An extremophile () is an organism that is able to live (or in some cases thrive) in extreme environments, i.e., environments with conditions approaching or stretching the limits of what known life can adapt to, such as extreme temperature, pressure, radiation, salinity, or pH level.
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. Dating back to more than 40 million years ago, extremophiles have continued to thrive in the most extreme conditions, making them one of the most abundant lifeforms.
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 Endospore that have been found that are 40 million years old on Earth—and we know they're very hardened to radiation."
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
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Acidophile: an organism with optimal growth at pH levels of 3.0 or below.
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Alkaliphile: an organism with optimal growth at pH levels of 9.0 or above.
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Capnophile: an organism with optimal growth conditions in high concentrations of carbon dioxide. An example would be Mannheimia succiniciproducens, a bacterium that inhabits a ruminant animal's digestive system.
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Halophile: an organism with optimal growth at a concentration of dissolved salts of 50 g/L (= 5% m/v) or above (for comparison, the ocean salinity is about 35 g/L (= 3.5% m/v)).
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Piezophile: an organism with optimal growth at hydrostatic pressures above 50 MPa (= 493 atm = 7,252 psi).
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Oligotroph: an organism with optimal growth in nutritionally limited environments.
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Osmophile: an organism with optimal growth in environments with a high sugar concentration.
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Piezophile or barophile: an organism with optimal growth in hydrostatic pressures above 10 MPa (= 99 atm = 1,450 psi).
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Polyextremophile (mixed Latin/Greek compound for affection for many extremes): not a well-defined category itself – an organism that qualifies as an extremophile under more than one category.
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Psychrophile or cryophile: an organism with optimal growth at temperatures of or lower.
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Radioresistant: organisms resistant to high levels of ionizing radiation, most commonly ultraviolet radiation. This category also includes organisms capable of resisting nuclear radiation, notably .
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Sulphophile: an organism with optimal growth conditions in high concentrations of sulfur. An example would be Sulfurovum epsilonproteobacteria, a sulfur-oxidizing bacteria that inhabits deep-water sulfur vents.
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Thermophile: an organism with optimal growth at temperatures above .
Overview
|
|
| Pyrolobus fumarii, Pyrococcus furiosus, Methanopyrus kandlerii, Thermophile |
| Rhodotorula glutinis, Psychrophile |
| Psychrobacter, Vibrio, Arthrobacter, Natronobacterium, Methanonatronarchaeum thermophilum, Alkaliphile |
| Picrophilus oshimae, Thermoplasma acidophilum , Acidophile |
1,500 to 6,000 Gy | Deinococcus radiodurans, Rubrobacter, Thermococcus gammatolerans |
5,000 Joule/m2 |
| Pyrococcus sp., Piezophile |
| Halobacteriaceae, Dunaliella salina, Halophile |
| Chroococcidiopsis, Xerophile |
| Desulforudis audaxviator, Halicephalobus mephisto, Mylonchulus brachyurus, unidentified arthropods |
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
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
Endolith 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 . Analysis showed that the small size of 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 photosynthesis within the simulation 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 Cleanroom, 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 to strong UV radiation.
In June 2016, scientists from Brigham Young University reported that 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
Extremophiles can also be useful players in the
bioremediation of contaminated sites as some species are capable of biodegradation under conditions too extreme for classic bioremediation candidate species. Anthropogenic activity causes the release of pollutants that may potentially settle in extreme environments as is the case with tailings and sediment released from deep-sea mining activity.
While most bacteria would be crushed by the pressure in these environments, piezophiles can tolerate these depths and can metabolize pollutants of concern if they possess bioremediation potential.
Hydrocarbons
There are multiple potential destinations for hydrocarbons after an oil spill has settled and currents routinely deposit them in extreme environments. Methane bubbles resulting from the Deepwater Horizon oil spill were found 1.1 kilometers below water surface level and at concentrations as high as 183
μmol per kilogram.
The combination of low temperatures and high pressures in this environment result in low microbial activity. However, bacteria that are present including species of
Pseudomonas,
Aeromonas and
Vibrio were found to be capable of bioremediation, albeit at a tenth of the speed they would perform at sea level pressure.
Polycyclic aromatic hydrocarbons increase in solubility and bioavailability with increasing temperature. Thermophilic
Thermus and
Bacillus species have demonstrated higher gene expression for the alkane mono-oxygenase
alkB at temperatures exceeding . The expression of this gene is a crucial precursor to the bioremediation process. Fungi that have been genetically modified with cold-adapted enzymes to tolerate differing pH levels and temperatures have been shown to be effective at remediating hydrocarbon contamination in freezing conditions in the Antarctic.
Metals
Acidithiubacillus ferroxidans has been shown to be effective in remediating mercury in acidic soil due to its
merA gene making it mercury resistant.
Industrial effluent contain high levels of metals that can be detrimental to both human and ecosystem health.
In extreme heat environments the extremophile
Geobacillus thermodenitrificans has been shown to effectively manage the concentration of these metals within twelve hours of introduction.
Some acidophilic microorganisms are effective at metal remediation in acidic environments due to proteins found in their periplasm, not present in any mesophilic organisms, allowing them to protect themselves from high proton concentrations.
are highly oxidative environments that can produce high levels of lead or cadmium.
Deinococcus radiodurans are resistant to the harsh conditions of the environment and are therefore candidate species for limiting the extent of contamination of these metals.
Some bacteria are known to also use rare earth elements on their biological processes. For example, Methylacidiphilum fumariolicum, Methylorubrum extorquens, and Methylobacterium radiotolerans are known to be able to use lanthanides as cofactors to increase their methanol dehydrogenase activity.
