台灣熱泉生態系統中微生物代謝之生物能量評估

Abstract

生存於熱泉生態系統中的嗜熱微生物，利用熱泉中的各種化學成分進行代謝作用維持生存，一方面受限於環境的地質地化條件而生長，另一方面又促進地質地化系統的循環。台灣地區無論在火成岩、變質岩或沉積岩地區均有溫泉出露，可提供嗜熱微生物多樣化的天然居所，是瞭解熱泉微生物族群變化與地質地化特性的關係，與討論不同區域中微生物代謝策略差異之好題材。本研究根據台灣各地溫泉的地球化學特性，計算反應自由能與能量通量，以期全面又快速的評估生態系統中微生物可以使用的代謝途徑，釐清微生物族群間的競爭關係，了解台灣熱泉生態系統中微生物族群結構與環境因子間的關連。 本研究依照地質條件將溫泉採樣區分成北部火成岩區、東部變質岩區與南部泥火山沉積岩區，共採集16處溫泉，測量熱泉中的各種陰陽離子、有機酸與溶解氣體，以及礦物種類。化學分析結果發現在台灣熱泉系統中出現的電子提供者包括H2、NH4+、Fe2+、H2S、CH4、有機酸，電子接受者包括O2、NO3-、Fe3+、SO42-、S0和CO2。取得完整的化學資料後，依據熱力學原理計算溫泉中各種化學反應的自由能與能量通量，配合微生物最低自由能需求的概念，推測微生物可能使用的代謝途徑，結果發現不同區域溫泉中微生物族群可用之代謝途徑與該途徑所能提供的能量通量，受控於溫泉中電子接受者與提供者的種類、濃度變化，而這些溫泉化學特徵的變化，可以歸因於地質條件的不同。進一步考量環境條件變化時，代謝反應的自由能與能量通量的變動，發現研究區域中微生物可能使用的代謝途徑大多能夠持續產能而被微生物所使用。對照已有的分子生物分析與微生物培養結果，大致與本研究結果相符，而微生物族群結構與物種間的競爭關係，亦可從地化條件與能量通量觀點討論之。

Thermophilic microorganisms using chemical compounds to carry on their metabolisms not only are limited by geological and geochemical characteristics but also enhance geological and geochemical cycling in hydrothermal ecosystems. Hot springs are widely hosted in igneous, metamorphic and sedimentary rocks in Taiwan. These diverse natural habitats for thermophilic microorganisms are ideal studying sites to understanding the interaction between microbial communities and geochemical circumstance in various geological regions. On the basis of geochemical compositions, free energy and energy flux could be calculated for certain reactions. Such evaluation would provide a complete and quick way to clarify the energetic metabolic pathways, to understand the competition among microbial populations and to reveal the relationship between community structures and environmental factors in various Taiwan hot springs ecosystems. This study sampled 16 hot springs derived from three geological domains, the northern volcanic region, the eastern metamorphic region and the western sedimentary region. Anions, cations, organic acids, dissolved gases, as well as minerals were measured in these hot springs. Chemical analyses showed that various electron donors, including H2, NH4+, Fe2+, H2S, CH4 and organic acids and electron acceptors, including O2, NO3-, Fe3+, SO42-, S0 and CO2 were available in Taiwan hot spring ecosystems. According to the thermodynamic principle and the concept of lowest free energy requirement, energetic metabolic pathways were revealed by the calculation of free energy and energy flux of various chemical reactions. Energetic metabolic reactions and their energy flux basically depended on the availability and concentration of electron acceptors and donors in different regions. Whether the metabolic reaction can continuously provide energy for the microorganism in the hot spring was also evaluated by the free energy and energy flux change due to certain geochemical variations. Most energetic metabolism pathways should be persistent despite of the dynamic change of chemical and physical conditions. These results were generally supported by molecular biology and microbial incubation analyses. The structure and the competition within microbial community were examined in the view of geochemical conditions and energy flux in the hydrothermal ecosystems.