Extremophiles – Definition, Classification, Examples

Extremophiles Definition

• Extremophiles are organisms that have evolved to thrive in settings that were once believed to be completely inhospitable.
• These habitats are unfriendly, reaching extremes of severe heat, acidity, pressure, and cold that are lethal to most other forms of life.
• Due to the fact that extremophiles inhabit the extreme ends of the spectrum, they can show the range of livable environments.
• It is essential to stress, however, that extremophiles are only “extreme” from an anthropocentric standpoint. For instance, while oxygen is essential to our survival and the majority of life on Earth, many creatures thrive in settings devoid of oxygen.
• Extremophiles can be categorised into two major groups: extremophilic and extremotolerant species.
• As suggested by the suffix “philic,” which translates to “loving,” extremophilic organisms require one or more extreme conditions to thrive, whereas extremotolerant organisms grow ideally in more “normal” conditions but are able to endure one or more severe physiochemical values.
• The majority of extremophiles are microorganisms belonging to the archaea domain of life.
• However, it would be inaccurate to suggest that extremophiles are restricted to this realm. Some extremophiles are classified as bacteria, whereas others are multicellular eukaryotes.

Importance in Research

• Extremozymes, the enzymes secreted by extremophiles that allow organisms to survive in hostile settings, are of tremendous interest to medical and biotechnological researchers.
• Perhaps they will be the key to developing genetically-based drugs and technology that can operate in severe environments.
• Extremophiles are of interest to astrobiologists due to their extraordinary resistance to freezing settings.
• Extremophiles or “psychrophiles” that thrive in such circumstances increase the likelihood of life on other planets, considering the majority of celestial bodies remain frozen.
• In addition, the metabolic features of such psychrophiles, such as the ability to utilise arsenic instead of phosphorus to produce energy, increase the likelihood of extraterrestrial life.
• And because extremophiles can reveal the range of conditions under which life is viable, they can also provide information about where and how to search for life on other solar bodies.

Classifications of Extremophiles

There are numerous types of extremophiles found all over the world, and each corresponds to the manner in which its environmental niche differs from mesophilic circumstances. These categories are not exhaustive. Many extremophiles are characterised as polyextremophiles because they belong to many groups. Thermococcus barophilus, for instance, is a thermophilic and piezophilic creature that lives in heated rocks deep beneath the Earth’s surface. At the summit of a mountain in the Atacama Desert, a polyextremophile may be a radioresistant xerophile, a psychrophile, and an oligotroph. It is generally known that polyextremophiles can withstand both high and low pH values. Similar to extremophiles and mesophiles, but with nanobe-sized dimensions, the labels extremonobes and mesonobes have also been proposed.

Classifications based on Solute Concentration and Water Activity

1. Halophile: To thrive, halophiles require high quantities of sodium chloride, often exceeding 0.2 M. Examples: Growing examples are Halobacterium, Dunaliella, and Ectothiorhodspira.
2. Osmotolerant: Capable of growing in a wide range of osmotic concentration or water activity. Example: Staphylococcus aureus, Zygosaccharomyces rouxii.
3. Xerophile: Organisms that thrive optimally at low water activity, often 0.85 or less. Examples: Xeromyces bisporus.

Classifications based on pH Tolerance

1. Acidophile: Growth is optimal for acidophiles between pH 0 and 5.5. This group consists of eukaryotes, bacteria, and archaea that can be found in sulfuric pools, polluted mine drainage sites, and even our stomachs. Acidophiles manage their pH levels by a number of specialised systems, some of which are passive (requiring little energy) and others of which are active (exerting energy). Passive techniques often entail reinforcing the cell barrier against the external environment. This may involve secreting a biofilm to prevent the diffusion of chemicals into the cell, or altering the cell membrane to include protective compounds like fatty acids. Some acidophiles can release chemicals that serve as buffers to increase their internal pH levels. Active pH regulation strategies comprise a hydrogen ion pump that continuously expels hydrogen ions from the cell.Example: Sulfolobus, Picrophilus, Ferroplasma, and Acontium are examples.
2. Alkaliphilic: optimal growth between pH 8.0 and 11.5 They utilise both passive and active processes to maintain homeostasis. Among passive methods is the accumulation of cytoplasmic polyamines within the cell. In alkaline settings, the polyamines’ abundance of positively charged amino groups acts as a buffer for the cytoplasm. Low membrane permeability, which impedes the passage of protons into and out of the cell, is another passive method. Active regulation utilises a sodium ion channel that transports protons into the cell. Example: Bacillus alcalophilus, Natronobacterium.
3. Neutrophile: Optimal growth occurs between pH ranges of 5.5-8. Example: Escherichia, Euglena, Paramecium.

