Editing Psychrophile

{{short description|Organism capable of growing and reproducing in the cold}}
[[File:Xanthoria elegans 97571 wb1.jpg|thumb|The [[lichen]] ''[[Xanthoria elegans]]'' can continue to [[photosynthesis|photosynthesize]] at −24&nbsp;°C.<ref name="BartákVáczi2007"/>]]

'''Psychrophiles''' or '''cryophiles''' (adj. ''psychrophilic'' or ''cryophilic'') are [[extremophile|extremophilic]] [[organism]]s that are capable of [[cell growth|growth]] and [[reproduction]] in low temperatures, ranging from {{cvt|−20|C|F}}<!--CONFLICT with stmt in body--><ref name="NeufeldClarke20132">{{cite journal |last1=Neufeld |first1=Josh |last2=Clarke |first2=Andrew |last3=Morris |first3=G. John |last4=Fonseca |first4=Fernanda |last5=Murray |first5=Benjamin J. |last6=Acton |first6=Elizabeth |last7=Price |first7=Hannah C. |year=2013 |title=A Low Temperature Limit for Life on Earth |journal=[[PLOS One]] |volume=8 |issue=6 |page=e66207 |doi=10.1371/journal.pone.0066207|pmid=23840425 |pmc=3686811 |bibcode=2013PLoSO...866207C |doi-access=free }}</ref> to {{cvt|20|C|F}}.<ref name=":3">{{cite journal |last1=Moyer |first1=Craig L. |title=Psychrophiles and Psychrotrophs |date=2017-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780128096338022822 |journal=Reference Module in Life Sciences |publisher=[[Elsevier]] |language=en |doi=10.1016/b978-0-12-809633-8.02282-2 |isbn=978-0-12-809633-8 |access-date=2022-05-22 |first2=Eric R. |last2=Collins |last3=Morita |first3=Richard Y.|url-access=subscription }}</ref><!--CONFLICT with stmt in body--> They are found in places that are permanently cold, such as the polar regions and the deep sea. They can be contrasted with [[thermophile]]s, which are organisms that thrive at unusually high temperatures, and [[mesophile]]s at intermediate temperatures. Psychrophile is Greek for 'cold-loving', {{etymology|grc|''{{wikt-lang|grc|ψυχρός}}'' ({{grc-transl|ψυχρός}})|cold, frozen}}.

Many such organisms are [[bacteria]] or [[archaea]], but some [[eukaryote]]s such as [[lichen]]s, [[snow algae]], [[phytoplankton]], fungi, and [[Chironomidae|wingless midges]], are also classified as psychrophiles.

==Biology==
[[File:Chlamydomonas nivalis.jpg|thumb|Snow surface with [[snow algae]] ''[[Chlamydomonas nivalis]]''.]]
===Habitat===
The cold environments that psychrophiles inhabit are ubiquitous on Earth, as a large fraction of the planetary surface experiences temperatures lower than 10&nbsp;°C. They are present in [[permafrost]], polar ice, [[glacier]]s, [[snow field|snowfields]] and [[deep ocean]] waters. These organisms can also be found in pockets of sea ice with high salinity content.<ref name="D'amico, Salvino 20063" /> Microbial activity has been measured in [[soil]]s frozen below −39&nbsp;°C.<!--CONFLICT with stmt in lead--><ref>{{cite journal |doi=10.1016/j.soilbio.2005.07.004 |title=Microbial activity in soils frozen to below −39°C |journal=Soil Biology and Biochemistry |volume=38 |issue=4 |pages=785–794 |year=2006 |last1=Panikov |first1=N.S. |last2=Flanagan |first2=P.W. |last3=Oechel |first3=W.C. |last4=Mastepanov |first4=M.A. |last5=Christensen |first5=T.R. }}</ref> In addition to their temperature limit, psychrophiles must also adapt to other extreme environmental constraints that may arise as a result of their habitat. These constraints include high pressure in the deep sea, and high salt concentration on some sea ice.<ref>{{cite journal |last1=Feller |first1=Georges |last2=Gerday |first2=Charles |date=December 2003 |title=Psychrophilic enzymes: hot topics in cold adaptation |journal=Nature Reviews Microbiology |volume=1 |issue=3 |pages=200–208 |doi=10.1038/nrmicro773|pmid=15035024 |s2cid=6441046 }}</ref><ref name="D'amico, Salvino 20063">{{cite journal |author1=D'Amico, Salvino |author2=Tony Collins |author3=Jean-Claude Marx |author4=Georges Feller |author5=Charles Gerday |year=2006 |title=Psychrophilic Microorganisms: Challenges for Life |journal=EMBO Reports |volume=7 |issue=4 |pages=385–9 |doi=10.1038/sj.embor.7400662 |pmc=1456908 |pmid=16585939 }}</ref>

