Abstract
摘要:人類在探訪找尋太陽系中或是地球早期可能之生命型態的時候,深部生物圈一向被認為是最佳的類比研究場所。我們對深部生物圈分佈範圍的瞭解,已經延伸至過去認為無法採樣或不適合生命生存的各式各樣環境,包括大陸地殼、大型火成侵入體、深海沉積物、含油層、以及海底熱液系統。在這些微生物可能居住的環境中,海底熱液系統是特別吸引人注目的,因為流體滲透入有浸透性之岩石並與之反應,產生豐富且具多樣性的養份,提供給可以獨立生存於地表光合作用外的微生物族群。海水與熱液系統逸出流體的混合,同時產生極大的溫度和化學的梯度,強烈的控制微生物的新陳代謝作用,使其呈現極不均勻的分布。雖然熱液噴出孔經常性輸出的高濃度溶質與還原性氣體,是控制周遭海水之流體化學及微生物生態系統的主因,流體及岩石的交互作用和其他物理作用(如地震),也扮演十分重要的角色。
我們提出一個三年型研究計劃,希望針對龜山島附近之海底熱液系統,以及即將於島上鑽穿表層數百公尺火成岩層至下部沉積岩層所取得之岩心,進行微生物組成與地球化學特性分析。我們推測這個區域的地下微生物生態系之新陳代謝及生理作用的表現,應與三個可能潛在之流體蘊藏體(包括天水、盆地滷水及海水)的化學特徵息息相關。不同的流體蘊藏體都和相同的熱源交互反應,產生獨特的水體及氣體化學特徵,孕育足以適應嚴峻環境壓力的微生物族群。由於安山岩基質所包裹之鐵鎂礦物可將水分子還原產生氫氣,而耗氫自營菌很可能是海殼沈積物與火成岩介面之特殊生態系統的主角之一,島上鑽探岩心之研究可以檢驗相關假說,包括是否耗氫自營菌的生物質量遠多於異營菌而組成食物鍊的底層?地質作用產生的氫氣是否支持此一特殊生態系統的運作?
本研究計畫之執行將對於我國參與隱沒、張裂相關構造系統之深部生物圈探勘,有重要的隱示。由於龜山島位於台灣東北外海十公里處,且現生的熱液噴出於水深十米的海床上,與深海熱液系統相比,此一地理上的優勢使其成為進行此類研究的最佳起點,不僅後勤支援容易、交通運送便宜,並且可以進行重複採樣。由於國際整合型海洋鑽探計畫近期內可能針對沖澠海漕進行數次的測試鑽井,同時深部生物圈又是國際整合型海洋鑽探計畫中的三大最重要的主題之一,本計畫之順利執行,將足以催化國內學者未來與國際整合型海洋鑽探計畫科學家進行合作研究。
Abstract: Deep biosphere has been regarded as the best analogue for searching the potential life on other extraterrestrial bodies in our solar system and for resolving the possible microbial ecosystem on early Earth. Its extent has been greatly expanded to various environments that were thought inaccessible and uninhabitable previously, including continental crust, large igneous intrusion, deep-sea sediments, oil reservoirs, and submarine hydrothermal vents. Of these potential habitats for microbes, submarine hydrothermal venting system appears to be particularly fascinating because fluid percolates through and reacts with permeable rocks, thereby generating abundant and various substrates to support microbial assemblages that can be fully independent from those fueled by solar-powered photosynthesis. The mixing of seawater with emanating hot fluid also creates steep temperature and chemical gradients in which metabolic capacity is strongly regulated and heterogeneously distributed. While the venting constantly delivers concentrated solutes and reducing gases to ambient seawater, fluid-rock interaction as well as other physical processes such as earthquakes appear to play an essential role on controlling fluid chemistry and microbial ecosystem.
We are proposing a three-year research in an attempt to recover microbial assemblages associated with hydrothermal fluids offshore Kuei-Shan-Tao and with drilled cores that may penetrate through the top 100’s meter thick volcanic successions to the underlying sedimentary formations on land. This sampling strategy is designed because we hypothesize that metabolic functions and physiological expressions of the subsurface ecosystem in this region are relevant with chemical characteristics of three potential fluid reservoirs, including meteoritic water, seawater, and basinal brine. Each individual component would react with a common heating source (magmatic intrusion), generating distinctive aqueous and gas chemistry which preferentially favor microbial assemblages capable of adapting to the severe stress. The field site is chosen partly due to the fact that mafic minerals (mainly pyroxene) encompassed by andesitic matrix are subject to H2 generation through reduction of H2O. The samples would allow to examine the hypotheses whether H2-consuming autotrophs dominate over heterotrophs and form the base of the food web and whether geologically-produced H2 provides excessive metabolic energy and the ecosystem becomes limited by the presence of electron acceptors.
The proposed research also bears several profound implications for exploring deep biosphere in the subduction-related tectonic setting. Since the Kuei-Shan-Tao is located only 10 km from northeastern Taiwan and active venting occurs at less than 10 m below sea level, the geographic advantage allows easier logistic support, cheaper transportation and repeated sampling when compared with deep-sea hydrothermal systems. While the Integrated Ocean Drilling Program (IODP) might initiate several test-drilling campaigns in the Okinawa Trough during the following years and “Deep Biosphere” has been highlighted as one of three most important themes in IODP, the proposed research can catalyze the potential collaboration with IODP scientists in the future.
Keyword(s)
地下生態系統
海底熱液系統
龜山島
subseafloor ecosystem
hydrothermal vent
Kuei-Shan-Tao