High Quality Content by WIKIPEDIA articles! Xerophiles are extremophilic organisms that can grow and reproduce in conditions with a low availability of water, also known as water activity. Water activity (aw) is a measure of the amount of water within a substrate that an organism can use to support sexual growth. Xerophiles are often said to be "xerotolerant", meaning tolerant of dry conditions. They can survive in environments with water activity below 0.8. Endoliths and halophiles are often xerotolerant.