===Adaptations===
Psychrophiles are protected from freezing and the expansion of ice by ice-induced [[desiccation]] and [[vitrification]] (glass transition), as long as they cool slowly. Free living cells desiccate and vitrify between −10&nbsp;°C and −26&nbsp;°C. Cells of multicellular organisms may vitrify at temperatures below −50&nbsp;°C. The cells may continue to have some metabolic activity in the extracellular fluid down to these temperatures, and they remain viable once restored to normal temperatures.<ref name="NeufeldClarke20132"/>

They must also overcome the stiffening of their lipid cell membrane, as this is important for the survival and functionality of these organisms. To accomplish this, psychrophiles adapt lipid membrane structures that have a high content of short, [[unsaturated fatty acids]]. Compared to longer saturated fatty acids, incorporating this type of fatty acid allows for the lipid cell membrane to have a lower melting point, which increases the fluidity of the membranes.<ref>{{cite journal |doi=10.1007/bf02705110 |title=A branched chain fatty acid promotes cold adaptation in bacteria |journal=Journal of Biosciences |volume=28 |issue=4 |pages=363–364 |year=2003 |last1=Chattopadhyay |first1=M. K. |last2=Jagannadham |first2=M. V. |pmid=12799482 |s2cid=44268024 }}</ref><ref>{{cite journal |doi=10.1021/acs.jpclett.0c01675 |title=Cryostabilization of the Cell Membrane of a Psychrotolerant Bacteria via Homeoviscous Adaptation |journal= J. Phys. Chem. Lett. |volume=11|pages=7709–7716 |year=2020 |last1=Erimban |first1=S. |last2=Daschakraborty |first2=S. |issue=18 |pmid=32840376 |s2cid=221305712 }}</ref> In addition, [[carotenoid]]s are present in the membrane, which help modulate the fluidity of it.<ref name=":0">{{cite journal |doi=10.1007/bf02705244 |title=Mechanism of bacterial adaptation to low temperature |journal=Journal of Biosciences |volume=31 |pages=157–165 |year=2006 |last1=Chattopadhyay |first1=M. K. |issue=1 |pmid=16595884 |s2cid=27521166 }}</ref>

[[Antifreeze proteins]] are also synthesized to keep psychrophiles' internal space liquid, and to protect their [[DNA]] when temperatures drop below water's freezing point. By doing so, the protein prevents any ice formation or recrystallization process from occurring.<ref name=":0" />

The enzymes of these organisms have been hypothesized to engage in an activity-stability-flexibility relationship as a method for adapting to the cold; the flexibility of their enzyme structure will increase as a way to compensate for the freezing effect of their environment.<ref name="D'amico, Salvino 20063"/>

