Data_Sheet_2_Zoantharian Endosymbiont Community Dynamics During a Stress Event.ZIP

Coral reefs are complex ecosystems composed of many interacting species. One ecologically important group consists of zoantharians, which are closely related to reef-building corals. Like corals, zoantharians form mutualistic symbioses with dinoflagellate micro-algae (family Symbiodiniaceae), but their associations remain underexplored. To examine the degree to which zoantharians exhibit altered symbiont dynamics under changing environmental conditions, we reciprocally transplanted colonies of Zoanthus sansibaricus between intertidal (2 m) and subtidal (26 m) depths within a reef in Okinawa, Japan. At this location, Z. sansibaricus can associate with three Symbiodiniaceae species from two genera distributed along a light and depth gradient. We developed species-specific molecular assays and sampled colonies pre‐ and post-transplantation to analyze symbiont community diversity. Despite large environmental differences across depths, we detected few symbiont compositional changes resulting from transplantation stress. Colonies sourced from the intertidal zone associated with mixtures of a “shallow” Symbiodinium sp. and a “shallow” Cladocopium sp. independent of whether they were transplanted to shallow or deep waters. Colonies sourced from the subtidal zone were dominated by a “deep” Cladocopium sp. regardless of transplant depth. Subtidal colonies brought to shallow depths did not transition to the presumably high-light adapted shallow symbionts present in the new environment, but rather bleached and died. These patterns mirror observations of highly stable coral-algal associations subjected to depth transplantation. Our results indicate that Zoanthus-Symbiodiniaceae symbioses remain stable despite stress, suggesting these important reef community members have relatively low capacity to shuffle to more stress-tolerant micro-algae in response to ongoing climate change.

珊瑚礁是由众多相互作用物种构成的复杂生态系统。
其中一类具有重要生态学意义的类群为花虫类（Zoantharians），它们与造礁珊瑚亲缘关系密切。与珊瑚类似，花虫类可与甲藻门微藻共生藻科（Symbiodiniaceae）形成互利共生关系，但二者的共生关联迄今仍未得到充分研究。
为探究花虫类在环境变化下共生体动态的改变程度，我们在日本冲绳的一处珊瑚礁内，将桑给巴尔花珊瑚（Zoanthus sansibaricus）的群落于潮间带（2米）与亚潮带（26米）水深之间开展双向移植实验。在该研究区域，桑给巴尔花珊瑚可与沿光照与水深梯度分布的2个属内的3种共生藻科物种形成共生关系。
我们开发了物种特异性分子检测方法，并在移植前后对珊瑚群落进行采样，以分析共生体群落的多样性。
尽管不同水深间环境差异显著，但我们未检测到由移植胁迫引发的共生体组成显著变化。
来自潮间带的群落与“浅海型”裸甲藻属（Symbiodinium）物种和“浅海型”枝孢藻属（Cladocopium）物种的混合类群形成共生，且这一关联不受移植至浅海或深海水深的影响。
来自亚潮带的群落则以“深海型”枝孢藻属（Cladocopium）物种为优势类群，且不受移植水深的影响。
被移植至浅海的亚潮带群落并未转向适应新环境高光条件的浅海型共生体，反而出现白化并死亡。
该模式与深度移植下珊瑚-藻类共生体高度稳定的观测结果一致。
我们的研究结果表明，花虫类-共生藻科的共生关系即使在胁迫下仍保持稳定，这意味着这类重要的珊瑚礁群落成员在应对持续气候变化时，转换为更耐胁迫微藻的能力相对有限。