Classifications based on Temperature

1. Psychrophile: A plant or animal that grows at 0 °C and has an optimal growth temperature of 15 °C or lower. This group is found in cold soils, permafrost, polar ice, cold ocean water, and alpine snow packs, and is comprised of bacteria, archaea, and eukarya. Extremozymes, which continue to operate at low temperatures and slightly more slowly at lower temperatures, contribute to their ability to survive in severe cold. Psychrophiles are also capable of producing proteins that are functional at low temperatures, and their plasma membranes contain high amounts of unsaturated fatty acids that protect the cells from the cold. Notably, some psychrophiles may replace the water in their bodies with the sugar trehalose, so preventing the production of dangerous ice crystals. Examples: Bacillus psychrophilus and Chlamydomonas nivalis are two examples.
2. Psychrotolerant: Able to grow between 0 and 7 degrees Celsius, with an optimal temperature range between 20 and 30 degrees Celsius and a maximum temperature around 35 degrees Celsius. Examples: Listeria monocytogenes, Pseudomonas fluorescens.
3. Mesophile: Having optimal growth between 20 and 45°C. Example: Escherichia coli, Trichomonas vaginalis.
4. Thermophile: Able to grow at 55°C or higher; optimal temperature is often between 55 and 65°C.
5. Hyperthermophile: Prefers temperatures between 85 and 113 degrees Celsius. Example: Sulfolobus, Pyrococcus, Pyrodictium.

Classifications based on Oxygen Concentration

1. Obligate aerobe: A microorganism wholly reliant on atmospheric O for growth. Micrococcus lutes, as well as the majority of protists and fungi.
2. Facultative anaerobe: Does not require oxygen for growth, but grows more efficiently in its presence. Example: Instances include Escherichia, Enterococcus, and Saccharomyces cerevisiae.
3. Aerotolerant anaerobe: Capable of thriving in the presence or absence of oxygen. Example: Streptococcus pyogenes.
4. Obligate anaerobe: An organism that cannot tolerate oxygen and dies in its presence. Example: Clostridium, Bacteroides, and Methanothermobacter are some examples.
5. Microaerophile: Microaerophiles require oxygen concentrations between 2% and 10% for growth and are harmed by oxygen concentrations of 20%. Example: Campylobacter, Spirillum volutans, Treponema pallidum.

Classifications based on Pressure

• Piezophile (barophile): Hydrostatic stresses increase the rate of growth. Example: Photobacterium profundum and Shewanella benthica are two examples.

Classifications based on Radiation

• Radiation resistant: Withstands strong gamma radiation exposures. Example: For instance, Deinococcus radiodurans and Thermococcus gamma-tolerance.

Other Types of Extremophiles

1. Capnophile: An organism that grows optimally at high carbon dioxide concentrations. Mannheimia succiniciproducens, a bacteria found in the digestive tract of ruminant animals, is an example.
2. Cryptoendolith: An organism that inhabits microscopic spaces within rocks, such as aggregate grain pores. These are also referred to as endoliths, a phrase that encompasses organisms inhabiting fissures, aquifers, and faults containing groundwater in the deep underground.
3. Hyperpiezophile: An organism that grows optimally at hydrostatic pressures more than 50 MPa (= 493 atm = 7,253 psi).
4. Hypolith: A subterranean creature that inhabits frigid deserts.
5. Metallotolerant: Able to tolerate high concentrations of dissolved heavy metals, such as copper, cadmium, arsenic, and zinc. Examples include Ferroplasma sp., Cupriavidus metallidurans and GFAJ-1.
6. Oligotroph: An organism that grows optimally in nutrient-poor settings.
7. Osmophile: An organism that thrives in settings with a high concentration of sugar.
8. Piezophile: An organism that grows optimally at hydrostatic pressures greater than 10 MPa (equal to 99 atm or 1,450 psi). Also known as a barophile.
9. Polyextremophile: A polyextremophile is an organism that meets the criteria for extremophile in more than one category.
10. Sulphophile: An organism that grows optimally in high sulphur concentrations. An illustration would be Sulfurovum Epsilonproteobacteria, a sulfur-oxidizing bacteria, inhabit sulphur vents in deep water.

Examples of Extremophiles

Snottite

• Snottites, also known as “snoticles,” are composed of colonies of cave-dwelling, single-celled, extremophilic bacteria.
• These colonies have the appearance of stalactites but the substance of snot. These bacterial colonies can endure, among other extreme physiochemical conditions, high levels of toxicity and acidity.
• They exist by converting volcanic sulphur compounds into energy and sulfuric acid waste through chemosynthesis.

