鐵對珊瑚共生藻於高溫高光逆境下的效應

Abstract

珊瑚共生藻在高光高溫下透過光系統電子傳遞鏈快速生成活性氧物種，由於活性氧物種的累積被認為是造成珊瑚白化的主因之一，因此移除活性氧物種相關金屬酶的充分表現相當重要。先前研究發現充足無機鐵供應是共生藻在高光高溫條件生長的關鍵因素，為驗證鐵是否能幫助緩解光和熱緊迫，本研究模擬自然環境下造成珊瑚白化的高光及高溫條件（30℃，1,000 μE m-2 s-1），並觀察不同濃度無機鐵（Fe'）所造成的影響。在各個程度的光及熱緊迫條件下，藉由培養不同屬的共生藻，觀察無機鐵（Fe'）濃度培養基質對於生長速率、SOD活性、光合作用效率表現（Fv/Fm）及細胞內鐵含量的影響。結果顯示，於間歇性熱緊迫下，共生藻的生長狀況相較持續性高溫有較好的生存狀況。耐熱性共生藻種Fugacium. kawagutii（F. k.）不論於間歇性或持續性熱緊迫皆展示出良好的生存狀態，而熱敏性共生藻種Breviolum minutum (B. m.) 及 Symbiodinium microadriaticum (S. m.) 於間歇性高溫中有稍微生長的趨勢，但無法生長於持續性高溫環境下，因此將兩種特性的共生藻種分別以不同的環境進行實驗。 熱敏性共生藻種B. m.及 S. m.無法生長於持續性熱緊迫情況，因此實驗以短暫性光及熱緊迫的方式進行。於正常溫度(26℃)，600及800 μE m-2 s-1的光強度下，高鐵組（1,000 pM Fe'）的B. m.相較於250 pM Fe'得以維持Fv/Fm。在不同溫度（30℃，34℃）及時長（2–6小時）的熱緊迫條件下，觀察其生長狀況及光合作用效率表現。結果顯示其於強光（1,000 μE m-2 s-1），34℃，6小時熱緊迫的條件下，Fv/Fm會明顯下降，但在回復正常溫度（26℃）及正常光照（200 μE m-2 s-1）後，給與高鐵條件（1,000 pM Fe'）的組別於一個禮拜後Fv/Fm有顯著恢復；同時亦觀察到細胞內金屬鐵的含量與鐵供給有相對應的表現。在藻液顏色上，B. m.高鐵組相較低鐵組有更明顯的藻綠色，也展現出恢復的趨勢。 耐熱性共生藻種F. k. 於持續性熱緊迫（30℃）且無進行溫度馴養的情況下，無論何種無機鐵濃度（10–400 pM），F. k.的細胞數均呈現下降的趨勢；有進行溫度馴養時，F. k.於100及400 pM Fe' 有稍微生長，10 pM Fe'則不生長，展現出不同的生長趨勢；而於間歇性熱緊迫組別（30℃），不論於100 pM Fe' 或10 pM Fe'，F. k.皆正常生長。特別於10 pM Fe'，間歇性相較於持續性熱緊迫表現出較高的SOD活性，由共生藻生長情形及SOD活性可推論10 pM Fe'不足以幫助F. k.抵禦持續性熱緊迫。 從共生藻於熱緊迫條件下的實驗結果顯示，不論是細胞生長狀況、SOD活性、Fv/Fm趨勢，皆展現了充足無機鐵供應對珊瑚共生藻細胞在光和熱逆境的修復中，扮演十分重要的角色。 相對於野外條件，已知開放性海洋表水的溶解態無機鐵濃度遠低於本實驗所使用的濃度水準，珊瑚礁生態系統的生物可利用濃度極可能不足以供應部分珊瑚共生藻在熱或光緊迫下有效移除活性氧物種所需。

The accumulation of reactive oxygen species (ROS) in Symbiodiniaceae is considered as a major cause resulting in coral bleaching. Under high light and temperature conditions, symbiotic dinoflagellates would rapidly generate ROS through electron transfer chain in photosystems. Sufficient expression of metalloenzymes related to ROS removal is thus essential to relieve the stresses. In this study, we designed experiments to investigate whether Fe can alleviate the effects of light and heat stress. We applied high light (1,000 μE m-2 s-1) and high temperature (30℃) conditions. Three different species of Symbiodiniaceae were grown under continuous or intermittent heat stress under different Fe availability levels. Their growth rates, SOD activities, photosynthetic performance (Fv/Fm), and cellular Fe quota were determined. We found that the two heat-sensitive species, Breviolum minutum (B. m.) and Symbiodinium microadriaticum (S. m.), could grow up under intermittent heat treatment but died under continuous heat treatment (1,000 μE m-2 s-1). However, the heat-tolerant species, Fugacium kawagutii (F. k.), could grow slowly under continuous heat treatment. Elevating Fe concentrations, the two heat-sensitive species were still unable to grow under high light and continuous heat stress but F. k. grew extremely well. We then conducted experiments with short-term light or heat stress. We found that the Fv/Fm levels of the two heat-sensitive species decreased sharply under extremely high light condition (1,000 μE m-2 s-1). At 600 and 800 μE m-2 s-1, only B. m. were able to maintain Fv/Fm levels while growing with elevated pM Fe' (1,000 pm) but its Fv/Fm levels decreased significantly in 250 pM Fe' level. For short term heat shock treatments, we found that Fv/Fm levels for the two heat-sensitive species significantly decreased right after 34℃ 6-h heat stress treatment but gradually recovered only for high Fe' treatments after switching back to non-stressed conditions. For intermittent heat treatment at 30℃, F. k. reached maximum growth rates at both 10 and 100 pM Fe' and showed much higher SOD activities than continuous heat treatments. Relatively, for continuous heat treatment, F. k. did not grow at 10 pM Fe' and grew slightly better under 100 pM and 400 pM Fe' levels. Overall, these findings support our hypothesis that sufficient Fe' levels are critical to sustain the growth or survival of some Symbiodiniaceae grown under the light and heat stresses. To our best knowledge, dissolved inorganic Fe concentrations in the surface water of the open ocean are significantly lower than the concentration level required by the Symbiodiniaceae needed to relieve the high heat and high light stresses applied in this study. Remained to be analyzed and validated in future studies, the concentration levels in coral reef ecosystems are highly likely below the levels needed to relieve the stresses.