Prokaryotic life has existed since the beginning of evolution. Their ability to adapt and mutate as per the environmental need is commendable. Some can tolerate the extreme environmental conditions whereas most need these conditions for growth. Thermophiles, acidophiles, and barophiles are some of the terms that denote organisms that endure the harsh environment, which are collectively termed extremophiles.
So, extremophiles thrive in extreme environments typically considered hostile to life. These environments include extreme temperatures, high or low pH levels, high salinity, high pressure, and even high radiation levels. The discovery of extremophiles has expanded our understanding of the limits of life and the conditions under which life can exist.
Extremophiles have high use in different fields because of their unique ability. From enzyme production to astrobiological studies, their applications to benefit human life is increasing significantly.
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Different Types of Extremophiles
As per their abilities to tolerate and thrive on different extreme environmental conditions, extremophiles are of various types. Acidophiles, thermophiles, barophiles, alkaliphiles, psychrophiles, halophiles, and radio-resistant organisms are some of the extremophiles.
Thermophiles are the organisms that thrive in extremely high temperatures, i.e., above 60°C (140°F) and up to around 122°C (252°F). These are present in hydrothermal vents, hot springs, and geothermal areas. Gases, minerals, and metals in the regions are the only sources of nourishment for the organism.
Thermoplasma acidophilus, Bacillus stearothermophilus, and Thermus aquaticus are examples of thermophiles. However, the thermophile that can thrive in temperatures as high as 121.11°C or 250°F is Methanopyrus kandleri.
Thermophiles are of different types; simple, extreme, and hyperthermophiles. Simple thermophiles tolerate temperatures from 50-64°C or 122-147.2°F. Extreme thermophiles tolerate or need high temperatures for survival, i.e., 65-79°C or 149-174.2°F. Hyperthermophiles can tolerate temperatures as high as 80°C or 176°F but not below 50°C.
Psychrophiles or Cryophiles are the organisms that love or require freezing temperatures, i.e., -20°C to +20°C. These organisms have 15°C or lower optimal growth temperature, with maximum growth temperature at about 20°C and minimum temperature at 0°C or lower. These are found in polar regions, deep-sea environments, and glaciers. Vibrio marinus and V psychroerythrus are the first true psychrophiles.
Here, Colwellia psychrerythraea isolated from Artic marine sediments was the first sequenced genome of psychrophile. The Artic permafrost bacteria, Planococcus halocryophilus, grows at the lowest growth temperature, i.e., -15°C with 50 days generation time. True psychrophiles that grow under sub-freezing temperatures have a longer generation time; Psychromonas ingraham grows after ten days at -12°C, and Psychrobacter arcticus grows at -10°C after 39 days of generation time. The only and first archaeon isolated is Methanogenium frigidum from Ace Lake Antarctic.
The organisms can thrive in highly acidic conditions, i.e., pH levels below 3. These grow in acid mine drainage, volcanic springs as well as acidic soils. Acidobacterium, Picrophilus, Ferroplasma, and Leptospirillum also grow in these acidic sites.
Acidithiobacillus thiooxidans isolated from soil-rock-sulfur composts was the first reported acidophile. Many acidophiles are metal resistant and can generate ATP or energy from metals like ferrous iron.
Most extreme acidophiles belong to the archeal group Acidianus, Metallosphaera, Pyrococcus, Desulfurococcus, Sulfurisphaera, Sulfolobus, Picrophilus, Stygiolobus, and Thermoplasma.
The organisms that adapt to highly alkaline environments, with pH above 9, are called alkaliphiles. These are found in soda lakes, alkaline lakes, and alkaline solids.
The aerobic alkaliphilic microorganisms include Bacillus, Micrococcus, Pseudomonas, Streptomyces, Bogoriella, Halomonas, Alkalibacillus, yeasts, and filamentous fungi isolated from various environments.
Vagococcus, Marinobacter, Alkalimonas, Paenibacillus, Rhodobaca, Dietzia, and Reseinatrobacter are isolated from the alkaline Lonar Lake of India. Whereas Alkaliphilic actinomycete, Bogoriella caseilytica was reported from a Soda lake in Africa. Likewise, Dietzia natronolimnaios was reported from an East African Soda Lake.
Other organisms include cyanobacteria like Spirulina platensis, Spirulina maxima, and Chorococcus species and anoxygenic phototrophic bacteria like Halorhodospira and Ectothiorhodospira.
