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.
Terms used to describe extremophiles include the following:
- Alkaliphile: An organism with optimal growth at pH values above 10.
- Barophile: An organism that lives optimally at high hydrostatic pressure.
- Endolith: An organism that lives inside rocks.
- Extreme Acidophile: An organism with a pH optimum for growth at, or below, pH 3.
- Halophile: An organism requiring at least 0.2M salt for growth.
- Methanogen: An organism which produces methane by reacting hydrogen and carbon dioxide.
- Thermophile or Hyperthermophile: An organism having a growth temperature optimum of 80 °C or higher.
- Oligotroph: An organism with optimal growth in nutrient limited conditions.
- Psychrophile: An organism having a growth temperature optimum of 15 °C or lower, and a maximum temperature of 20 °C.
- Toxitolerant: An organism able to withstand high levels of damaging agents. For example, living in water saturated with benzene, or in the water-core of a nuclear reactor (see Deinococcus radiodurans).
- Xerotolerant: An organism capable of growth at low water activity. For example, extreme halophile or endolith.
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.