本篇文章共分 3 段，將地下生命的存在證明方式、這些生命存在的區域，以及地下生命的類型一一加以說明陳述。

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Underground Life 地下生命

1. 地下生命存在之檢測

   Until about the late 1980s, most scientists believed that life was restricted to the top few meters of the soil or ocean sediments. The few reports of organisms being recovered from great depths within Earth were dismissed as contamination with material from the surface layers. Two technical developments changed this view. The first was the development of drilling techniques that gave confidence that samples could be retrieved from depth without contamination. Samples were recovered using a diamond-studded drill bit that headed a great length of rotating steel pipe from a drilling derrick. A concentrated tracer material was added to the lubricating fluid so that when a deep sample of rock was removed, any contaminated material could be identified and cut away to leave a pristine sample of rock from deep within Earth. The second development was the advent of techniques for identifying microorganisms without having to grow them in culture. All organisms contain DNA, and their presence can be revealed by dyes that either stain DNA directly or can be attached to nucleic acid probes. By varying the nucleic acid probe, scientists can demonstrate the presence of different types of microorganisms.

   直到 20 世紀 80 年代末，大多數科學家認為，生命只限於土壤或海洋沉積物的頂部幾米。少數關於從地球深處找到的生物體的報告被認為是被表層的物質所污染。兩項技術發展改變了這種觀點。首先是鑽探技術的發展，使人們相信樣品可以從深處被回收而不會受到污染。採集樣品時，使用了一個鑲有鑽石的鑽頭，該鑽頭從一個鑽井架上引出了很長的旋轉鋼管。一種濃縮的示蹤材料被添加到潤滑液中，這樣當一個深層的岩石樣本被取出時，任何被污染的材料都可以被識別並切掉，從而留下一個來自地球深處的原始岩石樣本。第二個發展是識別微生物的技術的出現，而不需要藉由培養方式來培養它們。所有的生物體都含有 DNA，它們的存在可以通過染料來顯示，這些染料可以直接對 DNA 進行染色，也可以與核酸探針相連。通過改變核酸探針，科學家可以證明不同類型的微生物的存在。

2. 地下生命存在之區域

   The first scientists to use these techniques were involved in the Subsurface Science Program of the United States Department of Energy (DOE). They were interested in the possibility that if organisms existed in the depths of Earth, they might degrade organic pollutants and help maintain the purity of groundwater or, rather less usefully, degrade the containers in which the DOE was proposing to deposit the radioactive waste from nuclear facilities. They demonstrated the presence of many different types of microorganisms in rocks at depths down to 500 meters beneath the surface. Since then, microbes have been discovered in many different types of rocks and deep within ocean sediments. The record depth at which life has been found is at the bottom of a South African gold mine, 3.5 kilometers below ground. Pressure and temperature increase as you go deeper into Earth. Some scientists think that subsurface bacteria could withstand temperatures as high as 150°C. This would allow organisms to exist to depths of about 7 kilometers beneath the seafloor and to 4 kilometers below the surface of the land. Although the organisms are often sparsely distributed, this is such an enormous volume that it has been estimated that the total biomass of deep subsurface organisms exceeds that of those living on, or just below, the surface.

   第一個使用這些技術的科學家參與了美國能源部 (DOE) 的地下科學計劃。他們感興趣的是，如果有機體存在於地球深處，它們可能會降解有機污染物並幫助維持地下水的純度，或者，更無益的是，降解能源部建議存放核設施放射性廢物的容器。他們證明了在地表下 500 米深處的岩石中存在著許多不同類型的微生物。從那時起，在許多不同類型的岩石和海洋沉積物深處都發現了微生物。發現生命的記錄深度是在南非一個金礦的底部，距離地面 3.5 公里。隨著進入地球深處，壓力和溫度都在增加。一些科學家認為，地表下的細菌可以承受高達 150℃ 的溫度。這將使生物體能夠存在於海底下約 7 公里的深度和陸地表面下 4 公里的深度。儘管這些生物通常分佈稀疏，但由於生活空間體積龐大，所以據估計，深層次表層生物的總生物量超過了生活在地表或正下方的生物。

3. 地下生命類型

   Bacteria are the most numerous of these subsurface organisms, but there are also fungi and protozoa. Some 10,000 strains of microorganism have been isolated from subsurface cores. Each gram of rock contains anything from 100 bacteria to 10 million bacteria (compared with more than 1 billion per gram in agricultural soils); ocean sediments contain even higher numbers. The protozoa feed on the bacteria, forming part of a simple subterranean food chain, but what do the bacteria feed on? Sedimentary rocks are formed from sands and from ocean, river, or lake sediments that have organic material trapped within them. Microbes living in pores within the sediments can utilize these ancient nutrients and grow. As sedimentary rocks are buried more deeply, they become increasingly compacted and their pores filled with minerals. The distribution of microorganisms is thus likely to become more patchy, condensed into the remaining pores and concentrations of nutrients. The bulk of Earth’s crust, however, consists of igneous rocks, such as granite and basalt, which are solidified from molten magma. These rocks were too hot to support life when they were first formed; the organisms that inhabit cracks and fissures within the rocks are carried there by the groundwater flowing through them. Subsurface bacteria do not just rely on nutrients trapped within the rock or carried there by groundwater. Some are chemotrophs, deriving their energy from the oxidation of iron or sulfur compounds and building organic material directly from the carbon dioxide and hydrogen gas dissolved in the rock. These bacteria excrete organic compounds that are then utilized by other types of bacteria. These ecosystems based on chemotrophic bacteria are completely independent of material and solar energy from the surface.

   細菌是這些地下生物中最多的，但也有真菌和原生動物，已經從地下巖芯中分離出大約 10,000 種微生物。每克岩石含有 100 到 1000 萬個細菌（相比之下，農業土壤中每克細菌超過 10 億個）；海洋沉積物中的細菌數量甚至更高。原生動物以細菌為食，形成一個簡單的地下食物鏈的一部分，但細菌以什麼為食呢？沉積岩是由沙子和海洋、河流或湖泊沉積物形成的，這些沉積物中夾雜著有機物質。生活在沉積物內孔隙中的微生物可以利用這些古老的營養物質並生長。隨著沉積岩被埋得越來越深，它們變得越來越緊湊，其孔隙被礦物填滿。因此，微生物的分佈可能會變得更加零散，濃縮在剩餘的孔隙和營養物質的濃度中。然而，地球地殼的大部分由火成岩組成，如花崗岩和玄武岩，它們是由熔融岩漿凝固而成。這些岩石在最初形成時溫度太高，無法支持生命；居住在岩石內的裂縫和裂隙中的生物是由流經它們的地下水帶到那裡的。地表下的細菌並不只是依靠岩石中截留的或由地下水帶到那裡的營養物質。有些是化能動物，從鐵或硫化合物的氧化中獲取能量，並直接從溶解在岩石中的二氧化碳和氫氣中製造有機物質。這些細菌排泄有機化合物，然後被其他類型的細菌利用。這些基於化學營養細菌的生態系統完全獨立於來自表面的物質和太陽能。