Acid mine drainage
Acid mine drainage is a major environmental concern associated with many metal mines. This is due to the fact that this highly acidic water can mix with groundwater, streams, and lakes. The drainage turns the pH in these water sources from a more neutral pH to a pH lower than 4. This is close to the acidity levels of battery acid or stomach acid. Exposure to the polluted water can greatly affect the health of plants, humans, and animals. However, a productive method of remediation is to introduce the extremophile,
Thiobacillus ferrooxidans. This extremophile is useful for its bioleaching property. It helps to break down minerals in the waste water created by the mine. By breaking down the minerals
Thiobacillus ferrooxidans start to help neutralize the acidity of the waste water. This is a way to reduce the environmental impact and help remediate the damage caused by the acid mine drainage leaks.
Oil-based, hazardous pollutants in Arctic regions
Psychrophile microbes metabolize hydrocarbons which assists in the remediation of hazardous, oil-based pollutants in the Arctic and Antarctic regions. These specific microbes are used in this region due to their ability to perform their functions at extremely cold temperatures.
Radioactive materials
Any bacteria capable of inhabiting radioactive mediums can be classified as an extremophile. Radioresistant organisms are therefore critical in the bioremediation of radionuclides. Uranium is particularly challenging to contain when released into an environment and very harmful to both human and ecosystem health.
The NANOBINDERS project is equipping bacteria that can survive in uranium rich environments with gene sequences that enable proteins to bind to uranium in mining effluent, making it more convenient to collect and dispose of.
Some examples are
Shewanella putrefaciens,
Geobacter metallireducens and some strains of
Burkholderia fungorum.
Radiotrophic fungi, which use radiation as an energy source, have been found inside and around the Chernobyl Nuclear Power Plant.
Radioresistance has also been observed in certain species of macroscopic lifeforms. The lethal dose required to kill up to 50% of a tortoise population is 40,000 roentgens, compared to only 800 roentgens needed to kill 50% of a human population.
Examples and recent findings
New sub-types of extremophiles are identified frequently and the sub-category list for extremophiles is always growing. For example, microbial life lives in the liquid
Bitumen lake,
Pitch Lake. Research indicates that extremophiles inhabit the asphalt lake in populations ranging between 10
6 and 10
7 cells/gram.
[ Microbial Life Found in Hydrocarbon Lake. the physics arXiv blog 15 April 2010.] Likewise, until recently,
boron tolerance was unknown, but a strong borophile was discovered in bacteria. With the recent isolation of
Bacillus boroniphilus, borophiles came into discussion.
Studying these borophiles may help illuminate the mechanisms of both boron toxicity and boron deficiency.
In July 2019, a scientific study of Kidd Mine in Canada discovered sulfur-breathing organisms which live below the surface, and which breathe sulfur in order to survive. These organisms are also remarkable due to eating rocks such as pyrite as their regular food source.[ World’s Oldest Groundwater Supports Life Through Water-Rock Chemistry , 29 July 2019, deepcarbon.net.][ Strange life-forms found deep in a mine point to vast 'underground Galapagos', By Corey S. Powell, 7 Sept. 2019, nbcnews.com.]
Biotechnology
The thermoalkaliphilic
catalase, which initiates the breakdown of hydrogen peroxide into oxygen and water, was isolated from an organism,
Thermus brockianus, found in Yellowstone National Park by Idaho National Laboratory researchers. The catalase operates over a temperature range from 30 °C to over 94 °C and a pH range from 6–10. This catalase is extremely stable compared to other catalases at high temperatures and pH. In a comparative study, the
T. brockianus catalase exhibited a half life of 15 days at 80 °C and pH 10 while a catalase derived from
Aspergillus niger had a half life of 15 seconds under the same conditions. The catalase will have applications for removal of hydrogen peroxide in industrial processes such as pulp and paper bleaching, textile bleaching, food pasteurization, and surface decontamination of food packaging.
DNA modifying enzymes such as Taq polymerase DNA polymerase and some Bacillus enzymes used in clinical diagnostics and starch liquefaction are produced commercially by several biotechnology companies.
DNA transfer
Over 65 prokaryotic species are known to be naturally competent for genetic transformation, the ability to transfer DNA from one cell to another cell followed by integration of the donor DNA into the recipient cell's chromosome.
Several extremophiles are able to carry out species-specific DNA transfer, as described below. However, it is not yet clear how common such a capability is among extremophiles.
The bacterium Deinococcus radiodurans is one of the most radioresistant organisms known. This bacterium can also survive cold, dehydration, vacuum and acid and is thus known as a polyextremophile. D. radiodurans is competent to perform genetic transformation.
Halobacterium volcanii, an extreme halophilic (salinity tolerant) archaeon, is capable of natural genetic transformation. Cytoplasmic bridges are formed between cells that appear to be used for DNA transfer from one cell to another in either direction.
Sulfolobus solfataricus and Sulfolobus acidocaldarius are hyperthermophilic archaea. Exposure of these organisms to the DNA damaging agents UV irradiation, bleomycin or mitomycin C induces species-specific cellular aggregation.
Extracellular membrane vesicles (MVs) might be involved in DNA transfer between different hyperthermophilic archaeal species.
See also
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Earliest known life forms
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Dissimilatory metal-reducing microorganisms
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Extremotroph
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List of microorganisms tested in outer space
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Mesophile, an organism that grows best in moderate temperatures
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Neutrophile, an organism that grows best in a neutral pH level
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RISE project
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Tardigrade
Further reading
External links
