View source for Biology:Xerophile

{{Short description|Organism that can grow and reproduce with low available water}}
A '''xerophile''' ({{etymology|gre|ξηρός : xērós|dry||φίλος : phílos|loving}})<ref>{{cite encyclopedia
 | title=xero-
 | encyclopedia=The New Oxford American Dictionary
 | edition=2nd
 | date=2005
 | publisher=Oxford University Press, Inc.
}}</ref> is an [[Earth:Extremophile|extremophilic]] organism that can grow and reproduce in conditions with a low availability of water, also known as [[Physics:Water activity|water activity]]. 

Water activity, a [[Physics:Thermodynamics|thermodynamical]] value denoted {{mvar|a<sub>w</sub>}}, is defined as the [[Physics:Partial pressure|partial water vapor pressure]] {{mvar|p}} in [[Physics:Thermodynamic equilibrium|equilibrium]] with the substance relative to (divided by) the (partial) [[Physics:Vapor pressure|vapor pressure]] of pure water {{math|''p*''}} at the same temperature: 
<math display="block"> a_w \equiv \frac{p}{p^*} </math>
The thermodynamical water activity is thus equal to the [[Physics:Relative humidity|relative humidity]] (RH), and the chemical activity of pure water is equal to one: {{nowrap|{{mvar|a<sub>w</sub>}} {{=}} 1.0.}}

When the atmosphere above a substance, or a [[Chemistry:Solution|solution]], is [[Physics:Vapor–liquid equilibrium|undersaturated]] in [[Physics:Water vapor|water vapor]] {{mvar|(p < p<sup>*</sup>)}}, its water activity is lower than one. 

Xerophiles are "xerotolerant", meaning tolerant of dry conditions. They can often survive in environments with water activity below 0.8; above which is typical for most life on Earth. Typically xerotolerance is used with respect to matrix drying, where a substance has a low water concentration. These environments include arid desert soils. The term [[Biology:Osmophile|osmophile]], or osmotolerant, is typically applied to [[Biology:Microorganism|microorganism]]s that can grow in [[Chemistry:Solution|solutions]] with high solute concentrations ([[Chemistry:Salt|salt]]s, [[Chemistry:Sugar|sugar]]s), such as halophiles.

The common food preservation methods of reducing water activity ([[Engineering:Food drying|food drying]]) may not always be sufficient to prevent the growth of xerophilic organisms, often resulting in food spoilage. Some mold and [[Biology:Yeast|yeast]] species are xerophilic. Mold growth on bread is an example of food spoilage by xerophilic organisms.{{Cn|date=October 2022}} 

Complete [[Medicine:Dehydration|dehydration]] based on the [[Engineering:Freeze-drying|freeze-drying]] technique with effective protection inside a tight [[Social:Packaging|packaging]] system, strictly impervious to water and atmospheric gases ({{O2}} and {{CO2|link=Carbon dioxide}}), may be required for long-term preservation of food and pharmacochemical substances ([[Chemistry:Antibiotic|antibiotic]]s, [[Biology:Vaccine|vaccine]]s…). Freeze drying can limit the microbial activity on the long term, as long as the product remains perfectly dry in a hermetically sealed and intact package, but it is not a [[Biology:Sterilization (microbiology)|sterilisation]] technique ''per se'', because after rehydration, even if many dehydrated cells suffer irreversible and lethal damages, some resistant [[Biology:Spore|spore]]s and [[Biology:Bacteria|bacteria]]l [[Biology:Endospore|endospore]]s can still be revived again, and multiplied, by means of [[Biology:Microbiological culture|microbiological culture]]s if the product was not initially sterilized by applying a proven technique.

Examples of xerophiles include ''Trichosporonoides nigrescens'',<ref>{{cite journal
|author=Hocking AD
|author2=Pitt JI
|title=''Trichosporonoides nigrescens sp.'' nov., a new xerophilic yeast-like fungus
|journal=Antonie van Leeuwenhoek
|date=December 1981
|volume=47
|issue=5
|pages=411–21
|pmid=7198892|doi=10.1007/BF00426003
|s2cid=10634262
}}</ref> ''[[Biology:Zygosaccharomyces|Zygosaccharomyces]]'', and [[Biology:Cactus|cacti]].

==See also==
* {{annotated link|Earth:Extremophile}}
* {{annotated link|Biology:Osmophile}}
* {{annotated link|Biology:Xerocole}}
* {{annotated link|Biology:Xerophyte}}

==References==
{{Reflist}}

{{Extremophile}}

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[[Category:Xerophiles| ]]
[[Category:Biology terminology]]
[[Category:Ecology terminology]]
[[Category:Extremophiles]]

{{Sourceattribution|Xerophile}}