Data from: Dominance of Endozoicomonas bacteria throughout coral bleaching and mortality suggests structural inflexibility of the Pocillopora verrucosa microbiome

The functional importance of Symbiodinium algal endosymbionts and a diverse suite of bacteria for coral holobiont health and functioning is widely acknowledged. Yet, we know surprisingly little about microbial community dynamics and the stability of host-microbe associations under adverse environmental conditions. To gain insight into the stability of coral host-microbe associations and holobiont structure, we assessed changes in the community structure of Symbiodinium and bacteria associated with the coral Pocillopora verrucosa under excess organic nutrient conditions. Pocillopora-associated microbial communities were monitored over 14 days in two independent experiments. We assessed the effect of excess dissolved organic nitrogen (DON) and excess dissolved organic carbon (DOC), the latter which was published recently. Exposure to excess nutrients rapidly affected coral health, resulting in two distinct stress phenotypes: coral bleaching under excess DOC on the one hand and severe tissue sloughing (> 90 % tissue loss resulting in host mortality) under excess DON. These phenotypes were accompanied by structural changes in the Symbiodinium community. In contrast, the associated bacterial community remained remarkably stable and was dominated by two Endozoicomonas phylotypes, comprising on average 90 % of bacterial sequences. This dominance of Endozoicomonas even under conditions of coral bleaching and mortality suggests the bacterial community of P. verrucosa may be rather inflexible, and thereby unable to respond or acclimatize to rapid changes in the environment, contrary to what was previously observed in other corals. In this light, our results add important detail to our understanding of the structural flexibility and stability of the coral holobiont. Coral holobionts might occupy structural landscapes ranging from highly flexible to rather inflexible with consequences for their ability to respond to environmental change.

虫黄藻（Symbiodinium）藻类内共生体与多样细菌类群对珊瑚共生功能体（coral holobiont）的健康及功能的重要性已获广泛认同。然而，我们对逆境环境下微生物群落动态以及宿主-微生物共生关系的稳定性却知之甚少。为深入探究珊瑚宿主-微生物共生关系的稳定性与共生功能体结构，我们针对过量有机营养条件下与疣状鹿角珊瑚（Pocillopora verrucosa）共生的虫黄藻与细菌群落结构变化展开了评估。本研究通过两组独立实验，对与该珊瑚共生的微生物群落进行了为期14天的监测。我们评估了过量溶解态有机氮（dissolved organic nitrogen, DON）与过量溶解态有机碳（dissolved organic carbon, DOC）的影响，其中过量溶解态有机碳的相关研究已于近期发表。暴露于过量营养环境会快速影响珊瑚健康，并催生两种截然不同的应激表型：过量溶解态有机碳环境下出现珊瑚白化，而过量溶解态有机氮环境下则发生严重的组织脱落（组织损失率超90%，最终导致宿主死亡）。这些表型伴随虫黄藻群落的结构变化。与之形成鲜明对比的是，共生细菌群落始终保持显著稳定，且由两种嗜内杆菌属（Endozoicomonas）菌型主导，其序列占比平均可达90%。即便在珊瑚白化乃至死亡的环境条件下，嗜内杆菌属仍占据主导地位，这表明疣状鹿角珊瑚的细菌群落可能缺乏足够的弹性，因而无法对环境的快速变化做出响应或产生适应性调整——这与此前在其他珊瑚物种中观测到的结果相悖。基于此，本研究结果为我们理解珊瑚共生功能体的结构弹性与稳定性提供了重要的细节补充。珊瑚共生功能体可能存在跨度极大的结构谱系，从高度弹性到相对固定，这将直接影响其应对环境变化的能力。