The organisms which require high saline (salty) environments like salt lakes, salt flats, and pans are called halophiles. Haloarchaea class and Nanohaloarchaeota subphylum are the only archaea that show halophilism. According to their requirements, halophiles have three categories; slight (0.34-0.85 M salt), moderate (0.85-3.4M), and extreme halophiles (3.4-5.1M salt).
Halophiles dominate the most hypersaline environments on Earth, with many surviving salt concentrations close to saturation levels. Eubacteria are mostly halotolerant in nature, which means these bacteria do not rely on salt to thrive but can tolerate some salt concentrations.
The organisms that can adapt and thrive in high-pressure environments are barophiles or piezophiles. These are found in areas like the deep sea where the pressure is several hundred times higher than Earth’s surface, i.e., above 380 atm. Halophiles cannot survive without high pressure, also called obligate barophiles. Halomonas Salaria, Gram-negative proteobacteria, is an example of obligate halophiles that requires 1000 atm pressure.
The barophiles are sensitive to ultraviolet rays and susceptible to UV radiation, because of which many of the barophiles grow in the dark. Other examples of barophiles include xenophyophores, found in the deepest ocean trench.
The organisms capable of thriving in high radiation levels, like ionizing radiation, are called radio-resistant organisms. These grow near nuclear reactors and waste storage sites in radioactive environments.
Deinococcus radiodurans is a bacterial species highly resistant to radiation levels up to 1.5✕105 rad (rad=radiation absorbed dose) of acute ionizing radiation but only 6000 rad/hour of chronic radiation. The bacteria grow in radiation-contaminated areas like deserts, oceans, and seas. This bacteria is highly resistant to hypertonic stress, oxidation as well as desiccation.
In addition to the Deinococcus species nitrogen-fixing cyanobacterium Anabaena species, Micrococcus, Bacillus, and Actinobacteria are known radio-resistant bacteria.
Those organisms that thrive in more than one extreme condition are called polyextremophiles. For example, most barophiles are psychrophilic,i.e., requiring low temperature for growth as well as high pressure.
Likewise, bacteria like Clostridium paradoxum are haloalkaliphilic moderately thermophilic organisms; Deinococcus radiodurans are radio-resistant, acidophiles, and psychrophiles.
Role of Extremophiles in Human Well-being
The study of extremophiles is significant in different fields like biotechnology, environmental science, and astrobiology. Their study has surely helped us understand the potential for life in various extreme environments on Earth. The roles of extremophiles in human well-being are as follows:
- Drug discovery: The pharmaceutical application of extremophiles depends on the unique compounds these organisms produce for living in extreme environments. The screening and study of the medicinal values of these compounds help in the discovery of new drugs in humans and also in veterinary treatment.
- Environmental applications: The demand for cleaning the Earth without any ramifications to its natural sources has exponentially risen in the past few years. The enzymes produced by extremophiles are significantly useful in bioremediation, which cleans up contaminated environments using living organisms. Some extremophiles can also degrade toxic pollutants like metal.
- Enzyme production: Extremophiles also produce some enzymes that can function in an extreme environment. The other term for these enzymes are extremoenzymes. These are highly stable and active even in high temperatures, high and low pH levels and are helpful in industries.
- Astrobiology: The study of extremophiles has given insights into the limit of life on Earth and the existence of life anywhere else in the universe. Research on extremophiles will surely guide the search for extraterrestrial life by designing an instrument that detects energy. The study will also help develop technologies and strategies for space exploration and habitation.
- Industrial process: The enzymes produced by extremophiles are useful in biofuel like ethanol. The extremophiles also have applications in producing textiles, chemicals, and other materials. These help in obtaining more sustainable and efficient alternatives to conventional methods.
- Agricultural and food production: Another sector that can benefit from extremophiles is agriculture and food production. The issues of decreased agricultural productivity due to various environmental and human factors and risk food security is at rise. The extremophiles can contribute to developing crop varieties resistant to extreme environmental conditions like drought or salinity.
- Biotechnological application: Biotechnological application of extremophiles includes methods like PCR, where amplification of DNA using Taq polymerase from thermophiles occurs, organisms used in mining under extreme conditions, and the production of carotenoids for cosmetic and food industries.
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