An extremophile is an organism, usually unicellular, which thrives in or requires "extreme" conditions. The definition of "extreme" is anthropocentric; to the organism itself its environment is completely normal. Thus, strictly, "extremophilic" labels should be used to describe the environment that an organism thrives in, regardless of how "normal" or "extreme" they may seem to human beings. For example, human beings are classified as a mesophilic aerobe.

When used in the context of describing organisms that thrive in environments that are extreme from human perspectives, many extremophiles are members of the Archaea family, and indeed the terms are occasionally used interchangeably to describe the many extremophilic bacteria and eukarya. Not all extremophiles are unicellular. Examples of extremophilic metazoa are the psychrophilic Grylloblattodea (insects) and antarctic krill (crustaceans).

Types of extremophiles

There are many different classes of extremophiles, each corresponding to the way its chosen environment differs from what is considered "normal" by other organisms. These classifications are not exclusive. Many extremophiles fall under multiple categories. For example, organisms living inside hot rocks deep under Earth's surface are both thermophilic and barophilic.

• Acidophile: An organism with an optimum pH level at or below pH 3.
• Aerobe: requires O₂ to survive.
• Alkaliphile: An organism with optimal growth at pH levels of 9 or above.
• Anaerobic: does not need O_{2 to survive.}
• _{Endolith: An organism that lives inside rocks.}
• _{Halophile: An organism requiring at least 0.2M of NaCl for growth.}
• _{Hypolith: An organism that lives inside rocks in cold deserts.}
• _{Mesophile: An organism that thrives in temperatures between 15-60 °C.}
• _{Metalotolerant : capable of tolerating high levels of heavy metals, such as copper, cadmium, arsenic, and zinc.}
• _{Microaerophilic: requires levels of O2 that are lower than atmospheric levels.}
• _{Oligotroph : An organism capable of growth in nutritionally limited environments.}
• _{Piezophile : An organism that lives optimally at high hydrostatic pressure. See also Barophile}
• _{Psychrophile: An organism that can thrive at temperatures of 15 °C or lower.}
• _{Radioresistant: resistant to high levels of ionizing radiation.}
• _{Thermophile: An organism that can thrive at temperatures between 60-80 °C.}
• _{Xerotolerant: requires very little water to survive.}

_{External links}

• _{http://www.nhm.ac.uk/zoology/extreme.html}
• _{Extremophiles 2002}
• _{DaveDarling's Encyclopedia of Astrobiology, Astronomy, and Spaceflight}