持久熱逆境決定共生藻洗牌程度與維氏腦紋珊瑚白化後之生存率

Abstract

由全球氣候變遷造成之海水暖化已然威脅造礁珊瑚的生存。共生藻洗牌為珊瑚宿主對抗熱逆境的數種機制之一，藉由改變體內既有不同抗熱性之共生藻相對豐度，來度過熱逆境所帶來之生存壓力。目前已知有少數幾種珊瑚具有執行共生藻洗牌能力，然而，熱逆境之歷程對於珊瑚執行共生藻洗牌之過程以及洗牌過後珊瑚宿主生存率之影響，至今還不清。本篇實驗採取位於墾丁核三廠出水口、入水口以及萬里桐之維氏腦紋珊瑚片段，於2014年與2015年分次進行野外三個地點間之交互移植實驗。由出水口採取之珊瑚片段，其體內共生藻族群以抗壓性強之D1/D1a型共生藻為主，而其餘兩地採取之珊瑚片段則以對熱敏感之C3/C3cc型共生藻為主。在2014年進行之實驗中，所有由入水口與萬里桐移植至出水口之珊瑚片段皆於七月產生白化現象（熱周度指標＝10.43），對比之下由出水口移植至其餘兩地之珊瑚片段則無此現象。所有白化之珊瑚片段中，共有40%完成共生藻洗牌，其體內共生藻族群轉為以D1/D1a型為主，然而在已完成洗牌之珊瑚片段中，有75%在熱逆境消散後死亡。在2015年進行之實驗中，由萬里桐移植至出水口之珊瑚片段共有73%產生白化現象（熱周度指標＝5.7），而其中又有73%完成共生藻洗牌，其體內共生藻族群轉為以D1/D1a為主並且全數存活。本篇研究結果證明了在面對氣候變遷影響下，維氏腦紋珊瑚具有進行共生藻洗牌以對抗海水暖化之能力，然而持久熱逆境可能扮演著決定最終共生藻洗牌成功率以及珊瑚宿主最終存活率的關鍵角色。此外，本篇研究亦在野外的條件下證明了體內以D1/D1a型共生藻為主之維氏腦紋珊瑚可有著與體內以C3/C3cc型共生藻為主之維氏腦紋珊瑚相等甚至更高的生長率，此結果暗示著位於出水口之維氏腦紋珊瑚族群可能已在長期熱逆境下藉由調適（acclimatization）或適應（adaptation）或兩者，發展出能更有效率地與D1/D1a型共生藻共生之機制。

Rising seawater temperature caused by climate change is threatening the survival of reef-building corals. One of the mechanisms for corals to overcome thermal stress is to shuffle in hospite, the relative dominance of Symbiodinium from thermally susceptible types to tolerant ones. Although the shuffling of Symbiodinium has been observed in a several of coral species, the influence of thermal history on the process of symbiont shuffling and subsequent survival of coral hosts remains unclear. In this study, we conducted in situ reciprocal transplantation experiments (RTE) on nubbins of brain coral, Platygyra verweyi, collecting from a nuclear power plant outlet (NPP-OL), inlet (NPP-IL) and Wanlitung (WLT) of Kenting National Park in 2014 and 2015. While the symbiont community of P. verweyi in NPP-OL was dominated by stress-tolerant types D1/D1a, the ones in NPP-IL and WLT were dominated by the thermal- susceptible types C3/C3cc. In 2014, all the nubbins transplanted from NPP-IL and WLT to NPP-OL bleached in July after the prolonged high seawater temperature (Degree Heating Week, DHW=10.43) in NPP-OL. In contrast, those ones transplanted from NPP-OL to the other two sites remained unbleached. Forty percent of bleached nubbins showed sign of shuffling from C3/C3cc to D1/D1a, although 75% of them died in the subsequent fall. In 2015, 73% of the RTE nubbins from WLT to NPP-OL bleached under less prolonged high seawater temperature (DHW=5.7) with 73% shuffling their symbiont community from C3/C3cc to D1/D1a dominant and surviving after a reduction of the thermal stress. Our results show that in the facing of climate change impact, P. verweyi is capable of shuffling symbiont community towards the stress-tolerant type in order to respond to the rising seawater temperature. However, the degree of prolonged thermal stress may play a crucial key role in determining the success of shuffling and final survival of the coral host. In addition, our study also indicates that the Symbiodinium D1/D1a dominant P. verweyi can have a comparable or higher growth rate than Symbiodinium C3/C3cc dominant one under field condition. This suggests a potential acclimatization and/or adaptation might occur in the P. verweyi population at NPP-OL after long-term association with Symbiodinium D1/D1a.