Studies on diversity and stress tolerance of coral-zooxanthella symbiotic system.

为了在细胞或组织水平上研究珊瑚-虫黄杆菌共生系统的漂白机理，我们建立了一个利用珊瑚细胞聚集体(组织球)的实验系统。从Fungia sp.分离的细胞中制备组织球。组织球存活曲线在31℃和25℃下有显著差异，31℃下组织球存活时间与组织球密度呈显著负相关，而25℃下组织球存活时间与组织球密度之间无显著相关性。抗氧化剂延长组织球在高温下的存活时间的发现表明，在应激条件下，虫黄藻产生了活性氧物种。为了检验对虫黄藻光系统II(PSII)的破坏是否是热诱导珊瑚漂白的主要步骤，我们评估了珊瑚Pachyseris rugosa共生藻中PSII的最大量子产率(Fv/Fm)或活性PSII中心(Fv/Fo)与最大电子传递速率(ETRmax)的关系。黑暗高温处理使ETRmax显著下降，但可逆性下降，而Fv/Fm无明显变化。至少需要60%的Fv/Fm或30%的活跃的PS II中心(Fv/Fo)来维持共生体中的ETRmax。这些结果表明，共生体光合器受热损伤的主要步骤涉及PSII以外的成分，很可能是在ETRmax降低的暗反应水平上，PSII损伤是次要的。我们计划使用这个实验系统来评估环境压力对与不同类型的虫黄藻相关的珊瑚的影响。

To investigate bleaching mechanisms of coral-zooxanthella symbiotic system at cellular or tissue level, we have established an experimental system using coral cell aggregates (tissue balls). Tissue balls prepared from dissociated cells of Fungia sp. and Pavona divaricata were exposed to either 31°C or 25°C. Survival curves of tissue balls were markedly different between 31°C and 25°C. There was a significant negative correlation between the survival time and zooxanthella density of tissue balls at 31°C, while no significant relationship was found at 25°C. The present results showed that zooxanthellae produced harmful substances and cause damage to coral ce Is under high temperature stress. The finding that the antioxidants extended the survival time of tissue balls at high temperature suggests that zooxanthellae produce reactive oxygen species under the stress condition.To examine whether damage to zooxanthellar photosystem II (PSII) is the primary step of thermally induced coral bleaching, we assessed the relationship between the maximum quantum yield of PSII (Fv/Fm) or active PSII centers (Fv/Fo) and maximum electron transport rate (ETRmax) in the symbiotic algae of the coral Pachyseris rugosa. The I h high temperature treatment in darkness produced a significant, but reversible decline in ETRmax without any change in Fv/Fm. A minimum of 60% of the Fv/Fm or 30% of active PS II centers (Fv/Fo) is required to maintain ETRmax in the symbionts. These results suggest that the primary step of heat-induced damage in the symbiont's photosynthetic apparatus involves a component beyond the PSII, probably at the level of the dark reaction as indicated by reduced ETRmax and the PSII damage is secondary.Planulae or primary polyps of Acropora nobilis and A. palifera could be infected with zooxanthellae from various hosts. We are planning to use this experimental system to assess effects of environmental stresses on corals associated with different types of zooxanthllae.