Artemia salina (Sea Monkey)

• The brine shrimp Artemia salina (sea monkey) can survive in environments with exceptionally high salt concentrations.
• These extremophiles inhabit salt lakes, salt marshes, oceans, and rocky coastlines.
• They can survive under nearly saturated salt concentrations. Their principal source of nutrition is green algae. Sea monkeys have an exoskeleton, antennae, complex eyes, segmented bodies, and gills, just like all crustaceans.
• Their gills assist them in surviving in saline conditions by absorbing and excreting ions and by creating concentrated urine.
• Sea monkeys procreate both sexually and asexually by parthenogenesis, similar to water bears.

Giant Tube Worms

• The gigantic tube worm is a deep-sea extremophile that lives near hydrothermal vents in conditions of extreme pressure, heat, and darkness.
• Near hydrothermal vents, the water can attain temperatures of 600 degrees Fahrenheit and pressures of about 9,000 psi! Without a digestive tract of their own, they rely on their symbiotic partners, extremophilic bacteria that reside in the midgut of the giant tube worm, to thrive in such harsh environments.
• The bacteria, which may make up as much as half of the worm’s mass, employ chemosynthesis to convert oxygen, hydrogen sulphide, and carbon dioxide into organic molecules that the worm can consume as sustenance.

Helicobacter pylori Bacteria

• Helicobacter pylori is a Gram-negative bacterium that thrives in the stomach’s highly acidic environment.
• These bacteria generate the hydrochloric acid neutralising enzyme urease. Some bacterial species comprise the microbiota of the stomach and can resist the stomach’s acidity.
• These bacteria prevent Helicobacter pylori and other pathogens from colonising the stomach. The spiral-shaped H. pylori bacteria burrow into the lining of the human stomach, causing ulcers and even stomach cancer.
• According to the Centers for Disease Control and Prevention (CDC), the bacteria are present in the majority of the world’s population, but they do not cause illness in the majority of these persons.

Tardigrades

• These eight-legged microorganisms, which are technically more extremotolerant than extremophilic, are among the most hardy organisms known to man.
• They have two survival plans, one for flooding and one for cold or drought. In the event of a flood, tardigrades inflate themselves like balloons so that they can float to the surface, where oxygen is available.
• Tardigrades have the extraordinary ability to replace more than 97% of the water in their bodies with trehalose in the event of drought or cold circumstances.
• This decreases the demand for water and prevents the formation of harmful ice crystals that would normally form with water.
• These species have survived temperatures ranging from -458 degrees Fahrenheit to 300 degrees Fahrenheit, pressures six times greater than those found in the deepest sections of the ocean, deadly doses of radiation, and even the vacuum of space by employing these survival tactics. The longer tardigrades remain in suboptimal settings, the less likely they are to survive.

Gloeocapsa Cyanobacteria

• Gloeocapsa is a genus of cyanobacteria that often inhabits moist, rocky shorelines. These cocci-shaped bacteria are capable of photosynthesis and contain chlorophyll a.
• Additionally, some have symbiotic interactions with fungus. Gloeocapsa cells are enveloped by translucent or brilliantly coloured gelatinous sheaths.
• It was discovered that Gloeocapsa species may survive in space for a year and a half. Gloeocapsa-containing rock samples were affixed to the outside of the International Space Station.
• These bacteria were able to endure harsh space conditions, including high temperature variations, exposure to vacuum, and radiation exposure.

Loricifera

• These microscopic organisms were first collected from the depths of a Mediterranean Sea basin where the salt-saturated brine they inhabit do not mix with or get watered down by the waters above it.
• They inhabited the marine sediment, flourishing in this salty, sulphuric, frigid, extremely pressured, and lightless habitat.
• This is feasible because, unlike humans, Loricifera use oxygen-free hydrogenosomes to make energy instead of mitochondria.

Grylloblattidae

• Grylloblattidae is a family of psychrophilic insects found on mountain peaks, glaciers, and ice sheets.
• Just above freezing, they like temperatures between 33.8 and 39.2 degrees Fahrenheit.
• When temperatures drop below freezing, these insects burrow beneath the snow and stay close to the ground; otherwise, they risk freezing to death.

References

• Cockell, Charles S, et al. “Exposure of Phototrophs to 548 Days in Low Earth Orbit: Microbial Selection Pressures in Outer Space and on Early Earth.” The ISME Journal, vol. 5, no. 10, 2011, pp. 1671–1682.
• Emslie, Sara. “Artemia Salina.” Animal Diversity Web.
• “Helicobacter Pylori and Cancer.” National Cancer Institute.
• https://www.sciencedirect.com/topics/immunology-and-microbiology/extremophile
• https://biologydictionary.net/extremophile/
• https://www.thoughtco.com/extremophiles-extreme-organisms-373905
• https://biolabtests.com/extremophiles/
• https://en.wikipedia.org/wiki/Extremophile
• https://www.britannica.com/science/extremophile