地外生命的類比：在深層生物圈中存活了四億年，以甲烷為食的細菌

原文網址：www.sciencedaily.com/releases/2017/05/170509083940.htm

地外生命的類比：在深層生物圈中存活了四億年，以甲烷為食的細菌

越來越多科學家意識到生物活動進行的主要地點並非地面，而是隱藏在地下數公里深的環境中，稱作「深層生物圈」。對此能量貧乏系統中存活的生物進行的研究，可以讓我們更加了解地球的生命起源，以及表面環境可能十分艱困，讓生物難以居住的其他星球演化出生命的方式。但我們對生存在我們腳下深處的古老生命，卻瞭解得極為稀少。

在地下1700公尺深處分布著無數裂隙，部分被生長於其中的晶體封閉。由林奈大學的Henrik Drake博士領導的國際研究團隊，追蹤此處由古代微生物進行的基礎代謝作用，包括生產與消耗一種溫室氣體――甲烷的反應。這是迄今在大陸地殼中，對古代微生物活動進行的最全面研究。在瑞典以花崗岩組成的數個研究地點，他們利用不同領域的研究方法，包含測量微小範圍的穩定同位素，輔以地質定年法測定由微生物活動形成的礦物。結果指出在岩床內部，廣泛分布著由微生物產生和消耗甲烷的作用。

Henrik Drake解釋他們是如何取出礦物的穩定同位素紀錄，以解讀古代微生物進行的作用：「從其他環境我們熟知在跟微生物作用有關的碳酸鹽礦物中，甲烷的形成與消耗會形成特徵性的同位素比例。我們對方解石晶體進行微觀分析，顯示其碳同位素組成的分布範圍相當極端，只能以微生物生產和消耗甲烷的作用來解釋。」

這項新知指出在自然界，甲烷的生成和吸收作用分布的時空尺度相當廣大，意謂著我們需要重新審視在龐大大陸地殼中進行的碳循環，且從長期來看，其可能在全球暖化中具有重要地位。研究共同作者，德國哥廷根大學的Christine Heim表示：「有趣的是我們能從相當深處的方解石中，發現來自地表的古代有機物(比如陸生植物)留下的生物訊號，因此該處微生物的營養來源至少有部分似乎是從地表來的。這道跟地表生物圈有關的連結，也許能解釋微生物活動訊號為何會在700至800公尺左右深的地方突然消失。」

研究中用到的晶體有相當細微的分層，因此需要發展出具有高空間解析度的新定年技術。研究人員利用這項技術，首次能直接界定微生物作用的時間，來指出生物活動於何時發生，結果為距今4億、3.5億和1.7億年前。顯然在漫長的地質歲月中，一直都有生命繁榮生長在大陸地殼的地底環境。它們給出的線索告訴我們在能量稀少的系統中，生命如何維繫自身存在。在尋找其他星球表面下方的環境有無生命時，也能用上這些線索。Henrik Drake總結：「我們涵蓋多重面向的研究方法，也能良好適用於地外天體環境的研究。」

哥廷根大學的共同作者Thomas Zack補充：「地球和其他星球中的裂縫可謂無所不在，而我們的發現指出這些地方可能是保有過去生物活動遺骸的完美墳場。之前有誰會想到呢？」

Methane-munching microbes living in the deep biosphere for 400 million years: An analogue for extra-terrestrial life

It is becoming more and more appreciated that a major part of the biologic activity is not going on at the ground surface, but is hidden underneath the soil down to depths of several kilometres in an environment coined the "deep biosphere." Studies of life-forms in this energy-poor system have implications for the origin of life on our planet and for how life may have evolved on other planets, where hostile conditions may have inhibited colonization of the surface environment. The knowledge about ancient life in this environment deep under our feet is extremely scarce.

In numerous cracks down to depths of 1700 meter that have been partly sealed by crystals grown in them, an international team of researchers led by Dr. Henrik Drake from Linnaeus University, Sweden, has traced fundamental ancient microbial processes, including production and consumption of the greenhouse gas methane. The multi-disciplinary approach included micro-scale measurement of stable isotopes coupled with geochronology within minerals formed in response to microbial activity at several Swedish granitic rock sites. This is the most extensive study on ancient microbial activity in the continental crust yet and the findings suggest that microbial methane formation and consumption are widespread in the bedrock.

Henrik Drake explains how they tapped the stable isotope archive of minerals to decipher ancient microbial processes: "It is well known from other environments that methane formation and consumption result in diagnostic isotope ratios in carbonate minerals formed in association with microbial processes. The micro-analyses within calcite crystals showed an extreme range in carbon-isotope compositions, which can only be explained by microbial methane formation and consumption."

This new knowledge of a deep source and sink for methane of widespread nature in space and time calls for a re-evaluation of the carbon cycling within the vast continental crust and may be significant in a long-term global warming perspective. Christine Heim of University of Göttingen, Germany, a co-author of the study, says: "It is intriguing that we could find biomarkers of ancient organic remains of surficial origin (e.g. land plants) preserved within calcite at great depth and that the nutrient source for the microbes at least partly seems to have been coming from the surface. This connection to the surface biosphere may explain why the marks of microbial activity abruptly disappear at around 700 to 800 m depth."

Direct timing constraint of the microbial processes reveals for the first time when the biologic activities occurred -- at ca 400, 350 and 170 Million years ago. This was facilitated by newly developed dating techniques of high spatial resolution, which is needed for delicately zoned crystals investigated in the study. Life in the subsurface environment of the continental crust has evidently thrived over geological eons, and provides clues about how life sustains in energy poor systems, which is relevant when searching for life in subsurface environments of other planets. Henrik Drake summarizes: "Our multi-phased methodology is clearly well suited for application to extra-terrestrial environments."

Co-author Thomas Zack from the University of Gothenburg adds: "Cracks in the Earth and on other planets are omnipresent, and our findings indicate that they may be the perfect graveyards for past biologic activities. Who would have thought that? "

原始論文；Henrik Drake, Christine Heim, Nick M.W. Roberts, Thomas Zack, Mikael Tillberg, Curt Broman, Magnus Ivarsson, Martin J. Whitehouse, Mats E. Åström. Isotopic evidence for microbial production and consumption of methane in the upper continental crust throughout the Phanerozoic eon. Earth and Planetary Science Letters, 2017; DOI: 10.1016/j.epsl.2017.04.034

引用自：Linnaeus University. "Methane-munching microbes living in the deep biosphere for 400 million years: An analogue for extra-terrestrial life." ScienceDaily. ScienceDaily, 9 May 2017.