Certain cryophiles, such as Gram-negative bacteria ''Vibrio'' and ''Aeromonas'' spp., can transition into a [[Viable but nonculturable|viable but nonculturable (VBNC)]] state.<ref>{{Cite journal|last1=Maayer|first1=Pieter De|last2=Anderson|first2=Dominique|last3=Cary|first3=Craig|last4=Cowan|first4=Don A.|date=May 15, 2015|title=Some like it cold: understanding the survival strategies of psychrophiles|journal=EMBO Reports|volume=15|issue=5|pages=508–517|doi=10.1002/embr.201338170|pmc=4210084|pmid=24671034}}</ref> During VBNC, a micro-organism can respire and use substrates for metabolism – however, it cannot replicate. An advantage of this state is that it is highly reversible. It has been debated whether VBNC is an active survival strategy or if eventually the organism's cells will no longer be able to be revived.<ref>{{cite journal |doi=10.3389/fmicb.2014.00258 |pmid=24917854 |pmc=4040921 |title=The importance of the viable but non-culturable state in human bacterial pathogens |journal=Frontiers in Microbiology |volume=5 |pages=258 |year=2014 |last1=Li |first1=Laam |last2=Mendis |first2=Nilmini |last3=Trigui |first3=Hana |last4=Oliver |first4=James D. |last5=Faucher |first5=Sebastien P. |doi-access=free }}</ref> There is proof however it may be very effective –  Gram positive bacteria Actinobacteria have been shown to have lived about 500,000 years in the permafrost conditions of Antarctica, Canada, and Siberia.<ref>{{cite journal |doi=10.1073/pnas.0706787104 |pmid=17728401 |pmc=1958816 |bibcode=2007PNAS..10414401J |title=Ancient bacteria show evidence of DNA repair |journal=Proceedings of the National Academy of Sciences |volume=104 |issue=36 |pages=14401–5 |last1=Johnson |first1=Sarah Stewart |last2=Hebsgaard |first2=Martin B. |last3=Christensen |first3=Torben R. |last4=Mastepanov |first4=Mikhail |last5=Nielsen |first5=Rasmus |last6=Munch |first6=Kasper |last7=Brand |first7=Tina |last8=Gilbert |first8=M. Thomas P. |last9=Zuber |first9=Maria T. |last10=Bunce |first10=Michael |last11=Rønn |first11=Regin |last12=Gilichinsky |first12=David |last13=Froese |first13=Duane |last14=Willerslev |first14=Eske |year=2007 |doi-access=free }}</ref>

===Taxonomic range===
Psychrophiles include bacteria, lichens, snow algae, phytoplankton, fungi, and insects.

Among the bacteria that can tolerate extreme cold are ''[[Arthrobacter]]'' sp., ''[[Psychrobacter]]'' sp. and members of the genera ''[[Halomonas]]'', ''[[Pseudomonas]]'', ''[[Hyphomonas]]'', and ''[[Sphingomonas]]''.<ref>{{cite journal |doi=10.1146/annurev-earth-040610-133514 |bibcode=2013AREPS..41...87S |title=Psychrophiles |journal=Annual Review of Earth and Planetary Sciences |volume=41 |pages=87–115 |last1=Siddiqui |first1=Khawar S. |last2=Williams |first2=Timothy J. |last3=Wilkins |first3=David |last4=Yau |first4=Sheree |last5=Allen |first5=Michelle A. |last6=Brown |first6=Mark V. |last7=Lauro |first7=Federico M. |last8=Cavicchioli |first8=Ricardo |year=2013 }}</ref> Another example is ''[[Chryseobacterium greenlandensis]]'', a psychrophile that was found in 120,000-year-old ice.

''[[Umbilicaria antarctica]]'' and ''[[Xanthoria elegans]]'' are lichens that have been recorded photosynthesizing at temperatures ranging down to −24&nbsp;°C, and they can grow down to around −10&nbsp;°C.<ref>{{cite journal |doi=10.1017/S1473550413000438 |bibcode=2014IJAsB..13..141C |title=The thermal limits to life on Earth |journal=International Journal of Astrobiology |volume=13 |issue=2 |pages=141–154 |last1=Clarke |first1=Andrew |year=2014 |url=http://nora.nerc.ac.uk/id/eprint/507274/1/Clarke.pdf |doi-access=free }}</ref><ref name="BartákVáczi2007">{{cite journal |last1=Barták |first1=Miloš |last2=Váczi |first2=Peter |last3=Hájek |first3=Josef |last4=Smykla |first4=Jerzy |title=Low-temperature limitation of primary photosynthetic processes in Antarctic lichens Umbilicaria antarctica and Xanthoria elegans |journal=Polar Biology |volume=31 |issue=1 |year=2007 |pages=47–51  |doi=10.1007/s00300-007-0331-x|bibcode=2007PoBio..31...47B |s2cid=46496194 }}</ref> Some multicellular eukaryotes can also be metabolically active at sub-zero temperatures, such as some conifers;<ref>{{cite book |title=Les résineux - Tome 1 : connaissance et reconnaissance |author=Riou-Nivert, Philippe |publisher=Institut pour le développement forestier |year=2001 |page=79}}</ref> those in the ''[[Chironomidae]]'' family are still active at −16&nbsp;°C.<ref>{{cite journal |doi=10.1038/310225a0 |bibcode=1984Natur.310..225K |title=A novel cold-tolerant insect found in a Himalayan glacier |journal=Nature |volume=310 |issue=5974 |pages=225–227 |last1=Kohshima |first1=Shiro |year=1984 |s2cid=35899097 }}</ref>

