An extremophile is an organism, usually unicellular but not always so, which thrives in or requires extreme conditions. The definition of extreme is anthropocentric, and this is because to the organism itself, its environment, not matter how extreme it may seem to be, is completely normal to itself. 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, most extremophiles are members of the Archaea family, although the terms are occasionally used interchangeably to describe the many extremophilic bacteria and eukarya. The Archaea are a major group of prokaryotes, which are unicellular (in rare cases, multicellular) organisms without a nucleus. This is in contrast to eukaryotes, organisms that have cell nuclei and may be variously unicellular or multicellular. Most prokaryotes are bacteria, and the two terms are often treated as synonyms. However, Carl Woese, originator of the RNA world hypothesis in 1967, has proposed dividing prokaryotes into the Bacteria and Archaea in 1977 because of the significant genetic differences between the two.
It is also important to note that not all extremophiles are unicellular. Examples of extremophilic metazoa are the psychrophilic Grylloblattodea (insects) and antarctic krill (crustaceans).
Different 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.
1. Acidophile: An organism which thrives under an environment with an optimum pH level at or below pH 3.
2. Aerobe: An organism which requires O2 to survive. There are 2 sub-categories of aerobes and they are the obligate aerobes and the facultative aerobes. Obligate aerobes require oxygen, while facultative aerobes can use oxygen, but also have other options. Almost all animals, most fungi and several bacteria are obligate aerobes. Being an obligate aerobe, although being advantageous from the energetical point of view, means also obligatory facing high levels of oxidative stress. Yeast, on the other hand, is an example of a facultative aerobe. Individual human cells are also facultative aerobes, as in they can switch to lactic acid fermentation if oxygen is not available. However, for the whole organism this cannot be sustained for long, and humans are therefore obligate aerobes.
3. Alkaliphile: An organism which thrives under an environment with an optimal pH levels of 9 or above, such as soda lakes and carbonate-rich soils.
4. Anaerobe: An organism which does not need O2 to survive. There are several sub-categories of anaerobes in existence. Aerotolerant organisms do not require oxygen, but are not affected by exposure to air. Microaerophiles are organisms that may use oxygen, but only at low concentrations (low micromolar range) and their growth is inhibited by normal oxygen concentrations (approximately 200 micromolar). Nanaerobes are organisms that cannot grow in the presence of micromolar concentrations of oxygen, but can grow with and benefit from nanomolar concentrations of oxygen. Certain anaerobic bacteria produce toxins, such as the tetanus or botulinum toxins, that are highly dangerous to higher organisms, including humans.
5. Endolith: An organism that lives inside rocks down to a depth of up to about 3 km, though it is unknown if that is their limit, or in the pores between mineral grains. Judging from the hyperthermophiles as discussed below, the temperature limit is at about 110°C, which limits the possible depth to 4 km below the continental crust, and 7 km below the ocean floor. Endolithic organisms have also been found in regions of low humidity.
6. Halophile: An organism which thrives in environments with very high concentrations of salt (NaCl) and requires at least 0.2 molar of salt for growth. Of particular note are the extreme halophiles or halobacteria, which require at least 2 molar of salt and are usually found in saturated solutions. These are the primary inhabitants of salt lakes and inland seas, such as the Dead Sea, where they tint the sediments bright colors.
7. Hypolith: An photosynthetic organism that lives inside or underneath rocks in climatically extreme in cold deserts. The rocks are generally translucent to allow for the penetration of light, such as quartz, which is one of the most common translucent rocks.
8. Mesophile: An organism that grows best in moderate temperature, neither too hot nor too cold, typically between 20 and 45 °C with an optimal temperature near 37 °C, which is the normal temperature of the human body. Most organisms that are pathogenic to humans are mesophiles. Organisms that prefer cold environments are termed psychrophilic and those preferring hot temperatures are termed thermophilic. A psychrophile is an organism which thrives at relatively cold temperatures. There are generally considered to be two groups of psychrophiles and they are name the classic psychrophiles and the psychrotrophs by food microbiologists. Classic psychrophiles are those organisms having a optimum growth temperature of 15°C or lower and do not grow in a climate beyond a maximum temperature of 20°C. They are largely found in icy places (such as in Antarctica) or at the freezing bottom of the ocean floor. Psychrotrophs, on the other hand, can grow at 0°C and up through approximately 40°C, and exist in much larger numbers than classic psychrophiles. They are of particular significance to food microbiologists as they can grow in refrigerated environments and cause food spoilage. A thermophile is an organism which thrives at relatively high temperatures, up to about 60 °C. Thermophiles have been found in various geothermally heated regions of the Earth such as hot springs like those in Yellowstone National Park and deep sea hydrothermal vents, and they are primarily responsible for producing the bright colors of the said waters. A hyperthermophile is an organism that thrives in extremely hot environments which are above 60°C. The optimal temperatures are between 80°C and 110°C. In fact, the recently-discovered Strain 121 has been able to double its population within 24 hours in an autoclave at 121°C, hence it was named as such. Many hyperthermophiles are also able to withstand other environmental extremes, such as high acidity or radiation levels. Hyperthermophiles were first discovered in the 1960s in hot springs in Yellowstone National Park, Wyoming. The most hardy hyperthermophiles are known live on the superheated walls of deep-sea hydrothermal vents, requiring temperatures of at least 90°C for survival.
9. Metalotolerant organism: An organism which is capable of tolerating high levels of heavy metals, such as copper, cadmium, arsenic, and zinc.
10. Oligotroph: An organism which is capable of growth in nutritionally limited environments.
11. Piezophile (also known as Barophile): An organism that lives optimally at high hydrostatic pressure, such as in high-pressure deep-sea environments, where pressure is well above atmospheric pressure. A pressure of 1 atmosphere is aproximately equivalent to 0.1 MPa. For every every km below ocean, the pressure increases approximately 10 MPa. For every km below earth’s crust, the pressure increases about 30 MPa. One strain of barophilic bacteria, Hirondellea gigas, was isolated from a sample of the world's deepest sediment, collected from the Mariana Trench, Challenger Deep, at a depth of 10,898 m. The Mariana Trench, Challenger Deep (11°22'N, 142°25'E) is the deepest ocean bottom in the world. Apparently, the Hirondellea gigas could grow only under pressure conditions of greater than 518 bars, that is approximately 50 MPa or more.
12. Radioresistant organism: An organism which is are capable of resisting very high levels of ionizing radiation, such as nuclear power plants. Please kindly refer to previous posting "Deinococcus radiodurans - The most radioresistant organism known to mankind".
13. Xerotolerant organism: An organism which can survive in environments where there is very little water. Water activity is a measure of the amount of water within a substrate that an organism can use to support growth. Xerotolerant organisms can survive in environments with water activity below 0.8. Endoliths and halophiles are xerotolerant.
N.B. Anthropocentrism is the practice, conscious or otherwise, of regarding the existence or concerns of human beings as the central fact of the universe.