Water Activity
Water is a basic requirement of all life as we know it. However, determining how much water is available to life is quite a complicated matter. Common measures of water levels are moisture content and relative humidity. However, these measures tell us how much water is present, not how much water is available to the microorganisms.

(Image Credit: Decagon Devices)

At a microscopic level, water not only has to be present, but it also has to be free and available, as opposed to water trapped or bound by other materials in the environment, which is not available to microorganisms. Water Activity (AW) is a measure of the amount of water actually available to microorganisms and a much better indicator of whether the specific habitat is capable of supporting life. AW is measured using a specialized water activity meter, which measures the dewpoint temperature of the air and the temperature of the sample in a sealer chamber. From these measurements, the water activity of the sample is then calculated. Pure water has an AW value of 1, and an AW value of 0.5 is thought to be the limit of microbial life. Under normal conditions, we observed AW values ranging from 0.3 to 0.8 for Dry Valley soils.