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Water is essential for all living organisms as it is crucial for many biomolecular processes, including protein folding and stability, enzymesubstrate interactions and the maintenance of cell structure1,2. Therefore, exposure to xeric conditions can markedly impede the function and survival of cells (FIG.1). Briefly, xeric stress, which is caused either by a lack of water (desiccation) or excessive solute concentrations (hypertonicity) in the surrounding environment, removes water from cells and thereby causes biochemical, metabolic, physical and physio logical stress. However, microorganisms that live in hyper-arid deserts, desiccated foods and macroorganisms, such as desert-dwelling plants, have developed intricate methods to survive when only trace amounts of water are available. Most bacteria cannot actively divide at water activity (aw)
values less than 0.91, and most fungi are not meta bolically active at <0.7 aw, but in many of the more extreme environments studied, microorganisms have been found to cope with water activity levels well below these limits3
(BOX1). These organisms are collectively called xerophiles (a term derived from the Greek xros or dry, and philos meaning loving), owing to the fact that they can survive under stringent xeric conditions. However, in practical terms, obligate xerophiles do not exist, as these organisms invariably have optimal physiological and biochemical activities at much higher water activity. Thus, the term xerotolerant might be more appropriate to describe organisms that are capable of withstanding xeric stress but do not require low water activity tothrive.
The arid zones in which xerotolerant organisms survive constitute approximately 10% of the land surface of the Earth and this area is predicted to increase substantially as a result of climate change and desertification
processes that are driven by human activity4,5 (BOX2).
Xerotolerant microorganisms are not restricted to desert soils; they can also survive in hypersaline aquatic environments, in which high salt concentrations limit water availability, and in preserved foods6. Therefore, understanding the physiological, biochemical and molecular adaptation mechanisms that are used by xerotolerant microorganisms is relevant for environmental, food and medical sciences. Recent advances in sequencing technologies and metagenomics have enabled us to explore and compare the microbial diversity that is associated with these extreme environments and gain insights into how microorganisms survive in these harsh ecological niches (TABLE1). For...