Extremophilic organisms do the unexpected. They love boiling or freezing temperatures, very salty, acid or alkaline surroundings, resist incredible pressures, dwell deep in earth’s crust or, last but not least, are at home in the most contaminated places mankind ever made.

An ever-increasing knowledge of them provides opportunities to respond to deeply human questions as much as to solve new and old necessities. Indeed, the study of extremophiles changed the tree of life, give us hints on the origin of life, allows us to explore how life could be on other planets and offers unprecedented chances to understand the basic mechanisms of metabolism and biochemistry.

We also find a wealth of solutions for our ever-growing needs, ranging from molecular biology to decontamination, promising increased efficiency in huge mining operations and ubiquitous household detergents, not to mention many other industrial processes.
Extremophiles can be classified according to the conditions in which they exist:

Thermophiles: Microorganisms having a growth temperature optimum of 50°C or higher. They are found in hot springs and other hot environments.

Hyperthermophiles: Microorganisms having a growth temperature optimum of 80 °C and above. They live in deep-sea vents and hot springs.

Halophiles: Microorganisms that live in salty environments. According to their salt requirements, moderate Halophiles grow between 2% and 5% salt concentrations (0.2-0.85 M) and extreme Halophiles grow optimally at salt concentrations above 20% (3.4-5.1M). They live in places like salt lakes and salt mines.

Acidophiles: Microorganisms with a growth optimum at or below pH 2. They live in places like sulphur springs or acidic mine waters.

Alkaliphiles: Microorganisms with optimal growth at pH above 10. They are found in places like soda lakes.

Psychrophiles: Microorganisms having an optimum growth temperature of 15°C or lower, some can survive at -10°C and are unable to grow above 20°C. They are found in sea ice as much as in Arctic and Antarctic ice packs.

Piezophiles: Microorganisms that live optimally at high hydrostatic pressures (up to 130 MPa). They are found in deep-sea hydrothermal vents. (Previously termed barophiles)

Xerophiles: Microorganisms resisting extremely dry environments, like for example the Chilean Atacama desert.

Other exotic extremophiles:

Heavy metal resistant: Microorganisms that thrive in high concentrations of heavy metals. Typical places are mineral deposits or mine industry waste.

Radiation resistant: Microorganisms that can live in environment with high levels of ionising radiation, with up to 20,000 Gy of Gamma radiation (Deadly dose for humans: 5 Gy). They live in environments close to nuclear reactors, but also have been found in natural environments.

Resistance to chemical extremes: Microorganisms have been found, which live in environments of high concentrations of toxic chemicals, like organic solvents and PCB.