籬枝軸孔珊瑚體內共生藻群聚在時間上的變動與光合生理差異

Abstract

墾丁潭子灣地區，籬枝軸孔珊瑚(Isopora palifera)生長在環境變化劇烈之上亞潮帶，常受溫度及光度等物理環境因子的擾動。籬枝軸孔珊瑚體內具有C與D兩種親緣型(phylotype)的共生藻。其中C親緣型對溫度的變動是屬於敏感型(thermal-sensitive)的藻類，而D親緣型為耐溫型(thermal-tolerant)藻類。本論文藉由籬枝軸孔珊瑚體內具有不同親緣型組合的共生藻群聚特性回答兩個問題。ㄧ、籬枝軸孔珊瑚體內C和D親緣型的組成，是否具有顯著的季節變動，這樣的變動是否為常態？ 二、具不同親緣型組成之共生藻群聚的籬枝軸孔珊瑚共生體（holobiont），由水下螢光測定儀現場測量測得光合生理是否不同？如有不同，是否與季節及環境因子（溫度與光度）的變動有關？ 由2006年3月到2007年3月的結果顯示，共生藻親緣型的群聚仍有顯著的季節性變化，但其優勢藻已由2000年的D親緣型轉變為2006年的C親緣型。推測可耐熱的D親緣型在1998年珊瑚白化事件影響後，因較能耐受逆境而成為籬枝軸孔珊瑚體內優勢共生藻，協助渡過白化與白化後的恢復期。過去五年並無顯著珊瑚白化，故共生藻經長時間的洗牌 (reshuffling)，重新以原本主要但對溫度較敏感的C親緣型為優勢。而共生藻密度與葉綠素含量都在水準內，顯示在本論文研究期間籬枝軸孔珊瑚並無珊瑚白化發生，表示珊瑚宿主不需要白化就可進行共生藻親緣型洗牌。 以整年的光合生理的資料顯示，共生藻的組成沒有反映在光合表現 Fv/ Fm (maximum quantum yield)上，但其值在七月顯著下降，而七月正值夏季最高水溫和日照強度的時期，可能造成共生藻光合作用抑制(photoinhibition)的現象；然而共生藻親緣型變動卻於九月才有最大的洗牌（C親緣型由 66.7%降為 25%；反之同時有 C+D系群由 33.3%升為 75%) 。這兩個月的時間差顯示共生體光合生理對逆境的反應快速，但要造成親緣型洗牌的效應較緩慢。共生體在不同的共生藻組成下，共生藻可維持穩定的能量供給宿主。綜合論本文所得資料，共生體光合表現的降低，可能是誘發共生藻洗牌的因素。

Isopora palifera inhabits the upper subtidal reefs where physical disturbances such as temperature and light intensity are high. In this thesis, I examined the temporal and photophysiological variations of symbiont communities in I. palifera. Monthly variation of Symbiodinium phylotypes in I. palifera collected from March 2006 to March 2007 indicated a significant seasonal fluctuation, a consistent pattern was also observed in the samples collected from January 2000 to July 2001 in Tantzei Bay, Kenting. Nevertheless, the dominant phylotype was completely shifted from Symbiodinium D to Symbiodinium C between these two sampling periods. This reshuffling provides the evidence that symbiont communities in I. palifera could return to the dominance of “cosmopolitan” Symbiodinium C, as seen in most of the Indo-Pacific corals, during the non-bleaching period. Whereas I. palifera shuffled the dominant symbiont to the thermal-tolerant phylotype D during the recovery period right after the mass coral bleaching in 1998 and 1999. Photophysiological responses measured in situ by Diving-PAM indicated that the tagged coral colonies showed a significant drop of maximum quantum yield (Fv/Fm) in July 2006. This drop may reflect the photoinhibition of Symbiodinium when seawater temperature and light intensity reached their threshold during summer. In contrast, the maximum reshuffling of Symbiodinoum phylotypes occurred in September (C phylotype reduced from 66.7% to 25%, but C+D phylotype increased from 33.3% to 75%). This time lag supported the scenario that phylotype reshuffling did not necessarily come with spontaneous photophysiological response. This result also implied that symbionts could provide stable energy source to the host with different symbiont compositions, and reducing photosynthetic capacity might trigger phylotype reshuffling in I. palifera.