康的珊瑚在控制孵化。我们还将调查世界各地的一些地方，以确定是否有非常相似的迹象疾病也与纤毛虫。其中一些疾病造成了严重的生态影响，例如，其中一种疾病（白色带状疾病）使埃尔克霍恩珊瑚不再是整个加勒比区域的主要珊瑚物种。由于这些疾病高度依赖于温度，我们将进行实验，以便更准确地模拟未来气候变化对珊瑚死亡率的影响。实验将区分温度对病原体活动增加的影响和宿主珊瑚敏感性的变化。我们将进一步研究易感性的变化，以确定珊瑚在健康条件下抵抗纤毛虫感染的可能机制。总之，这些研究将使我们能够从机理上理解温度如何影响疾病过程，因此我们可以模拟未来气候变化的影响，而不仅仅是模拟过去的历史。研究的最终综合将使我们能够从根本上重新评估过去20-30年珊瑚疾病的出现，预测未来的变化并提出潜在的管理解决方案。

Global CO2 emissions are produced mainly by western economies in the temperate zones, however the impacts of climate change are mainly being seen at the climatic extremes of the poles and tropical zones. While the poles are scarcely populated, coral reefs play a vital role in directly supporting at least 500 million people worldwide, despite only representing 0.1% of the world's ocean area. The Coral Triangle in south-east Asia alone includes over 100 million people who are almost entirely dependent on coastal resources. Coral diseases have contributed significantly to global declines in coral reefs (some scientists put the figure at about 40% loss over the last 40 years), leaving few options for coastal peoples of developing countries. Many scientists have linked the emergence of coral diseases to climate change that affects the overall health and disease resistance of the host as well as promoting the activity of some pathogens. However, our understanding of coral diseases is driven mainly on the assumption that most are caused by bacteria. Following a general lack of success in identifying causal agents using traditional culture-based approaches, our group began using modern culture-independent methods based on analysing bacterial DNA in environmental samples over ten years ago. While this work has been successful in advancing understanding of the microbial ecology of several common coral diseases, there have been few breakthroughs in determining the causal agents of disease. More recently, working on a NERC-funded project to investigate temperature stress effects on coral susceptibility to disease, we have discovered that several of the most important types of coral disease are associated with mass infections by protist pathogens, as well as bacteria. The protists (ciliates similar to Paramecium) act as pathogens kill the coral by ingesting the tissues. Ongoing work will address the relative changes in ciliate and bacterial pathogen populations during the disease process, but there is no doubt that the ciliates are important agents in disease transmission and pathology and may be the primary pathogens. We have also shown that these diseases are highly temperature-dependent, which may explain the global increase in disease prevalence in the last 20-30 years. The proposed study therefore addresses the ciliate diseases specifically and will test whether they are acting as primary pathogens (causal agents) of the disease or secondary, opportunistic pathogens invading the tissues after another primary (possibly bacterial) pathogen. To do this we will apply traditional Koch's postulates, isolating the potential pathogens in culture and innoculating healthy corals in controlled incubations. We will also survey a number of locations worldwide to determine whether diseases with very similar signs are also associated with ciliates. Some of these diseases have caused serious ecological impacts, for example one (White Band Disease) has elimnated elkhorn coral as the dominant coral species in the whole Caribbean region. Since the diseases are highly temperature-dependent, we will conduct experiments to allow us to more accurately model the impacts of future climate change scenarios on coral mortality. The experiments will distinguish the effects of temperature on increased pathogen activity and changes in host coral susceptibility. We will further investigate the changes in susceptibility to determine the likely mechanisms by which corals resist ciliate infections under healthy conditions. Together, these studies will allow a mechanistic understanding of how temperature affects the disease process, so we can model the effects of future climate change, rather than just model past history. The final synthesis of the research will allow us to fundamentally re-evaluate the emergence of coral diseases in the last 20-30 years as well as predict future changes and propose potential management solutions.