Boulotte Nadine M, Dalton Steven J, Carroll Andrew G, Harrison Peter L, Putnam Hollie M, Peplow Lesa M, van Oppen Madeleine Jh
National Marine Science Centre, School of Environment, Science and Engineering, Southern Cross University, Coffs Harbour, New South Wales, Australia.
Marine Ecology Research Centre, School of Environment, Science and Engineering, Southern Cross University, Lismore, New South Wales, Australia.
ISME J. 2016 Nov;10(11):2693-2701. doi: 10.1038/ismej.2016.54. Epub 2016 Apr 19.
Reef-building corals possess a range of acclimatisation and adaptation mechanisms to respond to seawater temperature increases. In some corals, thermal tolerance increases through community composition changes of their dinoflagellate endosymbionts (Symbiodinium spp.), but this mechanism is believed to be limited to the Symbiodinium types already present in the coral tissue acquired during early life stages. Compelling evidence for symbiont switching, that is, the acquisition of novel Symbiodinium types from the environment, by adult coral colonies, is currently lacking. Using deep sequencing analysis of Symbiodinium rDNA internal transcribed spacer 2 (ITS2) PCR amplicons from two pocilloporid coral species, we show evidence consistent with de novo acquisition of Symbiodinium types from the environment by adult corals following two consecutive bleaching events. Most of these newly detected symbionts remained in the rare biosphere (background types occurring below 1% relative abundance), but one novel type reached a relative abundance of ~33%. Two de novo acquired Symbiodinium types belong to the thermally resistant clade D, suggesting that this switching may have been driven by consecutive thermal bleaching events. Our results are particularly important given the maternal mode of Symbiodinium transmission in the study species, which generally results in high symbiont specificity. These findings will cause a paradigm shift in our understanding of coral-Symbiodinium symbiosis flexibility and mechanisms of environmental acclimatisation in corals.
造礁珊瑚拥有一系列的适应和适应机制来应对海水温度升高。在一些珊瑚中，通过其双鞭毛藻内共生体（共生藻属）的群落组成变化，耐热性会增加，但这种机制被认为仅限于珊瑚组织在早期生命阶段获得的已存在的共生藻类型。目前缺乏成年珊瑚群体从环境中获取新的共生藻类型（即共生体转换）的确凿证据。通过对两种鹿角珊瑚物种的共生藻核糖体DNA内部转录间隔区2（ITS2）PCR扩增子进行深度测序分析，我们发现了与成年珊瑚在连续两次白化事件后从环境中重新获得共生藻类型相一致的证据。这些新检测到的共生体大多仍处于稀有生物圈（相对丰度低于1%的背景类型），但有一种新类型的相对丰度达到了约33%。两种重新获得的共生藻类型属于耐热的D类群，这表明这种转换可能是由连续的热白化事件驱动的。鉴于研究物种中共生藻的母系传播模式，我们的结果尤为重要，这种模式通常会导致高度的共生体特异性。这些发现将使我们对珊瑚 - 共生藻共生关系的灵活性以及珊瑚环境适应机制的理解发生范式转变。
