Life, or at least life as we know it, depends on water. Water is the solvent in which the cytoplasmic enzymatic machinery functions. However, life on Earth has adapted to a variety of environments with low water activity. Availability of water can be reduced both by the presence of salts and other solutes in the cells' surroundings (‘osmotic water stress’) and by drought (‘matric water stress’). As biological membranes are permeable to water, intracellular water activity equals that of the outside medium. A water activity of 0.60–0.62 appears to be the lower limit for life. Salt lakes and other hypersaline environments are popu lated by a diverse world of microorganisms adapted to life at salt concentrations up to NaCl saturation. The study of those organisms growing at the highest salinities shows that the problem how to cope with low water activity can be solved in different ways.

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