Symbiodinium morphology data and statistics

Coral bleaching is the dysfunction of the coral-algal endosymbiosis and is characterized as a loss of <i>Symbiodinium</i>cells from host tissues or the loss of photosynthetic pigments. Symbiosis breakdown occurs as a result elevated temperature beyond the meta-organism’s thermal threshold. Despite the impacts of coral bleaching we have not resolved the tipping points and the mechanisms underlying bleaching responses. This study characterized the cellular responses of <i>Symbiodinium</i>sp. (C3) within <i>Acropora aspera</i>exposed to thermal stress. Thermally stressed corals experienced significant declines in <i>Symbiodinium</i>density and dark-adapted yield following exposure to bleaching at 34 °C for four consecutive days, equating to an ecological thermal stress of 4-degree heating weeks (DHW). However large impacts on <i>Symbiodinium</i>morphology and cell division were seen following exposure to temperatures between 2-3 °C (2 DHW) below the bleaching threshold, and by 4 DHW over 90% of <i>Symbiodinium</i>were degraded. While these sub-bleaching levels will not trigger bleaching alerts it is vital that we begin to quantify how these significant sub-bleaching levels will impact the fitness of <i>Symbiodinium</i>populations, their coral hosts and subsequently reefs worldwide.珊瑚白化是珊瑚与藻类内共生关系失调的表现，其特征为宿主组织内的虫黄藻（Symbiodinium）细胞流失，或是光合色素丢失。当水温超过共生复合体（meta-organism）的热阈值时，便会引发共生关系破裂。尽管珊瑚白化会造成诸多严重影响，但目前我们尚未明确其发生临界点与白化响应的潜在调控机制。本研究系统解析了暴露于热胁迫下的粗糙鹿角珊瑚（Acropora aspera）体内的虫黄藻菌株C3（Symbiodinium sp. C3）的细胞响应特征。当在34℃下连续暴露4天（对应生态热胁迫强度为4热积温周（DHW））时，经受热胁迫的珊瑚体内的虫黄藻密度与暗适应量子产量均出现显著下降。然而，当暴露于比白化阈值低2~3℃（对应2热积温周（DHW））的水温时，虫黄藻的形态与细胞分裂已受到显著影响；当热胁迫达到4热积温周（DHW）时，超过90%的虫黄藻已发生降解。尽管这类亚白化水平不会触发珊瑚白化预警，但我们亟需量化这些显著的亚白化胁迫会如何影响虫黄藻种群的适合度、珊瑚宿主的健康，乃至全球范围内的珊瑚礁生态系统。