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An extremophile is an organism that thrives in extreme conditions that are harmful to most life on Earth. In contrast, organisms from moderate temperature or neutral pH environments may be termed mesophiles or neutrophiles.
In the 1980s and 1990s, biologists found that microbial life has a capacity for surviving in extreme environments. These are niches that are extreme in some way. They may be extraordinarily hot, or cold, or dry, or under immense pressures, or very salty or acidic. Such conditions are inhospitable to complex organisms. Some scientists suggest that life may have begun on Earth in hydrothermal vents far below the ocean surface. High temperature habitats such as hot oceans, hot springs and deep ocean thermal vents would have occurred widely in the early Archaean eon (from 3.9 billion years ago). Adaptation to these conditions may have been crucial to the development of early forms of life.
Types of extremophiles
Most known extremophiles are microbes. The domain Archaea contains well-known examples, but extremophiles occur in bacteria as well. It is a mistake to use the term extremophile for all archaeans, as some are mesophilic. Nor are all extremophiles unicellular; protostome animals found in similar environments.
Some extremophiles fall under several categories. For example, organisms living inside hot rocks deep under Earth's surface are both thermophilic and barophilic.
- An organism with optimal growth at pH levels of 3 or below
- An organism with optimal growth at pH levels of 9 or above
- An organism that lives in microscopic spaces within rocks, fissures, aquifers, and faults filled with groundwater in the deep subsurface
- An organism requiring at least 0.2molar concentrations of salt (NaCl) for growth
- An organism that can thrive at temperatures between 80–122 °C, such as those found in hydrothermal systems
- An organism that lives inside rocks in cold deserts
- An organism (usually bacteria) whose sole source of carbon is carbon dioxide and exergonic inorganic oxidation (chemolithotrophs) such as Nitrosomonas europaea; these organisms are capable of deriving energy from reduced mineral compounds like pyrites, and are active in geochemical cycling and the weathering of parent bedrock to form soil
- capable of tolerating high levels of dissolved heavy metals in solution, such as copper, cadmium, arsenic, and zinc; examples include Ferroplasma sp. and Ralstonia metallidurans
- An organism capable of growth in nutritionally limited environments
- An organism capable of growth in environments with a high sugar concentration
- An organism that lives optimally at high hydrostatic pressure; common in the deep terrestrial subsurface, as well as in oceanic trenches
- An organism that qualifies as an extremophile under more than one category
- An organism that grows better at temperatures of 15 °C or lower; common in cold soils, permafrost, polar ice, cold ocean water, and in or under alpine snowpack
- Organisms resistant to high levels of ionizing radiation, most commonly ultraviolet radiation, but also including organisms capable of resisting nuclear radiation
- An organism that can thrive at temperatures between 60–80 °C
- Combination of thermophile and acidophile that prefer temperatures of 70–80 °C and pH between 2 and 3
- An organism that can grow in extremely dry, desiccating conditions; this type is exemplified by the soil microbes of the Atacama Desert
- "Mars Exploration - Press kit" (PDF). NASA. June 2003. http://marsrovers.jpl.nasa.gov/newsroom/merlaunch.pdf. Retrieved 2009-07-14.
- Gouy M. & Chaussidon M. 2008. Ancient bacteria liked it hot. Nature 451: p635.
- Cavicchioli R. & Thomas T. 2000. Extremophiles. In: J. Lederberg (ed) Encyclopedia of Microbiology, 2nd ed, Vol 2, pp317–337. Academic Press, San Diego.