An extremophile () is an organism that is able to live or even thrive in extreme environments, such as extreme temperature (hot or cold), pressure, radiation, salinity, or pH level. Extremophiles are mainly species of microorganisms – bacteria and chiefly archaea, but also include some eukaryotes such as some fungi, and tardigrades.
Since the definition of an extreme environment is relative to an arbitrarily defined standard, often an anthropocentric one, these organisms can be considered ecologically dominant in the evolutionary history of the planet. Extremophiles have continued to thrive in the most extreme conditions, making them one of the most abundant lifeforms. The study of extremophiles has expanded human knowledge of the limits of life, and informs speculation about extraterrestrial life. Extremophiles are also of interest because of their potential for bioremediation of environments made hazardous to humans due to pollution or contamination.
Characteristics
In the 1980s and 1990s, biologists found that microbial life can survive in extreme environments—niches that are acidic, extraordinarily hot, or with irregular air pressure for example—that would be inhospitable to complex organisms. Some scientists even concluded that life may have begun on Earth in hydrothermal vents far beneath the ocean's surface.
According to astrophysicist Steinn Sigurdsson, "There are viable bacterial spores that have been found that are 40 million years old on Earth—and we know they're very hardened to radiation." Some bacteria were found living in the cold and dark in a lake buried a half-mile deep under the ice in Antarctica, and in the Mariana Trench, the deepest place in Earth's oceans. Expeditions of the International Ocean Discovery Program found microorganisms in sediment that is below seafloor in the Nankai Trough subduction zone. Some microorganisms have been found thriving inside rocks up to below the sea floor under of ocean off the coast of the northwestern United States. According to one of the researchers, "You can find microbes everywhere—they're extremely adaptable to conditions, and survive wherever they are." A key to extremophile adaptation is their amino acid composition, affecting their protein folding ability under particular conditions. Studying extreme environments on Earth can help researchers understand the limits of habitability on other worlds.
Tom Gheysens from Ghent University in Belgium and colleagues showed that endospores from a species of Bacillus bacteria were viable after being heated to temperatures of .
Classification Definitions
Acidophile: an organism with optimal growth at high acidity – pH levels of 3.0 or below. Alkaliphile: an organism with optimal growth at low acidity – pH levels of 9.0 or above. Capnophile: an organism with optimal growth conditions in high concentrations of carbon dioxide. An example would be Mannheimia, a genus of bacteria that inhabit the digestive systems of cattle and sheep. Halophile: an organism with optimal growth at a concentration of dissolved salts of 50 g/L (= 5% m/v) or above (for comparison, the salinity of a coral reef is about 35 g/L (= 3.5% m/v)). Hyperpiezophile: an organism with optimal growth at hydrostatic pressures above 50 MPa (= 493 atm = 7,252 psi). Hyperthermophile: an organism with optimal growth at temperatures above . Metallotolerant: an organism capable of tolerating high levels of dissolved heavy metals in solution, such as copper, cadmium, arsenic, and zinc. Examples include Ferroplasma sp., Cupriavidus metallidurans and GFAJ-1. Oligotroph: an organism with optimal growth in nutritionally limited environments. Osmophile: an organism with optimal growth in environments with a high sugar concentration. Piezophile or barophile: an organism with optimal growth in hydrostatic pressures above 10 MPa (= 99 atm = 1,450 psi). Polyextremophile (mixed Latin/Greek compound for affection for many extremes): an omnibus category for organisms which qualify as extremophiles under more than one category. Psychrophile or cryophile: an organism with optimal growth at temperatures of or lower. Radioresistant: organisms resistant to high levels of ionizing radiation, most commonly ultraviolet radiation. This category also includes organisms capable of resisting gamma rays. Sulphophile: an organism with optimal growth conditions in high concentrations of sulfur. An example would be Sulfurovum epsilonproteobacteria, a sulfur-oxidizing bacteria that inhabits the ocean floor. Thermophile: an organism with optimal growth at temperatures above . Xerophile: an organism with optimal growth at water activity below 0.8.