[[File:Chlamydomonas3 (Antarctique).jpg|thumb|Psychrophilic [[algae]] can tolerate cold temperatures, like this ''[[Chlamydomonas]]'' green algae growing on snow in [[Antarctica]].]]
[[Microalgae]] that live in snow and ice include green, brown, and red algae. [[Snow algae]] species such as ''[[Chloromonas]] sp.'', ''[[Chlamydomonas]] sp.'', and ''[[Chlorella]] sp.'' are found in polar environments.<ref name="Davey Norman Sterk Huete‐Ortega pp. 1242–1255">{{cite journal | last1=Davey | first1=Matthew P. | last2=Norman | first2=Louisa | last3=Sterk | first3=Peter | last4=Huete-Ortega | first4=Maria | last5=Bunbury | first5=Freddy | last6=Loh | first6=Bradford Kin Wai | last7=Stockton | first7=Sian | last8=Peck | first8=Lloyd S. | last9=Convey | first9=Peter | last10=Newsham | first10=Kevin K. | last11=Smith | first11=Alison G. | title=Snow algae communities in Antarctica: metabolic and taxonomic composition | journal=New Phytologist | publisher=Wiley | volume=222 | issue=3 | date=27 February 2019 | issn=0028-646X | doi=10.1111/nph.15701 | pages=1242–1255| pmid=30667072 | pmc=6492300 }}</ref><ref name="Khan Dierssen Scambos Höfer pp. 133–148">{{cite journal | last1=Khan | first1=Alia L. | last2=Dierssen | first2=Heidi M. | last3=Scambos | first3=Ted A. | last4=Höfer | first4=Juan | last5=Cordero | first5=Raul R. | title=Spectral characterization, radiative forcing and pigment content of coastal Antarctic snow algae: approaches to spectrally discriminate red and green communities and their impact on snowmelt | journal=The Cryosphere | publisher=Copernicus GmbH | volume=15 | issue=1 | date=13 January 2021 | issn=1994-0424 | doi=10.5194/tc-15-133-2021 | pages=133–148| bibcode=2021TCry...15..133K | s2cid=234356880 | doi-access=free }}</ref>

Some [[phytoplankton]] can tolerate extremely cold temperatures and high salinities that occur in brine channels when [[sea ice]] forms in polar oceans. Some examples are [[diatoms]] like ''[[Fragilariopsis cylindrus]]'', ''Nitzchia lecointeii'', ''Entomoneis kjellmanii'', ''Nitzchia stellata'', ''Thalassiosira australis'', ''Berkelaya adeliense'', and ''Navicula glaciei''.<ref name="Lauritano Rizzo Lo Giudice Saggiomo p=1957">{{cite journal | last1=Lauritano | first1=Chiara | last2=Rizzo | first2=Carmen | last3=Lo Giudice | first3=Angelina | last4=Saggiomo | first4=Maria | title=Physiological and Molecular Responses to Main Environmental Stressors of Microalgae and Bacteria in Polar Marine Environments | journal=Microorganisms | publisher=MDPI AG | volume=8 | issue=12 | date=9 December 2020 | issn=2076-2607 | doi=10.3390/microorganisms8121957 | page=1957| pmid=33317109 | pmc=7764121 | doi-access=free }}</ref><ref name="Young Goldman Kranz Tortell pp. 172–181">{{cite journal | last1=Young | first1=Jodi N. | last2=Goldman | first2=Johanna A. L. | last3=Kranz | first3=Sven A. | last4=Tortell | first4=Philippe D. | last5=Morel | first5=Francois M. M. | title=Slow carboxylation of R ubisco constrains the rate of carbon fixation during A ntarctic phytoplankton blooms | journal=New Phytologist | publisher=Wiley | volume=205 | issue=1 | date=3 October 2014 | issn=0028-646X | doi=10.1111/nph.13021 | pages=172–181| pmid=25283055 | doi-access=free }}</ref><ref name="Young Kranz Goldman Tortell pp. 13–28">{{cite journal | last1=Young | first1=JN | last2=Kranz | first2=SA | last3=Goldman | first3=JAL | last4=Tortell | first4=PD | last5=Morel | first5=FMM | title=Antarctic phytoplankton down-regulate their carbon-concentrating mechanisms under high CO2 with no change in growth rates | journal=Marine Ecology Progress Series | publisher=Inter-Research Science Center | volume=532 | date=21 July 2015 | issn=0171-8630 | doi=10.3354/meps11336 | pages=13–28| bibcode=2015MEPS..532...13Y | s2cid=87147116 | doi-access=free }}</ref>

