珊瑚礁是組成沿岸海洋生態系的重要部分，它是由類似海葵的動物，也就是珊瑚蟲，歷經百萬年以上的時間構建而成。
珊瑚蟲會分泌碳酸鈣，可幫助形成我們稱之為珊瑚礁的結構。珊瑚礁被稱作是海洋生態系的雨林，因其供養為數眾多的其他動物，包括魚類、植物與無脊椎動物。珊瑚礁也支撐著許多熱帶島嶼國家人民的經濟，以其各式各樣資源提供人民生計所需。
氣候變遷≠全球暖化
氣候變遷是一個廣義的辭彙，涵蓋了我們地球上氣候正在發生的所有變化。氣候變遷愈來愈常與全球暖化混淆。事實上，全球暖化是氣候變遷的影響之一；而氣候變遷是地球環境（包括自然與人為過程）累進變遷的結果，地球環境變遷則是多種環境條件變動使然，其中之一便是全球暖化。全球暖化肇因於多種氣體與汙染物被排放進入大氣層，形成一個氣層，吸收太陽進入地球大氣層的熱。換句話說，全球暖化就像是處在溫室裡的狀態。由於氣候變遷，主要是因為人類活動的增加（釋放各種氣體到大氣層、伐木、污染、沿岸地區開發等等），最近幾年各種效應已然增加，像是全球暖化/全球寒冷化、紫外線輻射、增多的天然災害如洪患、旱災與颱風/颶風。
珊瑚健康
珊瑚的健康仰賴於其與單細胞植物的共生關係，這種植物即所謂的共生藻（Symbiodinium sp）。在此共生關係中，珊瑚蟲提供居所、保護與食物給這些微小的細胞，相對的，這些藻類植物細胞則藉由光合作用提供珊瑚蟲能量，並以其色素增添珊瑚蟲美麗的色彩。這個關係對於珊瑚蟲的正常功能與良好健康狀況都非常重要。此相結合的關係構成了所謂的共生功能體（珊瑚蟲+共生藻）。近年來，珊瑚的健康已經受到了來自於天然（海水溫度變化、鹽度與沉積作用等）與人為作用（污染）的不同壓力所影響。當珊瑚受到壓力作用而變得不健康時（參見圖二與四），共生關係將受影響，而珊瑚在失去其夥伴後將更難以存活。
珊瑚如何回應壓力？
珊瑚回應壓力有點像我們人類會做的那樣。它們在受損害位置會藉由累積色素，對壓力展現出某種免疫反應（圖三）。珊瑚受到微生物如細菌、病毒或黴菌的影響，會在受感染的位置因累積色素、增加黏液分泌或是珊瑚組織全數死亡（圖四），而在顏色上有所改變（圖三）。這情形在組織與細胞兩個層次上都一直可見。在這種情況下，珊瑚可能顯露出逆境反應，若非暫時性，便會直到病源的細菌效用消逝。
某些珊瑚種類也會分泌一層保護性的色素扮演太陽眼鏡的功能，防衛漸增的紫外線輻射影響。最為常見的反應是，當受到諸如增高的溫度、增多/減少的鹽度、海水pH值改變、沉積物、化學物或污染物所影響，珊瑚會白化（圖二），或者說因為珊瑚蟲與共生藻之間的共生關係被破壞而失去其色彩。當珊瑚發生白化現象，假如不是因為珊瑚蟲失去體內的共生藻細胞，就是因為共生藻的色素減退，或者是兩者兼具。
珊瑚白化時，此機制到底發生了什麼事，大家對其仍知之甚少。許多研究顯示出，在珊瑚遭受壓力時會有許多不同的生化與生理上的反應發生，包括珊瑚蟲和共生藻細胞兩者均釋放出毒性氧、排放不同種類的壓力蛋白，以及細胞與組織死亡。不同珊瑚種類面對壓力的反應，也要看珊瑚蟲所共生的共生藻種類。再次說明，與人類類似，對壓力源的反應結果並非所有的珊瑚種類都相同。這取決於不同的特性，諸如珊瑚蟲與共生藻是否強健、其對於特定壓力源之反應模式、它們所生存的周遭環境類型，以及珊瑚在面對壓力時可以多快做出反應等等。
就連比鄰而居的兩個珊瑚種類，在壓力下都可能會顯現出截然不同的反應，而使得個別珊瑚種類彼此差異的主要原因，現在仍無法全然被瞭解。實際上，因為珊瑚蟲與共生藻之間的共生關係，在珊瑚體內進行中的各種程序非常複雜，使得珊瑚共生體的生理實驗研究難以進行及呈現。
※本文與農委會林務局合作刊登
Coral reefs from an important part of coastal marine ecosystem. Coral reefs are built over millions of years by sea anemone like animals known as coral. Corals secrete calcium carbonate that helps in formation of structures known a reefs. Reefs are known as rain forests of marine ecosystem since they support numerous other animals which include fishes, plants and invertebrates. Coral reefs also support the economy of people of many tropical island nations by providing them with various resources for their livelihood.
