Water activity
Lower aw substances tend to support fewer microorganisms since these get desiccated by the water migration.
[citation needed] Water activity values can also help limit moisture migration within a food product made with different ingredients.
[citation needed] For many years, researchers tried to equate bacterial growth potential with water content.
[4] It is firmly established that growth of bacteria is inhibited at specific water activity values.
Studies in powdered milk show that viable cells can exist at much lower water activity values, but that they never grow.
Water activity values are obtained by either a resistive electrolytic, a capacitance or a dew point hygrometer.
The resistance is directly proportional to relative air humidity and therefore also to water activity of the sample (once vapor–liquid equilibrium is established).
This relation can be checked by either verification or calibration using saturated salt-water mixtures, which provide a well-defined and reproducible air humidity in the measurement chamber.
Such influences can easily be avoided by using chemical protection filters that absorb the volatile compound before arriving at the sensor.
[citation needed] Capacitance hygrometers consist of two charged plates separated by a polymer membrane dielectric.
[citation needed] Capacitance hygrometers are not affected by most volatile chemicals and can be much smaller than other alternative sensors.
The temperature at which dew forms on a clean surface is directly related to the vapor pressure of the air.
[citation needed] There is net evaporation from a solution with a water activity greater than the relative humidity of its surroundings.
In highly compacted bentonite and deep clay formations, microbial activity is limited by the lack of space and the transport of nutrients towards bacteria and the elimination of toxins produced by their metabolism is controlled by diffusion in the pore water.
So, "space and water restrictions" are two limiting factors of the microbial activity in deep sediments.
After the tantalizing detection of phosphine (PH3) in the atmosphere of Venus, in the absence of known and plausible chemical mechanism to explain the formation of this molecule, the presence of micro-organisms in suspension in Venus's atmosphere has been suspected and the hypothesis of the microbial formation of phosphine has been formulated by Greaves et al. (2020) from Cardiff University envisaging the possibility of a liveable window in the Venusian clouds at a certain altitude with an acceptable temperature range for microbial life.
[15] Hallsworth et al. (2021) from the School of Biological Sciences at Queen's University Belfast have studied the conditions required to support the life of extremophile micro-organisms in the clouds at high altitude in the Venus atmosphere where favorable temperature conditions might prevail.
Beside the presence of sulfuric acid in the clouds which already represent a major challenge for the survival of most micro-organisms, they came to the conclusion that the atmosphere of Venus is much too dry to host microbial life.
Indeed, Hallsworth et al. (2021) have determined a water activity of ≤ 0.004, two orders of magnitude below the 0.585 limit for known extremophiles.
[citation needed] Direct measurements of the Venusian atmosphere by spatial probes point to very harsh conditions, likely making Venus an uninhabitable world, even for the most extreme forms of life known on Earth.
The extremely low water activity of the desiccated Venusian atmosphere represents the very limiting factor for life, much more severe than the infernal conditions of temperature and pressure, or the presence of sulfuric acid.
Astrobiologists presently consider that more favorable conditions could be encountered in the clouds of Jupiter where a sufficient water activity could prevail in the atmosphere provided that other conditions necessary for life are also met in the same environment (sufficient supply of nutrients and energy in a non-toxic medium).
