Reef Corals: Symbiotic Dinoflagellate/Host Combinations and their Physiological Response to Environmental Change

珊瑚礁和相关珊瑚的生长、钙化、营养、繁殖、健康以及在许多情况下的形态是它们与共生甲藻共生的结果，共生甲鞭毛虫通常被称为“虫黄藻”。在过去的30年里，已经描述了几种不同的共生线虫，以及至少五个分子“分支”中的数十种遗传上不同的类型，至少四分之一的珊瑚物种能够在它们的胃皮细胞中容纳一种以上的共生线虫。新的分子技术使珊瑚礁生物学家在区分共生体之间的基因组相似性和差异性方面取得了快速进展，但关于藻类生理差异及其对宿主珊瑚生理(光合作用、生长速度、繁殖和宿主总体健康)的影响的研究远远落后。没有这样的信息，就不可能预测共生(或珊瑚宿主)从拥有哪些特定的共生生物组合中获得的好处。就物种数量和属间相互作用的复杂性而言，珊瑚礁构成了世界上最多样化的生态系统之一。然而，世界各地的珊瑚礁正在恶化：为大量鱼类和无脊椎动物提供珊瑚礁框架和栖息地的珊瑚礁群落覆盖率正在下降，经历了前所未有的疾病事件，并显示出明显的压力迹象，主要是对海洋变暖的反应。尤其令人担忧的是，珊瑚漂白现象增多，共生体失去了作为宿主生长、繁殖和发育所需碳(来自光合作用)的主要贡献者。在严重的漂白过程中，当白色骨骼穿透动物组织时，共生体密度下降，珊瑚组织生物量和能量储备减少，宿主生长停止，繁殖受到损害，部分或整个群体可能死亡。也有人认为，受到压力(即漂白)的珊瑚更容易生病。对珊瑚数量减少的原因以及可能促进珊瑚恢复的过程的了解很少。特别是对不同类型的共生菌与其宿主整合的复杂性和特异性，特别是哪些共生体组合能导致更大的宿主生长、繁殖和存活，以及这种最佳组合是如何建立和维持的，人们知之甚少。这项研究旨在解决珊瑚寄主/共生体关系的复杂性，记录不同共生体的能力：(1)向寄主提供营养(共生体的光合作用和光合作用产物从藻类向寄主的转移)，(2)影响寄主的生长，(3)稳定地栖息在不同的寄主上，(4)通过生产各种‘防晒霜’来提供对紫外线的保护，(5)在高于正常温度的情况下生存，(6)光适应不同的光照条件。实验的总体目标是能够模拟共生-寄主组合的季节性和长期变化，特别是关于完整系统因环境变化而发生的变化，环境变化可能导致完整联系的生理表现(包括生存)更大或更小。这些数据将使我们能够预测，在未来50-100年全球变暖的情况下，哪些协会能够在世界热带海洋的预期条件下幸存下来，以及是否可以“切换”到耐受程度更高的藻类类型。

The growth, calcification, nutrition, reproduction, health and, in many cases, morphological form of reef corals and related cnidarians is the result of their mutualism with symbiotic dinoflagellates of the genus Symbiodinium, often referred to as "zooxanthellae". Several different species of Symbiodinium, as well as dozens of genetically different types within at least five molecular "clades" have been described within the past 30 years, with at least a quarter of coral species able to harbor more than one type of Symbiodinium within their gastrodermal cells. New molecular techniques have enabled coral reef biologists to make rapid progress in distinguishing genomic similarities and differences among Symbiodinium symbionts, but research on differences in algal physiology and its repercussions on host coral physiology (photosynthesis, growth rates, reproduction, and general health of the host) lags far behind. Without such information, it is impossible to project what benefits to the symbiosis (or coral host) come from having which specific combinations of Symbiodinium. Coral reefs comprise one of the world's most diverse ecosystems, both in terms of number of species and complexity of interactions among genera. Yet, coral reefs worldwide are deteriorating: the coral colonies that provide reef framework and habitat to multitudes of fish and invertebrates are decreasing in percent cover, experiencing unprecedented incidents of disease, and showing obvious signs of stress largely in response to ocean warming. Especially alarming is the increased occurrence of coral bleaching in which symbionts are lost as the principal contributors of carbon (from photosynthesis) for host growth, reproduction and development. Upon severe bleaching, when the white skeleton shows through the animal tissue as symbiont densities decline, coral tissue biomass and energy stores decrease, host growth ceases, reproduction is impaired, and portions or entire colony may die. It is also thought that stressed (i.e. bleached) corals are more susceptible to disease. Understanding of the causative factors for the decline of corals and the processes that might promote their recovery is very poor. Particularly little is known the of the complexity and specificity of the integration of the different types of Symbiodinium with their hosts, especially which combinations of symbionts lead to greater host growth, reproduction and survival and how such optimal combinations are established and maintained. This research is designed to resolve the complexities of the coral host/symbiont relationship, in terms of documenting the ability of different symbionts: (1) to provide nutrition to their host (photosynthesis of symbiont and translocation of photosynthate from alga to host), (2) to influence growth of the host, (3) to stably inhabit (=infect) different hosts, (4) to provide protection from ultra-violet light by the production of various 'sun screens', (5) to survive periods of higher-than-normal temperatures, (6) to photoadapt to different light conditions. The overall goal of the experiments is to be able to model seasonal and long-term changes in symbiont-host combinations, especially in regard to changes in the intact system in response to environmental change that might lead to greater or diminished physiological performance (including survival) of the intact association. The data will allow us to predict which associations are capable of surviving conditions expected in the world's tropical oceans over the next 50-100 years of global warming, and whether "switching" to algal types with higher tolerance levels might be a viable outcome.