''[[Penicillium]]'' is a genus of fungi found in a wide range of environments including extreme cold.<ref>{{cite journal |doi=10.5958/2230-732x.2014.00258.7 |title=Extremophiles: An Overview of Microorganism from Extreme Environment |journal=International Journal of Agriculture, Environment and Biotechnology |volume=7 |issue=2 |pages=371 |year=2014 |last1=Gupta |first1=G.N. |last2=Srivastava |first2=S. |last3=Khare |first3=S.K. |last4=Prakash |first4=V. }}</ref>

Among the psychrophile insects, the [[Grylloblattidae]] or ice crawlers, found on mountaintops, have optimal temperatures between 1–4&nbsp;°C.<ref name="grimaldi">{{cite book|title=Evolution of the Insects|author1=Grimaldi, David |author2=Engel, Michael S. |date=2005 |chapter=Polyneoptera: Grylloblattodea: The Ice Crawlers |pages=222–224 |publisher=[[Cambridge University Press]]| location=New York City |isbn=9780521821490}}</ref> The wingless midge (Chironomidae) ''[[Belgica antarctica]]'' can tolerate salt, being frozen and strong ultraviolet, and has the smallest known genome of any insect. The small [[genome]], of 99&nbsp;million [[base pair]]s, is thought to be adaptive to extreme environments.<ref>{{cite web|last1=Gough|first1=Zoe|title=Antarctic midge has smallest insect genome|url=https://www.bbc.co.uk/nature/28525963|publisher=BBC|access-date=14 January 2018|date=12 August 2014}}</ref>

==Psychrotrophic bacteria==
Psychrotrophic microbes are able to grow at temperatures below {{convert |7 |C |sigfig=3}}, but have better growth rates at higher temperatures. Psychrotrophic bacteria and fungi are able to grow at refrigeration temperatures, and can be responsible for food spoilage and as [[foodborne illness|foodborne pathogens]] such as ''[[Yersinia]]''. They provide an estimation of the product's shelf life, but also they can be found in soils,<ref>{{cite journal |doi=10.1111/j.1365-2672.1970.tb02215.x |title=An Ecological Study of the Psychrotrophic Bacteria of Soil, Water, Grass and Hay |journal=Journal of Applied Bacteriology |volume=33 |issue=2 |pages=420–435 |year=1970 |last1=Druce |first1=R. G. |last2=Thomas |first2=S. B. |pmid=5448255 }}</ref> in surface and deep sea waters,<ref>{{cite journal |doi=10.1007/s10126-001-0050-1 |pmid=14961338 |title=Characterization of Psychrotrophic Bacteria in the Surface and Deep-Sea Waters from the Northwestern Pacific Ocean Based on 16S Ribosomal DNA Analysis |journal=Marine Biotechnology |volume=3 |issue=5 |pages=454–462 |year=2001 |last1=Radjasa |first1=Ocky Karna |last2=Urakawa |first2=Hidetoshi |last3=Kita-Tsukamoto |first3=Kumiko |last4=Ohwada |first4=Kouichi |bibcode=2001MarBt...3..454R |s2cid=23054036 |url=http://eprints.undip.ac.id/355/1/Abstract_of_01OckyMarBiotech-2001.PDF }}</ref> in Antarctic ecosystems,<ref>{{cite web |url=http://es.scribd.com/doc/33323896/Psychrotrophic-Bacteria |title=Psychrotrophic bacteria isolated from Antarctic ecosystems |date=2007 |first1=A. |last1=Correa-Guimaraes |first2=J. |last2=Martín-Gil |first3=M. C. |last3=Ramos-Sánchez |first4=L. |last4=Vallejo-Pérez |publisher=Department of Forestry, Agricultural and Environmental Engineering, ETSIA, Avenida de Madrid, 57, Palencia, Spain}}</ref> and in foods.<ref>{{cite web |url=http://www.encyclopedia.com/doc/1G1-14605181.html |title=Psychrotrophic Bacteria in Foods: Disease and Spoilage. – Food Trade Review |publisher=Encyclopedia.com |date=1993-09-01 |access-date=2010-09-01}}</ref>

