Xerotolerant bacteria: surviving through a dry spell

Key Points Xerotolerant microorganisms are extremophiles that can survive in environments with extremely limited water availability. Despite their importance to these ecosystems, xerotolerant bacteria have been largely overlooked. A high diversity of xerotolerant bacteria can be found in many different extreme environments, including hot and cold environments, such as the Atacama and Antarctic deserts. In these biomes, xerotolerant microorganisms survive in sheltered geological niches that allow for biological activity. Dormancy and sporulation are common behavioural responses to desiccation that enable xerotolerant microorganisms to react to sporadic cycles of rainfall and drought by remaining in an inert metabolic state. Xerotolerant bacteria use several physiological mechanisms to prevent cell disruption and water loss, including phospholipid modifications to maintain membrane fluidity, the secretion of water-retaining extracellular polymeric substances (EPS), and the accumulation of compatible solutes that preserve the osmotic potential across the membrane. For xerotolerant microorganisms, DNA and protein stability are crucial to ensure that cellular activity is resumed under favourable conditions. Consequently, most molecular adaptations to xeric stress involve the upregulation of proteins that are stable under low water activity and that preserve the integrity of DNA through physical protection and repair. Although xerotolerant bacteria are unique in their capacity to survive in environments in which water is scarce, many of the adaptive mechanisms that they use are also triggered by other abiotic stresses that are present in these environments. Therefore, these mechanisms are part of broader adaptive response that enables the survival of microorganisms in extreme biomes. Understanding the ecology and function of dry-adapted communities is important for understanding and preventing desertification. In this Review, Lebre, De Maayer and Cowan discuss the adaptations that enable xerotolerant bacteria to survive extreme dry conditions and highlight insights from recent metagenomic and transcriptomic studies. Water is vital for many biological processes and is essential for all living organisms. However, numerous macroorganisms and microorganisms have adapted to survive in environments in which water is scarce; such organisms are collectively termed xerotolerant. With increasing global desertification due to climate change and human-driven desertification