An extremophile is an organism, usually unicellular, which thrives in or requires 'extreme' conditions that would exceed optimal conditions for growth and reproduction in the majority of mesophilic terrestrial organisms.

Most extremophiles are microbes. The domain Archaea is known for widespread extremophily, but extremophiles are present in numerous and diverse genetic lineages of both the bacteria and archaea. Occasionally the terms archaea and extremophile are used interchangably, but this usage is incorrect; there are many mesophile archaeans and many extremophile bacteria. Additionally, not all extremophiles are unicellular. Examples of extremophilic metazoa are the Pompeii worm, the psychrophilic Grylloblattodea (insects), antarctic krill (crustaceans) and the Tardigrade.

Types of extremophiles

There are many different classes of extremophiles, each corresponding to the way its environmental niche differs from those of the majority of terrestrial mesophile 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.
• Alkaliphile: An organism with optimal growth at pH levels of 9 or above.
• Barophile: Bacteria which live in environments characterized by high gas or liquid pressure; synonymous with piezophile.
• Endolith: An organism that lives in microscopic spaces within rocks, such as pores between aggregate grains. These may also be called cryptoendoliths. This term also includes organisms populating fissures, aquifers, and faults filled with groundwater in the deep subsurface.
• Halophile: An organism requiring at least 2M of salt, NaCl, for growth.
• Hyperthermophile: An organism that can thrive at temperatures between 80-121 °C, such as those found in hydrothermal systems.
• Hypolith: An organism that lives inside rocks in cold deserts.
• Lithoautotroph: An organism (usually bacteria) whose sole source of carbon is carbon dioxide and exergonic inorganic oxidation (chemolithotrophs) such as Nitrosomonas europea These organisms are capable of deriving energy from reduced mineral compounds like pyrites, and are active in geochemical cycling and the weathering of parent bedrock to form soil.
• Metalotolerant: capable of tolerating high levels of dissolved heavy metals in solution, 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 Common in the deep terrestrial subsurface, as well as in oceanic trenches.
• Polyextremophile: An organism that can survive different extreme conditions.
• Psychrophile/Cryophile: An organism that grows better at temperatures of 15 °C or lower. Common in cold soils, permafrost, polar ice, cold ocean water, and in/under alpine snowpack.
• Radioresistant: resistant to high levels of ionizing radiation, most commonly ultraviolet radiation but also includes organisms capable of resisting nuclear radiation.
• Thermophile: An organism that can thrive at temperatures between 60-80 °C.
• Xerophile: An organism that can grow in extremely dry, dessicating conditions. This type is exemplified by the soil microbes of the Atacama Desert.

Extremophiles and Astrobiology

Astrobiology is the field concerned with forming theories about the distribution, nature, and future of life in the universe. In it, microbial ecologists, astronomers, planetary scientists, geochemists, philosophers, and explorers cooperate to constructively guide the search for life on other planets. Astrobiologists are particularly interested in studying extremophiles, as many organisms of this type are capable of surviving in environments similar to those known to exist on other planets. For example, Mars may have regions in its deep subsurface permafrost that could harbor endolith communities. The subsurface water ocean of Jupiter's moon Europa may harbor life, especially at hypothesized hydrothermal vents at the ocean floor. Extraterrestrial life is expected to be largely microbial, and as conditions as forgiving as those found on Earth are rare in known planetary systems, extremophiles and their biology are thought to represent appropriate analogues to possible extraterrestrial life.

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 - such as spores - which can survive in harsh environments.

External links

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

MicrobiologyExtremophiles

Extremophile | Extremófilo | Extrémophile | Extremofiel | 極限環境微生物 | Ekstremofiili | Extremofil | 嗜極生物