Psychrotrophic bacteria are of particular concern to the [[dairy industry]].<ref>{{cite web |url=http://www.leonthemilkman.com/2006/03/18/the-case-of-psychrotrophic-bacteria/ |title=The case of Psychrotrophic bacteria |publisher=Leon the Milkman's Blog |date=2006-03-18 |access-date=2010-09-01 |archive-url=https://web.archive.org/web/20110713201215/http://www.leonthemilkman.com/2006/03/18/the-case-of-psychrotrophic-bacteria/ |archive-date=2011-07-13 |url-status=dead }}</ref>{{self-published inline|date=November 2018}} Most are killed by [[pasteurization]]; however, they can be present in milk as post-pasteurization contaminants due to less than adequate sanitation practices. According to the Food Science Department at [[Cornell University]], psychrotrophs are bacteria capable of growth at temperatures at or less than {{convert |7 |C |sigfig=3}}. At freezing temperatures, growth of psychrotrophic bacteria becomes negligible or virtually stops.<ref>[https://web.archive.org/web/20100623132829/http://foodscience.cornell.edu/cals/foodsci/extension/upload/Bact-Milk-Shelf-Life-Doc.doc Steven C. Murphy, "Shelf Life of Fluid Milk Products – Microbial Spoilage", Food Science Department, Cornell University.]. Retrieved 22 November 2009.</ref>

All three subunits of the RecBCD enzyme are essential for physiological activities of the enzyme in the Antarctic ''[[Pseudomonas syringae]]'', namely, repairing of DNA damage and supporting the growth at low temperature. The RecBCD enzymes are exchangeable between the psychrophilic ''P. syringae'' and the mesophilic ''E. coli'' when provided with the entire protein complex from same species. However, the RecBC proteins (RecBCPs and RecBCEc) of the two bacteria are not equivalent; the RecBCEc is proficient in DNA recombination and repair, and supports the growth of ''P. syringae'' at low temperature, while RecBCPs is insufficient for these functions. Finally, both helicase and nuclease activity of the RecBCDPs are although important for DNA repair and growth of ''P. syringae'' at low temperature, the RecB-nuclease activity is not essential in vivo.<ref>{{cite journal |doi=10.1371/journal.pone.0009412 |pmid=20195537 |pmc=2828478 |bibcode=2010PLoSO...5.9412P |title=All Three Subunits of RecBCD Enzyme Are Essential for DNA Repair and Low-Temperature Growth in the Antarctic Pseudomonas syringae Lz4W |journal=PLOS ONE |volume=5 |issue=2 |pages=e9412 |last1=Pavankumar |first1=Theetha L. |last2=Sinha |first2=Anurag K. |last3=Ray |first3=Malay K. |year=2010 |doi-access=free }}</ref>

== Psychrophilic microalgae ==
[[File:Broken pack ice with cryopelagic antarctic diatoms.jpg|thumb|Antarctic [[diatom]] algae covering the underwater surface of broken [[sea ice]] in the [[Ross Sea]].]]
Microscopic algae that can tolerate extremely cold temperatures can survive in snow, ice, and very cold seawater. On snow, cold-tolerant algae can bloom on the snow surface covering land, glaciers, or sea ice when there is sufficient light. These snow algae darken the surface of the snow and can contribute to snow melt.<ref name="Khan Dierssen Scambos Höfer pp. 133–148" /> In seawater, phytoplankton that can tolerate both very high salinities and very cold temperatures are able to live in sea ice. One example of a psychrophilic phytoplankton species is the ice-associated diatom ''[[Fragilariopsis cylindrus]]''.<ref name="Lauritano Rizzo Lo Giudice Saggiomo p=1957" /> Phytoplankton living in the cold ocean waters near [[Antarctica]] often have very high protein content, containing some of the highest concentrations ever measured of enzymes like [[RuBisCO|Rubisco]].<ref name="Young Goldman Kranz Tortell pp. 172–181" />

