Extremophile

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An extremophile is an organism, usually unicellular, which thrives in or requires "extreme" conditions. It is important to note that the definition of "extreme" in this context is anthropocentric; from the point of view of the organism, its environment is completely normal.

Because many extremophiles are Archaea and most known archaea are extremophilic, on occasion, the terms are used interchangeably. However, this is not strictly correct and many bacteria and eukarya are extremophilic. Additionally, not all extremophiles are unicellular. Examples of extremophilic metazoa are the Pompeii worm, the psychrophilic Grylloblattodea (insects), antarctic krill (crustaceans) and the Tardigrade.

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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.

Image:Pompeii worm colony.jpg

Acidophile: An organism with an optimum pH level at or below pH 3.
Alkaliphile: An organism with optimal growth at pH levels of 9 or above.
Autotroph: An organism (usually bacteria) who sole source of carbon is carbon dioxide.
Endolith: An organism that lives inside rocks.
Halophile: An organism requiring at least 2M of NaCl for growth.
Hyperthermophile: An organism that can thrive at temperatures between 80-110 °C.
Hypolith: An organism that lives inside rocks in cold deserts.
Lithoautotroph: An organism (usually bacteria) who sole source of carbon is carbon dioxide and exergonic inorganic oxidation (chemolithotrophs) such as Nitrosomonas europea
Metalotolerant: capable of tolerating high levels of heavy metals, such as copper, cadmium, arsenic, and zinc.
Oligotroph: An organism capable of growth in nutritionally limited environments.
Piezophile: An organism that lives optimally at high hydrostatic pressure. See also Barophile
Polyextremophile: An organism that can survive different extreme conditions.
Psychrophile/Cryophile: 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.
Xerophile: An organism that can grow in environments with a low water activity.

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Bacteria on the Moon

Apollo 12 astronauts retrieved parts from the lunar probe Surveyor 3 for analysis. A common bacteria, Streptococcus mitis, was unintentionally present inside the spacecraft's camera at launch. Around 50 to 100 of these bacteria survived dormant in this harsh environment for three years, to be detected when Apollo 12 brought the camera back to Earth. Many bacteria have dormant forms which can survive in harsh environments, and merely being dormant is not sufficient to make an organism be considered an extremophile.

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