Why are archaea called extremophiles?

Archaea are called extremophiles because many cultured representatives were first discovered and described in environments characterized by extreme conditions, such as high temperatures, high salinity, or high acidity.1

For example, some archaea thrive in hydrothermal vents and hot springs, with species like Pyrolobus fumarii capable of living at temperatures as high as 113 °C (235 °F). Others, such as species of Picrophilus, are highly acid-tolerant and can grow at a pH of approximately 0, while halophilic archaea flourish in environments with high salt concentrations.1

It is important to note, however, that these extremophilic organisms represent only a minority of the total diversity within the domain Archaea. Modern culture-independent techniques have revealed that archaea are ubiquitous in global habitats, including cold and temperate ecosystems, where they fulfill vital ecological roles.1