Deep subsurface microbes
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Legend/credit: Electron micrograph of the Ebola Zaire virus. This was the first photo ever taken of the virus, on 10/13/1976. By Dr. F.A. Murphy, now at U.C. Davis, then at the CDC.
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In the introduction, briefly describe the habitat that is the topic of this page. Introduce the habitat, its ecological significance, and the importance of microorganisms in this environment. (What processes do they carry out? What functions do they perform?)
Microbes have found a way to exist in every corner of the planet, and humans have found ways to utilize their incredible diversity for thousands of years. From fermenting cheese and wine, to the production of pharmaceuticals, microbes have been utilized for a myriad of reasons. It was not until the late 1920's however, that scientists thought to look deep within the body of our earth itself for a new source of potential biodiversity.
Deep subsurface microbes first came into question when the American geologist Edson S. Bastin questioned why samples of water extracted from oil fields contained hydrogen sulfide and bicarbonates. Armed with the knowledge that certain species of bacteria can derive energy from reducing sulfur compounds in the absence of oxygen, he concluded that there must be populations of these bacteria living in the underground oil reserves, degrading the organic components of oil as a carbon source, and reducing sulfur compounds for energy. By 1926, Bastin and his colleague, Frank E. Greer has cultured sulfur reducing bacteria from samples taken from the groundwater of an oil deposit several hundred meters below the surface. Bastin and Greer's initial deduction was that the bacteria were the ancestors of those buried up to 300 million years ago when the organic materials constituting the oil deposit were buried.
The deeps subsurface ecosystem begins at about 50m below the surface of earths crust, and extends variably downward, up to 2.8km (1.7mi). The organisms live within the flooded pore space within the rocks and live by reducing inorganic compounds found in the rock. These microbes have some very specific adaptations that allow them to proliferate in such a hostile environment. Radiation resistant, thermophilic, anaerobic microorganisms with a large emphasis on DNA repair mechanisms survive for decades and centuries instead of hours or days. With such a surprising diversity of organisms in such an extreme environment, the deep subsurface has been the subject of many studies in the recent years. They carry out processes that alter the chemical makeup of minerals, degrade pollutants, and alter the mineral content of ground water. Studies are being done to search for deep subsurface microbes that produce antibiotics and heat stable enzymes, and for those that assist in the degradation of toxic substances.
Perhaps the most incredible thing about the microbes found in the deep subsurface, is that the majority of the populations can thrive indefinitely without any input from the earth's surface. That being said, they are effectively 100% disconnected from the rest of life as we know it.
Intense pressures, high temperatures, limited livable space, and limited nutrient availability are all factors that microbes living in this environment must adapt to. It seems that the largest limitation to microbial life in this habitat is temperature, which increases with depth. The highest temperature generally accepted as the livable range for microorganisms in this habitat is 110 degrees C  In oceanic crusts, the temperature of the subsurface increases at a rate of about 15 degrees C per kilometer of depth, giving a maximum livable depth of about 7 kilometers. In the continental crusts the rock warms at a significantly faster rate, about 25 degrees C per kilometer, resulting in a maximum livable depth of approximately 4 kilometers . Microbes in these environments can only exist where water fills the pore spaces of rocks. In the marine subsurface, this is rarely an issue, but in continental subsurface, there tends to be a bit more variability in groundwater dispersion.
Subsurface Aquifers/Hydrothermal Waters
Hydrothermal waters spewing from deep sea vents Commonly found in deep aquifers, deep subsurface microbes provide a significant impact on the chemistry of the groundwater available in these aquifers.
Sedimentary Basins/Oil Reservoirs
In sedimentary basins and oil reservoirs, it is thought that the microbial communities are remnants from when their ancestors were buried underneath the sediment or organic matter. The nutrients contained within these ecosystems is typically organic matter produced by plants existing at the time when the layers were exposed at the surface. Energy is derived from the reduction of organic compounds in oil or sediments, as well as from inorganic compounds such as sulfur, iron, and manganese . As depth increases, available pore space and nutrient availability decreases, so the metabolic rate of the communities slows down significantly. As the rock becomes increasingly compacted, the colonizable areas become increasingly patchy and isolated, resulting in a plethora of microcommunities surrounding the available sources of nutrients.
Crystalline Metamorphic and Igneous Rocks
Igneous rocks are perhaps the most hostile environment in which deep subsurface microbes exist. Due to the processes needed to create igneous rocks, i.e. extreme temperature and pressure, these habitats are effectively sterilized at their creation. Microbes can only colonize the rocks when they have been removed from the hostile surroundings, usually by tectonic processes. Once the rocks cool to below the temperature threshold for microbial life (as far as we know it) the microbes must be transported there by the infiltration of groundwater from above. The groundwater penetrates microscopic fissures and spaces between the crystals and the microbes begin to take hold.
Due to the lack of organic materials in these igneous rocks, the microbial communities are comprised primarily of autotrophs. Their primary source of energy is hydrogen gas, which is produced by reducing iron and sulfur in the presence of oxygen poor water, and gather carbon from carbon dioxide. These microbes, termed "acetogens", excrete organic compounds that can then be utilized by other microbes. These environments are often referred to as "SLiMEs", which stands for subsurface lithoautotrophic microbial ecosystems. These ecosystems can exist indefinitely without any input from the surface.
Thermophillic metal reducers proliferate throughout the range of deep subsurface microbes.
[Sample reference] Takai, K., Sugai, A., Itoh, T., and Horikoshi, K. "Palaeococcus ferrophilus gen. nov., sp. nov., a barophilic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney". International Journal of Systematic and Evolutionary Microbiology. 2000. Volume 50. p. 489-500.
http://wvlc.uwaterloo.ca/biology447/modules/module6/Scientific_American_Article_Microbes.htm Fredrickson, J., Onstott, T. "Microbes Deep Inside the Earth." "Scientific American". 1996.
Amend, J. P., & Teske, A. (2005). Expanding frontiers in deep subsurface microbiology. Palaeogeography, Palaeoclimatology, Palaeoecology, 219(1-2), 131-155. Elsevier. Retrieved from http://linkinghub.elsevier.com/retrieve/pii/S0031018204005954
 http://www.episodes.co.in/www/backissues/303/202.pdf Dong, H., Yu, B., "Geomicrobiological processes in extreme environments: A review" Episodes: Journal of International Geosciences. 2007. Volume 30. p. 202-216.
Edited by <Craig Mack>, a student of Angela Kent at the University of Illinois at Urbana-Champaign.