不同熱經驗的珊瑚面對氣候變遷情境下的反應研究

Abstract

珊瑚礁生態系含有豐富的生物多樣性並具有相當重要的生態與經濟價值，向來有「海洋中的熱帶雨林」之稱。然而隨著全球氣候變遷與人為活動影響逐漸加劇，珊瑚也持續面臨難以忽視的威脅。尤其是全球暖化造成的海水溫度上升，即使僅攝氏1~2度的溫度上升也可能形成大規模的珊瑚白化事件。然而，隨著珊瑚中共生群聚的組成不同、生長區域的熱環境差異以及本身的生活史策略，不同的珊瑚也有著不同程度的熱忍受能力。本研究為了能更深入瞭解生活於不同熱環境歷史中的不同物種所發展的調適能力，並進而預測未來氣候變遷下可能的影響，使用細枝軸孔珊瑚與鐘形微孔珊瑚兩種形態及生活史策略相異的物種進行熱逆境實驗。實驗所用的珊瑚採自水溫擾動變異性高的環境(核三廠出水口)以及相較穩定的環境(萬里桐)，並模擬三種不同的熱逆境狀態，分別為過去的白化溫度—以1998年聖嬰現象所帶來的大規模珊瑚白化事件為參考(29 ºC)，現今的珊瑚白化溫度—以近兩年來白化事件為參考(31 ºC)，以及IPCC所預測未來升溫溫度—以IPCC於2050年所預測之上升溫度為參考(33 ºC)，與實驗期間維持水溫不變的控制組（以近期年均水溫為參考26 ºC)，並將在環境溫度下馴化一週的珊瑚於水缸中逐步升溫至實驗溫度，維持溫度一週後降回控制組溫度並等其恢復。於實驗期間取樣分析其總蛋白質表現量、共生藻種類、密度以及葉綠素濃度，以及每日進行共生藻光合作用效率的測量，以瞭解實驗物種的生理響應以及調適能力。而觀察結果顯示，控制組、29 ºC以及31 ºC的處理組，兩種珊瑚經一週熱逆境後皆仍未有明顯白化，33 ºC的處理組中鐘形微孔珊瑚發生白化但仍存活至實驗結束，而來自萬里桐與出水口的細枝軸孔珊瑚則有不同程度的耐熱反應，出水口的細枝軸孔珊瑚即使與較耐熱的共生藻屬共生，並且白化反應較萬里桐的珊瑚慢，仍在加溫一週後死亡，推測在溫度壓力下珊瑚宿主雖能與較耐熱的共生藻共生而提升其熱耐受能力，但此適應或調適能力或許無法趕上未來環境的變遷，並且珊瑚宿主本身的熱耐受能力可能對於未來可否生存佔有更重要的關鍵。

It is known that corals are at substantial risk due to rising ocean temperatures associated with climate change. However, the threshold temperature inducing coral bleaching varies among different coral species, due to their partnership with specific symbionts, and exposure to different temperature regimes based on location. In this study, I aim to investigate how the above factors and their interaction influence coral’s reponse to future temperature scenarios. I used corals with different life-history strategies: Acropora nana (competitive) and Porites lutea (stress tolerant) from two different thermal environments in the Kenting National Park. One is the outlet of the 3rd NPP (larger temperature fluctuations) and the other is Wanlitong (relatively stable thermal environment compared to the 3rd NPP). Experiment to simulate three different thermal stress conditions; past (temperature of 1998 mass coral bleaching event, 29 ºC), present (current bleaching temperature, 31 ºC) and the future (IPCC predictions for 2050, 33 ºC), was carried out by exposing coral nubbins to different temperature stress for one week in 600L tanks with continuous seawater flow system. Results from PAM measurements showed that no significant stress response was found in 31 and 29 ºC treatment for both coral species. However, at 33 ºC treatment, nubbins of A. nana experienced high mortality with bleaching occurring sooner in coral nubbins from Wanlitong compared to those from the Outlet, which associated with more heat tolerant Symbiodiniaceae genus. Whereas, nubbins of P. lutea from both locations showed sign of bleaching without any mortality. Results of this study give insights on the species specific acclimatization and/or adaptation potential play an important role in response to the climate change induced seawater anomalies in the future, and some coral species might not survive in the future seawater anomalies even associated with heat tolerant Symbiodiniaceae genus.