Examples of extremophile species: Factor Example environment / source Limits Organisms High temperature Hydrothermal vents to Pyrolobus fumarii, Pyrococcus furiosus, Methanopyrus kandlerii Low temperature Permafrost to Rhodotorula glutinis Alkaline waters Soda lakes pH ~11 Psychrobacter, Vibrio, Arthrobacter, Natronobacterium Acidic waters Volcanism and acid mine drainage pH 0.06 to 1.0 Picrophilus oshimae Ionizing radiation Cosmic rays and NORMs 1,500 to 6,000 Gy Deinococcus radiodurans, Rubrobacter, Thermococcus gammatolerans UV radiation The Sun 5,000 J/m2 High pressures Mariana Trench 1,100 bar Pyrococcus, Colwellia High salinity waters Soda lakes and Saline lakes aw ~ 0.6 Halobacteriaceae, Dunaliella salina, Vibrio Desiccation Atacama Desert (Chile), McMurdo Dry Valleys (Antarctica) ~60% relative humidity Chroococcidiopsis
Polyextremophiles There are many classes of extremophiles that range all around the globe; each corresponding to the way its environmental niche differs from mesophilic conditions. These classifications are not exclusive. Many extremophiles fall under multiple categories and are classified as polyextremophiles. For example, organisms living inside hot rocks deep under Earth's surface are thermophilic and piezophilic such as Thermococcus barophilus. A polyextremophile living at the summit of a mountain in the Atacama Desert might be a radioresistant xerophile, a psychrophile, and an oligotroph. Polyextremophiles are well known for their ability to tolerate both high and low pH levels. Note that "tolerant" or "resistant" organisms are not necessarily extremophiles: tolerant or resistant organisms may survive despite harsh conditions instead of thriving in harsh conditions. For example, the tardigrade (Tardigrada spp.), despite being highly resistant to many stresses, is not an extremophile properly speaking.
In astrobiology Astrobiology is the multidisciplinary field that investigates how life arises, distributes, and evolves in the universe. Astrobiology makes use of physics, chemistry, astronomy, solar physics, biology, molecular biology, ecology, planetary science, geography, and geology to investigate the possibility of life on other worlds and recognize biospheres that might be different from that on Earth. Astrobiologists are interested in extremophiles, as it allows them to map what is known about the limits of life on Earth to potential extraterrestrial environments For example, analogous deserts of Antarctica are exposed to harmful UV radiation, low temperature, high salt concentration and low mineral concentration. These conditions are similar to those on Mars. Therefore, finding viable microbes in the subsurface of Antarctica suggests that there may be microbes surviving in endolithic communities and living under the Martian surface. Research indicates it is unlikely that Martian microbes exist on the surface or at shallow depths, but may be found at subsurface depths of around 100 meters.
Recent research carried out on extremophiles in Japan involved a variety of bacteria including Escherichia coli and Paracoccus denitrificans being subject to conditions of extreme gravity. The bacteria were cultivated while being rotated in an ultracentrifuge at high speeds corresponding to 403,627 g (i.e. 403,627 times the gravity experienced on Earth). P. denitrificans was one of the bacteria which displayed not only survival but also robust cellular growth under these conditions of hyperacceleration which are usually found only in cosmic environments, such as on very massive stars or in the shock waves of supernovas. Analysis showed that the small size of prokaryotic cells is essential for successful growth under hypergravity. The research has implications on the feasibility of panspermia.
On 26 April 2012, scientists reported that lichen survived and showed remarkable results on the adaptation capacity of photosynthetic activity within the simulation time of 34 days under some conditions similar to those on Mars in the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center (DLR).
On 29 April 2013, scientists at Rensselaer Polytechnic Institute, funded by NASA, reported that, during spaceflight on the International Space Station, microbes seem to adapt to the space environment in ways "not observed on Earth" and in ways that "can lead to increases in growth and virulence".
On 19 May 2014, scientists announced that some microbes, like Tersicoccus phoenicis, may be resistant to methods usually used in spacecraft assembly clean rooms, giving rise to speculation that such microbes could have withstood space travel and are present on the Curiosity rover now on the planet Mars.
On 20 August 2014, scientists confirmed the existence of microorganisms living half a mile below the ice of Antarctica.
In September 2015, scientists from CNR-National Research Council of Italy reported that S. soflataricus survived under Martian radiation at a wavelength that was considered lethal to most bacteria. This discovery is significant because it indicates that not only bacterial spores, but also growing cells can resist strong UV radiation.
In June 2016, scientists from Brigham Young University reported that endospores of Bacillus subtilis were able to survive high speed impacts up to 299±28 m/s, extreme shock, and extreme deceleration. They pointed out that this feature might allow endospores to survive and to be transferred between planets by traveling within meteorites or by experiencing atmosphere disruption. Moreover, they suggested that the landing of spacecraft may also result in interplanetary spore transfer, given that spores can survive high-velocity impact while ejected from the spacecraft onto the planet surface. This is the first study which reported that bacteria can survive in such high-velocity impact. However, the lethal impact speed is unknown, and further experiments should be done by introducing higher-velocity impact to bacterial endospores.
In August 2020 scientists reported that bacteria that feed on air discovered 2017 in Antarctica are likely not limited to Antarctica after discovering the two genes previously linked to their "atmospheric chemosynthesis" in soil of two other similar cold desert sites, which provides further information on this carbon sink and further strengthens the extremophile evidence that supports the potential existence of microbial life on alien planets. Text and images are available under a Creative Commons Attribution 4.0 International License.
The same month, scientists reported that bacteria from Earth, particularly Deinococcus radiodurans, were found to survive for three years in outer space, based on studies on the International Space Station. These findings support the notion of panspermia. Text and images are available under a Creative Commons Attribution 4.0 International License.
Bioremediation