Extremophile
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.
Importance of enzymes from thermophiles
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.
The most commonly used DNA polymerase for the polymerase chain reaction technique is Taq DNA polymerase, originally isolated from Thermus aquaticus, a bacterial species found in surface aquatic locations such as Yellowstone National Park hot springs. For a few PCR applications, the lack of proofreading by Taq DNA polymerase is a problem.
The DNA polymerase from Thermococcus litoralis was shown to have a proofreading exonuclease activity. Fidelity of DNA synthesis by the Thermococcus litoralis DNA polymerase--an extremely heat stable enzyme with proofreading activity (Mattila P, Korpela J, Tenkanen T, Pitkanen K. Nucleic Acids Res. 1991 Sep 25;19(18):4967-73.) Thermococcus litoralis was isolated from a deep sea hydrothermal vent. This DNA polymerase is marketed as "Vent" polymerase.
Taq DNA polyerase is adequate for most PCR, but one study ("DNA polymerases as engines for biotechnology" by Hamilton SC, Farchaus JW, Davis MC. in "Biotechniques" 2001 31:370-6, 378-80, 382-3). reported that higher fidelity thermostable DNA polymerases such as Vent account for as much as 30% of DNA polymerase sales
An order of extremophile Metazoa is the psychrophilic Grylloblattodea insects.