台灣泥火山與熱泉之微生物生態系統與碳硫循環

Abstract

陸域泥火山是氣體、流體與沉積物的混合物由地底下噴發至地表並堆積所形成的特殊地貌，也是甲烷逸散的重要通道；而地底下經加熱的流體在地表出露則形成熱泉。兩者均包含還原流體自深部地層上升的過程，並混合了來自上部地層低溫且含氧的地表水，可形成具有溫度和氧化還原電位之梯度變化的環境，並支持多樣的微生物生存。研究此等環境中的微生物生態、探討微生物對甲烷濃度變化、溫度、氧化還原電位之反應，不僅有助於評估微生物對甲烷排放的貢獻度，也可增進瞭解微生物與自然環境的交互作用。據此，本研究結合了地球化學分析、分子生物與培養的方法，冀望藉由多重佐證，以了解陸域泥火山及熱泉環境中，微生物的生態系統及生物地球化學循環。 本論文之第一部份，藉台灣西南部的新養女湖泥火山為研究模板，討論微生物組成、代謝作用、甲烷循環和流體過程彼此之間的連結關係。地化和分子生物的結果共同指出，此地沉積物縱剖面具有一致的地化特徵及微生物分佈的分區，由淺至深分別為：有氧型甲烷氧化作用、硫酸還原作用、由硫酸還原所支持的厭氧型甲烷氧化、及甲烷生成作用。此分區現象儘管與海洋環境相似，但硫酸來源推測是由地表的蒸發作用與礦物氧化作用共同富集而來;且沉積物中不僅有來自深部的熱生成甲烷，還富含大量的當地甲烷菌生成的甲烷;儘管這些甲烷最終逸散至大氣中，據通量計算微生物的活動也相當程度地降低了甲烷逸散量。 第二部份，將取自關子嶺滲氣噴泉的熱泥漿培養於一系列的不同溫度中，用以探討深部來源的硫酸還原菌，對於遷移至地表過程中可能遇到的溫度變化之反應。研究結果顯示硫酸還原反應在40­80oC均可發生，菌相組成因溫度而異，培養在最佳生長溫度範圍的菌種則成為該溫度的優勢種。發酵作用可將大分子有機物質分解，反應產生的電子可傳遞給硫酸還原菌使用，藉以支持整體群落的能量來源。 第三部份採集大屯山火山區的熱泉池水與底泥，分析懸浮性以及底生性微生物之種類與代謝差異，並測量不同深度的池水與底泥的化學特性，旨在探討微生物的分佈受環境化學因子和氧化程度的影響。結果顯示好氧的 Sulfolobus spp. 為懸浮性微生物的優勢菌種，而厭氧的 Caldisphaera spp. 則為重要底棲性族群，此兩類古菌均以元素硫為能量來源；組成分析指出懸浮性微生物社群與底生性質者歧異，需氧特性也不等，此分佈差異與池水或底泥中的含氧量相關，根據計算可知溶氧會在水層中大量被微生物或非生物作用消耗，進而限制了好氧性族群的分佈範圍。 總結而言，不論是陸域泥火山系統或是熱泉系統，其中的微生物代謝活動，皆同時受到來自地表與地底的能量來源、溫度、氧化還原程度等因子的控制，並與甲烷和硫化物的循環有緊密的關聯性。

Terrestrial mud volcanoes (MVs) and hot springs are prominent surface relief formed by the rapid release and accumulation of a mixture of sediments, fluids, and gases along fracture networks tapping into deep structures and/or petroleum/gas reservoirs. The interaction between the deeply-sourced, reducing fluids and low-temperature, oxidizing surface environment generates dynamic redox and temperature gradients that would facilitate the proliferation of diverse microorganisms. How microbial communities exploit such chemical and physical disequilibria and contribute to the attenuation of methane emission is rarely explored. This study combined geochemical analyses, molecular diversity surveys, and microbial activity measurements to explore fluid processes, and community compositions, functions and activities, aiming at better understanding of microbial ecosystems and geochemical cycles in mud volcano and hot spring systems. In the first part, the Shin-Yan-Ny-Hu Mud Volcano of southwestern Taiwan was chosen to represent a model system that could witnesses how methane and other elements are cycles by the interactions between microbial and geochemical processes. Geochemical and molecular results indicated the compartmentalization of aerobic methane oxidation, sulfate reduction, sulfate-dependent methane oxidation and methane production along a depth profile. Such metabolic zonation, although resembling those in marine setting, is driven by the downward percolation of sulfate generated by surface evaporation and mineral oxidation and the upward migration of methane primarily produced by in situ methanogenesis. In the second part, hot muddy fluids emanating from the Kuan-Tzu-Ling seep were incubated at a wide range of temperatures to uncover the responses of indigenous sulfate-reducing communities to the potential temperature changes during transportation. The results demonstrate that specific groups of sulfate reducers could rapidly respond to temperature fluctuations and persist in accordance with their physiological limits. These communities were sustained by exploiting recalcitrant organic carbon that has survived long-term geological degradation. In the third part, three paired water and sediment samples from the hot ponds in the Tatun Volcano Group were collected in order to assess the distributions and metabolic roles of microbial communities and correlate their functional capabilities with the redox potential along depth. Molecular analyses revealed that aerobic, sulfur-oxidizing Sulfolobus spp. and obligate anaerobic, sulfur-reducing Caldisphaera spp. prevailed in water and sediments, respectively. A segregation of bottom-dwelling archaea from planktonic ones was revealed and could be accounted for by the oxygen affinities inherited in individual archaeal members. In conclusion, microbial metabolic networks in MV and hot spring ecosystems are strongly influenced by temperature and redox potential and are tightly connected with carbon and sulfur contents.