Organisms which inhabit extreme environmental conditions are called extreme lovers or Extremophiles. Such conditions are inhospitable for other organisms and they do not thrive in such habitat. Generally, microorganisms are known to be Extremophilic; both Archaebacteria and Eubacteria and their viruses represent extremophilic genera. Very few organisms from Eukaryotes: Algae (Chlamydomonas and Chlorobium), Pompeii worms, Crustacean like Antarctic krill and some insects (Cockroach and Grylloblatta) are known to be extremophiles. Plant kingdom also consists of extremophytes like Arabidopsis and Thellungiella. Recently, scientists Lowenstein and Schubert reported presence of 34000 years old living bacteria inside salt crystals from the Death Valley, California. It is amazing to know that how did these bacteria manage to thrive inside salt crystals for so many years. Discoverers explained that bacteria are not alone inhabitants of crystal but algal species has also been accompanying them! This is very splendid example of extremophilic association. Various types of extremophiles have been classified into different groups depending upon their habits and habitat. Let's know about some extraordinary extremophiles.
The organisms living in extremities of physico-chemical conditions are:
1. Extreme pH: Organisms living at pH of 3 or below (highly acidic) are Acidophiles (Acidophyllum, Acidocella) while as those living at pH of 9 or above (highly alkaline) are known as Basophiles or Alkalophiles (Natronobacterium, Bacillus).
2. Extreme temperature: Organisms living in high temperatures of 55-80˚C and as high as 80 to 130˚C are called as thermophiles (Thermus, Chloroflexus) and hyperthermophiles (Pyrococcus, Methanopyrus) respectively. They are found in hydrothermal vents deep under the ocean, hot water springs or also near the site of volcanic eruption.
3. Extreme pressure: Barophilic organisms live under extreme pressure; are found underground rock subsurface (oil reservoir) or deep sea water (Antarctic lake, depth more than 4000m). They are adapted to sustain pressure as high as 100MPa. The principle barophilic genus include Methanococcus sp. The subcategory of barophiles is known Piezophiles (Shewanella); they are found in deep oceanic trenches.
4. Extreme cold: Cryophiles or Psychrophiles are cold loving microbes. They live in cold temperatures of less than -15˚C. The habits include cold deserts of Himalayas, Alps, Antarctica, Arctic land and ocean waters. The well known psychrophilic bacterial and fungal genera include Hyphomonas, Sphingomonas and Chrysosporium.
5. Extreme dry: Organisms living in extremely dry regions of Earth are known as Xerophiles (Trichosporonoides, Wallemia, and Artemia) and ofcourse; they inhabit most of the deserts (Atacama, Sahara, Kalahari, Thar etc) on the Earth.
6. Heavy metals: Some genera like Ferroplasma, Cupriavidus and Ralstonia are capable of tolerating higher concentrations of heavy metals like copper, cadmium, nickel, arsenic and zinc are also extremophiles.
7. High salt concentration: Salt (Sodium chloride, NaCl) concentration higher than 0.2M is required for growth of halophile organisms like Salinibacter, Halobaterium, Chromohalobacter and Halorubrum. They can tolerate NaCl concentration upto 5M.
8. High sugar concentration: Osmophilic organisms are capable of living in medium of high sugar concentration; such as Starmerella and Zygosaccharomyces yeasts can sustain in honey and fruit juice concentrates.
9. Unusual growth substrates: These bacteria derive energy from reduced mineral/gaseous forms of iron, nitrogen, sulfur or manganese instead of using usual form of energy (carbon from carbon dioxide). The principle bacterial genera include Hydrogenomonas, Leptospirillium, Methanobacter, Methanosarcina, Nitrobacter, Nitrosomonas, Desulfovibrio including remaining genera of Iron and Sulfur bacteria.
10. Unusual habitats: Organisms live in microspaces within rocks, rock fissures, compact soil, aquifers, vents or faults and deep into the Earth's crust; they are called as Endoliths/ Hypoliths. Prokaryotic cyanobacteria are endolithic.
11. Polluted environment: Microbes usually resist ultraviolet radiation and some were also reported to thrive in nuclear radiation. They are called as radiation resistant microbes (Deinococcus radioresistans). Microbes have also been isolated from oil spilled ocean waters (Geotrichum, Aeromonas, Rhizopus), soils containing relatively higher concentration of pesticides or from recalcitrant aquatic and soil bodies (Veillonella, Micrococcus, Pseudomonas, Serratia).
Sometimes, extremophiles adapt themselves to live in more than one type of extreme conditions; they are termed as polyextremophiles or cosmopolitan (Chroococcidiopsis, Thermoacidiphilus). Thermoacidophilic bacteria are able to live in very hot as well as very acidic environment. In all categories of extremophiles, organisms are able to grow, reproduce and survive similarly like other microbes from normal ecosystems. Extremophiles also have important biotechnological applications which are enlisted here:
1. Pharmaceuticals, antibiotics and other novel drugs
2. Surfactants for oil recovery
3. Metal extraction and recovery
4. Coal desulfurization
5. Environmental biosensors
6. Source of heat, cold, alkali and acid resistant enzymes and proteins
7. Methane production
8. Pigments for electrical generation
9. Bioremediation agents in waste treatment
10. Evidence for Earth's formation, evolution and history
11. Astrobiology: comparative studies and evidence for the presence of life on Mars, Jupiter, its moons and other plants.
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