== Psychrotrophic insects ==
[[File:Belgica antarctica mating.jpg|thumb|upright|The wingless midge ([[Chironomidae]]) ''[[Belgica antarctica]]''.]]
Insects that are psychrotrophic can survive cold temperatures through several general mechanisms (unlike opportunistic and chill susceptible insects): (1) chill tolerance, (2) freeze avoidance, and (3) freeze tolerance.<ref name=":1">{{Cite journal|last=Sinclair|first=B.|date=1999|title=Insect cold tolerance: How many kinds of frozen?|url=https://www.eje.cz/artkey/eje-199902-0009_Insect_cold_tolerance_How_many_kinds_of_frozen.php|journal=Eur. J. Entomol.|volume=96|pages=157–164}}</ref> Chill tolerant insects succumb to freezing temperatures after prolonged exposure to mild or moderate freezing temperatures.<ref name=":2">{{Cite journal|last=Bale|first=J.|date=1996|title=Insect cold hardiness: A matter of life and death|url=https://www.eje.cz/artkey/eje-199603-0009_Insect_cold_hardiness_A_matter_of_life_and_death.php|journal=Eur. J. Entomol.|volume=93|pages=369–382}}</ref> Freeze avoiding insects can survive extended periods of time at sub-freezing temperatures in a supercooled state, but die at their [[supercooling]] point.<ref name=":2" /> Freeze tolerant insects can survive ice crystal formation within their body at sub-freezing temperatures.<ref name=":2" /> Freeze tolerance within insects is argued to be on a continuum, with some insect species exhibiting partial (e.g., ''[[Tipula paludosa]]'',<ref>{{Cite journal|last1=Todd|first1=C.|last2=Block|first2=W.|date=1995|title=A comparison of the cold hardiness attributes in larvae of four species of Diptera|journal=CryoLetters|volume=16|pages=137–146}}</ref> ''[[Hemideina thoracica]]''<ref name="SinclairWorland2002">{{cite journal|last1=Sinclair|first1=Brent J.|last2=Worland|first2=M. Roger|last3=Wharton|first3=David A.|title=Ice nucleation and freezing tolerance in New Zealand alpine and lowland weta, ''Hemideina'' spp. (Orthoptera: [[Stenopelmatidae]]) |journal=Physiological Entomology|volume=24|issue=1|date=March 1999|pages=56–63|issn=0307-6962|doi=10.1046/j.1365-3032.1999.00112.x|s2cid=85725823}}</ref>
), moderate (e.g., ''[[Cryptocercus punctulatus]]''<ref name="HamiltonMullins1985">{{cite journal|last1=Hamilton|first1=R. L.|last2=Mullins|first2=D. E.|last3=Orcutt|first3=D. M.|title=Freezing-tolerance in the woodroachCryptocercus punctulatus (Scudder)|journal=Experientia|volume=41|issue=12|year=1985|pages=1535–1537|issn=0014-4754|doi=10.1007/BF01964793|s2cid=40606283}}</ref>), and strong freezing tolerance (e.g., ''[[Eurosta solidaginis]]''<ref name="BaustNishino1991">{{cite book|last1=Baust|first1=John G.|last2=Nishino|first2=Misako|title=Insects at Low Temperature|chapter=Freezing Tolerance in the Goldenrod Gall Fly (Eurosta solidaginis)|year=1991|pages=260–275|doi=10.1007/978-1-4757-0190-6_11|isbn=978-1-4757-0192-0}}</ref> and ''[[Syrphus ribesii]]''<ref name="HartBale1997">{{cite journal|last1=Hart|first1=Andrew|last2=Bale|first2=Jeffrey|title=Evidence for the first strongly freeze-tolerant insect found in the U.K.|journal=Ecological Entomology|volume=22|issue=2|year=1997|pages=242–245|issn=0307-6946|doi=10.1046/j.1365-2311.1997.t01-1-00048.x|bibcode=1997EcoEn..22..242H |s2cid=85423418}}</ref>'')'', and other insect species exhibiting freezing tolerance with low supercooling point (e.g., ''[[Pytho deplanatus]]''<ref name="Ring1982">{{cite journal|last1=Ring|first1=Richard A|title=Freezing-tolerant insects with low supercooling points|journal=Comparative Biochemistry and Physiology Part A: Physiology|volume=73|issue=4|year=1982|pages=605–612|issn=0300-9629|doi=10.1016/0300-9629(82)90267-5}}</ref>).<ref name=":1" />

