Summary For the most‐extreme fungal xerophiles, metabolic activity and cell division typically halts between 0.700 and 0.640 water activity (approximately 70.0–64.0% relative humidity). Here, we investigate whether glycerol can enhance xerophile germination under acute water‐activity regimes, using an experimental system which represents the biophysical limit of Earth's biosphere. Spores from a variety of species, including As...

Summary Since a key requirement of known life forms is available water (water activity; a w ), recent searches for signatures of past life in terrestrial and extraterrestrial environments have targeted places known to have contained significant quantities of biologically available water. However, early life on E arth inhabited high‐salt environments, suggesting an ability to withstand low water‐activity. The lower limit of wat...

Summary Diverse parameters, including chaotropicity, can limit the function of cellular systems and thereby determine the extent of E arth's biosphere. Whereas parameters such as temperature, hydrophobicity, pressure, pH , H ofmeister effects, and water activity can be quantified via standard scales of measurement, the chao‐/kosmotropic activities of environmentally ubiquitous substances have no widely accepted, universal scal...