"Extremophile" is a term that refers to bacteria that are able to grow and sustain in extremely harsh environments when compared to the environments that are termed as favourable for the growth of bacteria. Organisms have been discovered in the volcanic outlets, in the cold of Antarctic and Arctic regions, on the bottom of oceans, in deep sea hydrothermal vents, in very dry environments, inorganic environments such as acidic, alkaline and salt which are detrimental to most life forms, in lethal ionizing radiation environments and also in rocks extending far down inside the earth. It was not until the 1970's that these organisms were recognised. Thermophiles were the first extremophiles to be discovered. The domain archaea consists of extremophiles and many eukaryotes are also known to live in such harsh environments. There is also prosperity of organisms and unique living organisms, known as tube worms, growing around deep sea hydrothermal vents. These organisms are sustained without receiving energy from the sun. The discovery of extremophiles has increased speculation of chances for bacterial life on planets such as Mars, Europa (moon of Jupiter) and other stellar bodies.

The unparallel enzymes "extremoenzymes" used by the extremophiles to carry out their biochemical processes in harsh environments are useful in biotechnological processes. The property of capability of surviving under hard conditions by the enzymes, such as ability to function at very high pressure and temperature are major tools in biotechnolological research. Popular example is the so called taq polymerase enzyme isolated from the extremophile Thermus aquaticus is an essential part in PCR (polymerase chain reaction) technique that has brought radical change in biotechnology. And the most extraordinary microbe Deinococcus radiodurans is able to withstand high levels of lethal ionizing radiation.

Extremophile is a union of the suffix 'phile'-meaning 'lover of' and a prefix particular to their environments. Some categories of extremophiles include:

• Acidophiles - organisms living in acidic environments. They normally are found surrounding geothermal vents which are active and the pH of the same are below 5 which is acidic. They also occur in contaminated places where mining or industrial activity has left acidic wastes.
• Alkaliphiles - organisms living in typically alkali environments where the soil is abundant with carbonate having a pH of above nine. The enzymes that function at such high alkaline state are explored in laundry by detergent manufacturers which operate at alkaline pH.
• Halophiles - salt-loving organisms or bacteria such as Halobactreium salinarum grow in environments where the Na concentration is very high such as in Dead Sea or Great Salt Lake.
• Thermophiles - organisms living in high temperature of 80oC (177oF) such as in hot springs of Yellowstone National Park, Black Smokers etc.
• Psychrophiles - these organisms are very low temperature dwelling organisms. Example, Polaromonas vacuolata has maximum growth at 4oC (39.2oF) which just above the ice point of the water. Due to this unique ability to survive, these bacteria are being applied in enzymatic processes that operate near freezing temperature and also industrially being applied in the cold cycle washing machines.
• Endoliths - organisms living inside the rocks and carry out anaerobic form of respiration.
• Xerophile -organisms capable of growing at very low water activity such as in tough deserts.
• Barophiles (piezophile) - organisms living in high hydrostatic pressure such as in very depth of terrestrial surfaces and oceanic trenches.
• Methanogens - organisms that produce methane from the reaction between hydrogen and carbondioxide.
• Metallotolarent - organisms which are capable of withstanding high levels of dissolved toxic metals in solution such as cadmium, arsenic etc
• Oligotroph - organisms capable of growing in nutrition restrained conditions.
• Radioresistant - organisms capable of surviving high levels of toxic and fatal ionizing radiations.

These awesome organisms don't just survive in these harsh brutal environments they develop, propagate and do best in extreme environments than in any other place. They show characteristics in between bacteria and eukaryotes. The study of the extreme environments and the organisms that live in those environments has a tremendous merit for us. Exploring extreme environments are necessary to understanding biology, it is also essential in search for traces of life on other planets. The study of extremophiles helps us to know evolution that is the process of evolution and to study the primitive earth because early planet was an extreme habitat. Extremophile study is also important part of astrobiology and their enzymes find numerous commercial applications.

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