台灣產微孔珊瑚共生體之共生藻群聚多樣性與熱逆境生理

Abstract

熱帶與亞熱帶造礁珊瑚與其體內共生藻所組成的“共生體”，其生理極限溫度約在攝氏28~30度左右，當海表面溫度升高攝氏0.5-1度將對珊瑚共生體造成生理上的熱逆境，熱逆境持續的結果將造成珊瑚產生“白化”現象。面對氣候變遷可能在本世紀末引以海水溫度升高攝氏1.8~4度，探討生活不同“溫度背景”的珊瑚共生體在熱逆境下如何調適或適應作出反應及其可能的機制，是珊瑚礁保育與永續重要的課題。本論文以在不同海域溫度背景的團塊型微孔珊瑚共生體為對象探討共生藻的多樣性，以及是否展現不同的熱逆境生理反應能力。將台灣南部恆春核三廠出水口（年均溫攝氏27.5 ± 1.7度）與北部鼻頭角(年均溫攝氏23.8 ± 3.82度）的團塊型微孔珊瑚共生體採回實驗室，已變性梯度電泳與核苷酸序列定序分析微孔珊瑚體內共生藻型的多樣性，並以水缸實驗探討原生溫度背景相異的微孔珊瑚共生體對熱逆境的反應。共生藻多樣性的調查顯示，在37個微孔珊瑚樣本中可鑑定出C15型、C55、C55-1與六種未命名的共生藻單型，而微孔珊瑚體內共生藻型在臺灣南北的地理分佈上有顯著差異，南部以C15原型為主，而北部多為未知的新共生藻單型為主。以共生藻最大光合作用效率量測微孔珊瑚共生體對熱逆境的反應結果顯示，鼻頭角的微孔珊瑚對於熱逆境的耐受度顯著優於核三廠出水口的群體。分析共生藻密度及葉綠素濃度隨時間的變動顯示，鼻頭角的微孔珊瑚共生體的葉綠素濃度隨著時間而增加，但共生藻密度下降，相反的，核三廠出水口的微孔珊瑚共生體體內共生藻密度隨時間而增加，葉綠素濃度並改變。綜合以上的結果顯示，團塊微孔珊瑚(P. lobata)在台灣南北海域不同的背景溫度下已發展出與不同型共生藻的共生能力，而且其對於熱逆境的反應策略也有所不同。此外，由於珊瑚宿主及環境不同，核三廠出水口三米水深與七米水深的微孔珊瑚共生體在光合作用效率上存在著本質上的差異，但其對於熱逆境的反應是相同的。總結本論文從分子與生理證據皆證實，即使是高度共生藻藻型專一性的團塊微孔珊瑚在小尺度的地理差異上也發現因適應不同的環境溫度導致共生藻組成族群之差異，而不同共生藻型對於熱逆境的光合生理也發展出不同的策略。Corals and their symbiotic algae (genus Symbiodinium), collectively known as coral holobionts, live close to their physiologically-limit of sea surface temperature (SST) between 28 0C to 30 0C in the tropical and subtropical water. Increasing of 0.5-1 0C above the sea surface temperature (SST) will cause physiological stress of coral holobionts and, as the consequence, breakdown of symbiotic relationship (also known as “bleaching”). Understanding how coral holobionts with different SST “background” respond to the thermal stress is the key to identify the strategies of future survival of coral holobionts and function of coral reef ecosystem under the impact of climate change. In this study first conducted Symbiodinium diversity surveys of Porites corals from the Bietou (BT) in the northern Taiwan and Nuclear power plant outlet (OL) of Kenting National Park, in southern Taiwan, where yearly mean SST 23.8 ± 3.8 0C and 27.5 ± 1.7 0C, respectively. 37 coral samples were examined, C15, C55, C55-1, and six C15-related new Symbiodinium types were identified in Porites. A significant difference in Symbiodinium type compositions was found in Porites between BT and NPP-OL with the six C15-related types dominant in BT. Analysis of the maximum quantum yield (Fv / Fm) in Porites exposed to different temperature treatments in the tanks showed that Porites of NPP-OL was more sensitive to the thermal treatments than those of the BT during thermal stress. In addition, NPP-OL Porites collected from 7 m displayed a significantly higher Fv / Fm than those from 3 m, which might due to difference of the host species. Analysis of Symbiodinium density and chlorophyll pigments concentration showed that BT population displayed higher concentration of chlorophyll pigments than OL population. However, NPP-OL population increased Symbiodinium density instead of chlorophyll pigments when facing thermal stress. Results from this thesis suggested that Porites living in different thermal background could associate with different Symbiodinium C15-related types. Meanwhile, different C15-related types might have variety of photosynthesis responses in assisting coral hosts to survive under the thermal stress caused by rising sea surface temperature.