Extremophiles are microorganisms that thrive in environmental conditions hostile to most life, including extremes of temperature (both high and low), acidity or alkalinity, salt concentration, and pressure. Most extremophile microorganisms are prokaryotes belonging to the domains Bacteria and Archaea that have adapted to survive in ecological niches that would be deadly to most other life forms. One well-known extremophile is Thermus aquaticus, which grows in geothermal springs at 70°C (158°F) and uses the process called chemosynthesis to convert the springs' heat and mineral compounds into metabolic energy. Another interesting extremophile is Desulforudis audaxviator, which lives 3.2 kilometers (2 miles) beneath the Earth's surface and utilizes radioactive decay from uranium to generate energy. The unique metabolic and enzymatic strategies that these microorganisms employ to withstand extreme conditions have made them objects of intense interest to basic research and innovative biotechnology. Nevertheless, because the harsh conditions under which extremophiles live make their discovery difficult, it is estimated that only 1% of them have been isolated from nature. See also: Archaea; Bacteria; Bacterial physiology and metabolism; Bacteriology; Biochemistry; Ecology; Ecosystem; Microbial ecology; Microbiology; Population ecology
In general, the extremophiles fall into three categories:
(1) Thermophiles and psychrophiles: Thermophiles are heat-tolerant and grow best at sites of geothermal activity, including volcanic fields, hydrothermal vents, and hot springs, where temperatures range from 70 to 100°C (158 to 230°F). Because these environments are also routinely poor in oxygen, thermophiles have evolved anaerobic mechanisms of metabolism based on sulfur instead of oxygen. In contrast, psychrophiles are cold-loving microorganisms that flourish in polar regions, especially in the waters of the Antarctic Ocean and deep-sea sediments, which maintain temperatures of –2 to 4°C (28 to 39°F). See also: Geologic thermometry; Hydrothermal vent; Spring (hydrology); Sulfur; Thermal ecology; Volcanology
(2) Alkaliphiles and halophiles: Alkaliphiles and halophiles live, respectively, under conditions of high alkalinity (pH values greater than 9) and high salinity (concentrations greater than 20% salt). Alkaliphiles are often found in alkaline groundwater, soda lakes, and deserts, whereas halophiles colonize high-salt environments, including hypersaline lakes (for example, the Dead Sea), evaporation ponds, and saline soils. See also: Halophilism (microbiology); pH; Salt (chemistry)
(3) Acidophiles and barophiles: Acidophiles are adapted for life in the acidic conditions (pH values less than 4) that prevail in volcanic areas, sulfuric pools, and geysers. The extremophiles that thrive under high pressure (more than 300 bars or 290 atmospheres) in the deep sea are known as barophiles. Often found on ocean floors, they have been collected at depths of 3000–10,000 meters (9840–32,800 feet). See also: Geyser; Pressure; Seawater fertility
Extremophiles and the molecules that they produce have found applications in various industries. Because the enzymes extracted from extremophiles display high stability under harsh conditions, they can be beneficial to biotechnologists and industrial chemists who need robust reagents. Enzymes of extremophiles have become attractive sources of novel catalysts for use in pharmaceuticals, food engineering, biodegradation and decontamination of toxic substances, detergents, and pulp and paper manufacturing. A DNA polymerase derived from Thermus aquaticus, for example, is crucial to the polymerase chain reaction (PCR) technique that molecular biologists use to copy fragments of genetic information. See also: Biodegradation; Biotechnology; Catalysis; Detergent; Enzyme; Food engineering; Paper; Polymerase chain reaction (PCR)
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