Extremophiles, from the Greek extremus “extreme” and philia “loving,” are organisms that thrive under conditions that are considered detrimental for most known organisms. These organisms can live in habitats where no other organisms can, and are capable of tolerating a wide range of hostile environmental conditions. These organisms are termed based on the conditions in which they grow, thus, some organisms are thermophiles, halophiles, psychrophiles, acidophiles, halophiles etc.; however, there are extremophiles that are able to grow in more than one habitat and are termed polyextremophiles. Microbes are extremely adaptable to harsh environment conditions and it is believed that extremophiles could be found in every unimaginable place on Earth.
Most extremophiles are microbes; however, not all are unicellular organisms, there are some which are protostome animals, such as the Antarctic krill, the Pompeii worm, Tardigrades, among others, and are found in similar conditions as unicellular extremophiles. Extremophiles are very common and new types and subtypes of extremophiles are constantly classified. In the present, the sub category of extremophiles is continuously getting bigger.
Types of extremophiles
There exist various types of extremophiles, all of which are classified depending on the environment on which they thrive. Halophiles are organisms that can tolerate high concentrations of salt. Halophilic microorganisms, such as halobacterium are adapted to survive in extreme salty concentrations, such as those in pools of saltwater used in the harvesting of salt. The Dead Sea and The Great Salt Lake in Utah U.S., where salt concentrations are high, are other examples of habitats where halobacterium is found. Halobacteium contains high concentrations (4-7 M) of potassium ions, which promote osmotic balance with the external medium and helps stabilize enzymes and the transport of proteins.
Psychrophiles
One of the most important factors determining the growth and survival of extremophiles is temperature. Extremophiles can befound living in environments of below zero to the boiling point of water and above. Psychrophiles may grow, reproduce and survive well at temperatures of -15 °C. These organisms are common in the Arctic and Antarctic habitats. The enzymes transport and protein synthetic systems of psychrophiles function well at low temperatures. Their cell membranes remain semi-liquid under cold conditions, but will become disrupted at temperatures of 20 °C and above.
Thermophiles
These organisms can grow at temperatures of 55 °C and over. Most thermophiles are prokaryotic archaea, which are the most heat tolerant of all microorganisms. The protein synthetic systems of psychrophiles remain stable at high temperatures and their cell membranes remain functional due to high levels of saturated lipids, with high melting gradients. Hyperthermophiles can live in temperatures of between 80-120 °C, such as the bacteria Pyrococcus Abysii and Pyrodictium occultum, which are common in the ocean’s depths, such as in hydrothermal vents.
PH
Extremophiles can live in environments of high acidity and alkalinity. Acidophiles are organisms with a range growth of pH 0-5.5. Alkalophiles have a range growth of pH 8.5-12. High acidity environments are usually associated with heavy metal pollution. Acidophiles thriving in low pH conditions include fungi, acidophilic green algae and acidothermophilic red algae. Archaea are often found in extreme environments, such as sewage, bogs, salt brines, hot acid springs and ocean depths; however, they can also be found in less extreme habitats, such as lakes, soils and in the intestines of cattle. Some species of algae may adapt to acid or alkaline surroundings.
Alkalophiles
Alkalophilic microorganisms, including bacteria, archaeans and blue-green algae, are found in environments, such as hot springs, soda lakes and buried in the Earth’s crust. These organisms grow best at pH levels of over 8.5, but alkaline levels may reach values of pH 12. Organisms are able to maintain their internal pH close to neutrality in freshwater environments in varied ways. Extreme alkalophiles, including Bacillus alcalophilus maintains a neutral pH by the exchange of internal sodium ions for external protons. When the internal pH suffers fluctuations, internal metabolic processes may come into play.
Halophiles
The majority of halophiles live in salt solutions ranging from 2-20% of a saturated NaCl solution. Extreme halophiles usually occur in hypersaline ocean waters of 20% NaCl up to saturation. Halophiles most commonly occur in salt lakes, soda lakes and underground salt water lakes. Halophile organisms include archaeans, such as Halobacterium, bacteria and algae, including Dunaleilla viridis. Hypersaline organisms can be aerobic and anaerobic. Halobacterium is capable of thriving in such extreme salt concentrations by concentrating KCl intracellularly in order to maintain osmotic (water) balance. Some species of halophiles produce gas vacuoles to remain afloat on a body of water.
Polyextremophiles are known for ability to survive in both low and high pH concentrations. One such organism was found at the top of a mountain in the Atacama Desert. These organisms possess the ability to endure harsh environments as a xerophile, psychrophile, oligotroph and radioresistant. Psychrophiles occur in frozen environments. The requirement of water allows these organisms to thrive in water pockets surrounded by ice or within cold melting water, arising during brief summer seasons. Microorganisms were recently isolated from ice cores extracted from up to 300 meters depth in Vostock, Russia.
Although extremophiles have been living on Earth for a long time, it was not until a few decades ago that scientists discovered them for the first time. Some of these extremophiles live in inhospitable environments dangerous for most other organisms. Scientists think that some of these habitats may resemble those in the early life stage of the Earth. Recently, there have been new findings of microbial life forms thriving in places, such as one half mile deep into the ice in Antarctica and deep in the ocean at the Marian Trench. According to hhmi.org, you can find microbes everywhere, since they’re extremely able to adapt and survive in the most inhospitable conditions.