Climate change
Climate change is a broad term that includes all the changes that is happening to the climate of our planet Earth. More often climate change is confused to global warming. In fact, global warming is one of the effects of climate change. While climate change is a result of cumulative changes in the earth’s environment （both as a result of natural and anthropogenic processes） resulting in alteration of various environmental conditions of which one is global warming. Global warming is due to release of various gases and pollutants in to the atmosphere where they from a layer and trap the heat of sun which enter to the earth’s atmosphere. In other words, global warming is like a condition in a green house. As a result of climate change, mainly due to increased activity of humans （release of various gasses into the atmosphere, deforestation, pollution, coastal development etc） various effects like global warming / cooling, ultraviolet irradiation, increased natural calamities like flood, droughts, typhoons/cyclones has increased in recent years.
Health of coral
Health of coral is dependent on its symbiotic association with a single celled plant cell known as zooxanthellae （Symbiodinium sp）. In this association, coral animal provides a home and protection and food to those tiny cells and in turn the plant cells which are algae, provide energy by photosynthesis and beautiful color through their pigments to coral animal. This relationship is very important to the proper functioning and good health condition （Fig. 1） of coral animal. This combined association forms what is known as holobiont （coral animal + zooxanthellae）. In recent years health of corals have been affected by various stressors as a result of natural （seawater temperature change, salinity, sedimentation, etc） and anthropogenic influences （pollution）. When corals become unhealthy （Fig. 2 and 4） due to effect of stress, the symbiotic relationship is affected and it becomes difficult for corals to survive without their partners.
How corals respond to stress?
Corals respond to stress somewhat similar to as we humans do. They show a kind of immune response to stress by accumulating pigments at the site of injury （Fig. 3）. Corals that are effected by microorganisms like bacteria, virus or fungi show a change in color at the site of infection （Fig. 3） by accumulation of pigments or increased mucus secretion or complete death of coral tissue （Fig. 4）. This has been seen to occur both at the tissue and cellular level. In this case, the coral may show a stress response either temporarily or until the effect of pathogenic bacteria persists. Some coral species also secrete a layer of protective pigments that act as a sunscreen to ward of the effect of increased ultraviolet radiation. The most common response, when corals are affected by stressors like increased temperature, increase/decrease in salinity, pH changes of seawater, sedimentation or chemicals and pollutants, is bleaching （Fig. 2） or in other words loss of their color as a result of breakdown in the symbiotic relationship between the coral animal and zooxanthellae. When corals bleach, it is either due to loss of the presence of zooxanthellae cells inside the coral animal or due to reduction of the pigments of the zooxanthellae or both. This mechanism of what exactly happens when corals bleach is still poorly understood. Many studies have shown that, there are many different biochemical and physiological responses that occur when coral is under stress including release of toxic oxygen both by coral animal and zooxanthellae cells, release of various kind of stress proteins as well as cell and tissue death. Response of a coral species to stress also depends on the coral animal and the type of the zooxanthellae it is associated with. Again, similar to humans, the effect of stressors is not the same for all the coral species. It depends on various characteristics such as, sturdiness of coral animal as well as zooxanthellae, the way they respond to a particular stress, the type of surrounding environment in which they exist and speed at which a coral can respond to stress etc. Two coral species living besides each other can show completely different response to stress and this aspect of what makes each coral species different is still not fully understood. In fact, due to the symbiotic association between coral animal and zooxanthellae, the various processes undergoing inside a coral is so complex that, more often research on this aspect has been difficult to perform.
Climate change and coral health
As discussed above, coral health is influenced by various natural and anthropogenic factors and climate change as a result of human activity is one of the most important factors that is affecting coral health and its proper functioning. As health of a coral is influenced by environmental conditions, increasing seawater temperatures is taking its toll on the survival of corals. Cumulative effect of various stressors will lead to reduced functioning of coral animal and in turn effect the total health of a coral reef.
※本文與農委會林務局合作刊登
