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An extremophile is an organism which thrives under "extreme" conditions; the term frequently refers to unicellular organisms. Extremophiles often require the extreme condition for growth. The definition of "extreme" is anthropocentric, of course, to the organism itself its environment is completely normal. Many extremophiles are members of the Archaea family, and indeed the term is occasionally used as a synonym for archaea even though there are many which live in non-extreme environments.

Most extremophiles are microorganisms. For example, the presently known upper temperature limit is 113 °C for archaea, 95 °C for bacteria, and 62 °C for single-celled eukaryotes, in contrast to metazoans (multi-cellular eukaryotes) which are unable to grow above 50 °C. Note that many species can survive extreme conditions in a dormant state, but are not capable of growing or reproducing indefinitely under those conditions. These are not necessarily considered extremophiles.

Terms used to describe extremophiles include the following:

Some extremeophiles may fall under multiple categories, for example living inside hot rocks deep under Earth's surface. Organisms which are not extremophiles are called mesophiles[?].

The enzymes isolated from some extremeophiles have proven to be of great use in the biotechnology industry, able to function under conditions that would denature enzymes taken from most "normal" organisms. For example, a commonly used DNA polymerase for polymerase chain reaction is Taq polymerase, originally isolated from Thermus aquaticus[?] found in deep ocean hydrothermal vents[?].

An order of extremophile Metazoa is the psychrophilic Grylloblattodea insects.

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