The following points highlight the six important groups of microorganisms found in extreme environment. The groups are: 1. Acidophiles 2. Alkalophiles 3. Halophiles 4. Thermophiles and Hyperthermophiles 5. Psychrophiles 6. Barophiles.
Group # 1. Acidophiles:
Microorganisms that have their growth optimum between about pH 0 and 5.5. Several species of Thiobacillus and archaebacterial genera including Sulfolobus and Thermoplasma are acidophilic. Many fungi also grow optimally at pH 5 or below and a few grow well at pH values as low as 2.
Group # 2. Alkalophiles:
Microorganisms that prefer the pH range of 8.5 to 11.5 for their growth and survival are called alkalophiles. Alkalophiles live in soils laden with carbonate and in Soda lakes, and most of them are aerobic or facultative anaerobic. Bacillus alkalophilus, B. firmus RAB. B. sp. No. 81 and B. sp. No. C-125 are some alkalophiles.
Group # 3. Halophiles:
Microorganisms which grow optimally at high levels of sodium chloride (NaCl) or other salt concentration (about 0.2 M). Extreme halophiles have adapted to hypertonic, saline concentrations between about 2 M (approx 12%) to about 6.2 M (about 37%).
Halobacterium, Natronobacteriuin, Methanogens, etc. are the examples of extreme halophiles. Halobacterium occurs in the Dead Sea (a salt lake between Isreal and Jordan and the lowest lake in the world), the Great Salt Lake in Utah and other high salt aquatic habitats.
Group # 4. Thermophiles and Hyperthermophiles:
Microorganisms that can grow at temperatures of 55°C or higher (the minimum being usually around 45°C, the optimum being between 55-65°C, maximum being 80°C).
They fluorish in habitats including composts, self-heating hay stacks, hot water lines, and hot springs. Thermoplasma acidophilum (minimum 45°C, optimum 59° C. maximum 62°C) and Bacillus stearothermophilus (minimum 30°C, optimum 60-65°C. maximum 75°C) are the examples of thermophiles.
Microorganisms that have growth optima between 80°C and about 113°C are called hyperthermophiles. They usually do not grow well below 55°C. Pyrococcus abyssi and Pyrodictium occultum exemplify marine hyperthermophiles isolated from hot areas of the seafloor.
Group # 5. Psychrophiles:
Microorganisms that grow well at 0°C possess an optimum growth temperature of 15°C or lower and the maximum temp, for growth is around 20°C. They are usually found growing in Arctic and Antarctic habitats because 90% of the ocean is 5°C or colder.
Psychrophiles are widespread among bacterial genera such as Pseudomonas, Vibrio, Alcaligenes, Bacillus, Photobacterium, Arthrobacter. Chlamydomonas nivalis is a psychrophilic alga that produces bright red spores which turn the show field pink in appearance.
Group # 6. Barophiles:
Microorganisms that grow at high atmospheric pressure of 400-500 atmosphere at 2-3°C. Such conditions exist in deep sea habitat about 100 metres in depth. Barophiles have been found among several bacterial genera such as Photobacterium, Shewanella, Colwellia. Archaebacterial genera such as Pyrococcus and Methanococcus (M. jannaschii) are thermobarophiles.