View source for 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 [[extremophile|extremophilic]] organism that can grow and reproduce in conditions with a low availability of water, also known as [[water activity]]. 

Water activity, a [[thermodynamics|thermodynamical]] value denoted {{mvar|a<sub>w</sub>}}, is defined as the [[Partial pressure|partial water vapor pressure]] {{mvar|p}} in [[Thermodynamic equilibrium|equilibrium]] with the substance relative to (divided by) the (partial) [[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 [[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 [[Solution (chemistry)|solution]], is [[Vapor–liquid equilibrium|undersaturated]] in [[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 [[osmophile]], or osmotolerant, is typically applied to [[microorganism]]s that can grow in [[Solution (chemistry)|solutions]] with high [[solute]] concentrations ([[salt]]s, [[sugar]]s), such as [[halophiles]].

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

Complete [[dehydration]] based on the [[freeze-drying]] technique with effective protection inside a tight [[packaging]] system, strictly impervious to water and atmospheric gases ({{O2}} and {{CO2|link=Carbon dioxide}}), may be required for long-term [[Food preservation|preservation]] of food and [[wikt:pharmacochemical| pharmacochemical]] substances ([[antibiotic]]s, [[vaccine]]s…). Freeze drying can limit the microbial activity on the long term, as long as the product remains perfectly dry in a [[hermetic seal|hermetically sealed]] and intact package, but it is not a [[Sterilization (microbiology)|sterilisation]] technique ''per se'', because after rehydration, even if many dehydrated cells suffer irreversible and lethal damages, some resistant [[spore]]s and [[bacteria]]l [[endospore]]s can still be revived again, and multiplied, by means of [[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> ''[[Zygosaccharomyces]]'', and [[Cactus|cacti]].

==See also==
{{Wiktionary|xerophile}}
* {{annotated link|Extremophile}}
*[[Ombrophobe]]
* {{annotated link|Osmophile}}
* {{annotated link|Xerocole}}
* {{annotated link|Xerophyte}}

==References==
{{Reflist}}

{{Extremophile}}

{{-}}

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