Relevance of Earth-Bound Extremophiles in the Search for Extraterrestrial Life.

The recent discovery of extrasolar Earth-like planets that orbit in their habitable zone of their system, and the latest clues of the presence of liquid water in the subsurface of Mars and in the subglacial ocean of Jupiter's and Saturn's moons, has reopened debates about habitability and limits of life. Although liquid water, widely accepted as an absolute requirement for terrestrial life, may be present in other bodies of the solar system or elsewhere, physical and chemical conditions, such as temperature, pressure, and salinity, may limit this habitability. However, extremophilic microorganisms found in various extreme terrestrial environments are adapted to thrive in permanently extreme ranges of physicochemical conditions. This review first describes promising environments for life in the Solar System and the microorganisms that inhabit similar environments on the Earth. The effects of extreme temperatures, salt, and hydrostatic pressure conditions on biomolecules will be explained in some detail, and recent advances in understanding biophysical and structural adaptation strategies allowing microorganisms to cope with extreme physicochemical conditions are reviewed to discuss promising environments for life in the Solar System in terms of habitability.