==Psychrophile versus psychrotroph==

In 1940, ZoBell and Conn stated that they had never encountered "true psychrophiles" or organisms that grow best at relatively low temperatures.<ref>{{cite journal |author=Ingraham, J. L. |title=Growth of psychrophilic bacteria |journal=Journal of Bacteriology |volume=76 |issue=1 |pages=75–80 |year=1958 |doi=10.1128/jb.76.1.75-80.1958 |pmc=290156 |pmid=13563393}}</ref> In 1958, J. L. Ingraham supported this by concluding that there are very few or possibly no bacteria that fit the textbook definitions of psychrophiles. Richard Y. Morita emphasizes this by using the term ''psychrotroph'' to describe organisms that do not meet the definition of psychrophiles. The confusion between the terms ''psychrotrophs'' and ''psychrophiles'' was started because investigators were unaware of the thermolability of psychrophilic organisms at the laboratory temperatures. Due to this, early investigators did not determine the cardinal temperatures for their isolates.<ref>{{cite journal |author=Morita, Richard Y. |title=Psychrophilic bacteria |journal=Bacteriological Reviews |volume=39 |issue=2 |pages=144–67 |year=1975 |doi=10.1128/br.39.2.144-167.1975 |pmc=413900 |pmid=1095004}}</ref>

The similarity between these two is that they are both capable of growing at zero, but optimum and upper temperature limits for the growth are lower for psychrophiles compared to psychrotrophs.<ref name="Russell, N. J. 1237">{{cite journal |vauthors=Russell NJ, Harrisson P, Johnston IA, Jaenicke R, Zuber M, Franks F, Wynn-Williams D|title=Cold Adaptation of Microorganisms [and Discussion] |journal= Philosophical Transactions of the Royal Society of London |series=Series B Biological Sciences |volume=326 |pages=595–611 |number=1237, Life at Low Temperatures |year=1990 |jstor=2398707 |doi=10.1098/rstb.1990.0034|pmid=1969649 |bibcode=1990RSPTB.326..595R |doi-access= }}</ref> Psychrophiles are also more often isolated from permanently cold habitats compared to psychrotrophs. Although psychrophilic enzymes remain under-used because the cost of production and processing at low temperatures is higher than for the commercial enzymes that are presently in use, the attention and resurgence of research interest in psychrophiles and psychrotrophs will be a contributor to the betterment of the environment and the desire to conserve energy.<ref name="Russell, N. J. 1237"/>

==See also==
* [[Chionophile]]
* [[Extremophile]]
* [[Halophile]]
* [[Ice algae]]
* [[Mesophile]]
* [[Osmophile]]
* [[Pathogenic microorganisms in frozen environments]]
* [[Thermophile]]
* [[Xerophile]]

==References==
{{Reflist}}

==Further reading==
* {{cite book | author=Bej, Asim K. | author2=Jackie Aislabie | author3=Ronald M. Atlas | title=Polar Microbiology: The Ecology, Biodiversity and Bioremediation Potential of Microorganisms in Extremely Cold Environments | publisher=Crc Pr Inc | date=15 December 2009 | isbn=978-1420083842}}
* {{Cite journal |author=Murata, Yoshinori |display-authors=etal |title=Genome-wide expression analysis of yeast response during exposure to 4C |journal=Extremophiles |date=2006 |volume=10 |pages=117–128 |doi=10.1007/s00792-005-0480-1 |pmid=16254683 |issue=2 |arxiv=1109.6589 |s2cid=11658804 }}
* {{Cite journal |author=Mikucki, J. A. |display-authors=etal |title=A contemporary microbially maintained subglacial ferrous 'ocean' |journal=Science |volume=324 |pages=397–400 |date=2009 |doi=10.1126/science.1167350 |pmid=19372431 |issue=5925|bibcode=2009Sci...324..397M |s2cid=44802632 }}
* {{cite journal |author=Sandle, T. |author2=Skinner, K. |year=2013 |title=Study of psychrophilic and psychrotolerant microorganisms isolated in cold rooms used for pharmaceutical processing |journal=Journal of Applied Microbiology |volume=114 |issue=4 |pages=1166–1174 | doi=10.1111/jam.12101|pmid=23216715 |s2cid=26032521 }}

{{Extremophile}}

[[Category:Microbial growth and nutrition]]
[[Category:Psychrophiles| ]]
[[Category:Cryobiology]]