Abstract
Earth contains several environmental extremes which are uninhabitable for
most of the living beings. But, astonishingly, in the last few decades, several organisms
thriving in such extreme environments have been discovered. “Extremophiles”,
meaning “Lovers of Extremities” are the entities that are especially adapted to live in
such harsh environmental conditions in which other entities cannot live. The discovery
of extremophiles has not only boosted the biotech industry to search for new products
from them, but also made researchers to think for the existence of extra-terrestrial life.
The most inhospitable environments include physical or chemical extremities, like high
or low temperatures, radiation, high pressure, water scarcity, high salinity, pH
extremes, and limitation of oxygen. Microorganisms have been found to live in all such
environmental conditions, like hyperthermophiles and psychrophiles, acidophiles and
alkaliphiles. Bacteria like Deinococcus radiodurans, which is able to withstand
extreme gamma radiation, and Moritella sp., able to grow at atmospheric pressure of
>1000 atm, have been reported. Environments like the Dead Sea, having saturated
NaCl concentrations, hold extreme halophiles like Halobacterium salinarum. Highly
acidic environments, like the Rio-Tinto River in Spain or Danakil depression in
Ethiopia harbour acidophiles with growth optima of pH zero, or close to it. Bacillus
alcalophilus, and Microcystis aeruginosa on the other hand inhabit natural alkaline
soda lakes where pH can reach about 12.0. A number of anaerobic prokaryotes can live
in complete anoxic environments by using terminal electron acceptors other than
oxygen. In this chapter, we shall discuss very briefly the diversity of all extremophiles
and their mechanism(s) of adaptation.