Psychrophile
Psychrophiles or cryophiles (adj. psychrophilic or cryophilic) are
Many such organisms are bacteria or archaea, but some eukaryotes such as lichens, snow algae, phytoplankton, fungi, and wingless midges, are also classified as psychrophiles.
Biology
Habitat
The cold environments that psychrophiles inhabit are ubiquitous on Earth, as a large fraction of the planetary surface experiences temperatures lower than 10 °C. They are present in
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 °C and −26 °C. Cells of multicellular organisms may vitrify at temperatures below −50 °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.[2]
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,
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.[4]
Certain cryophiles, such as Gram-negative bacteria Vibrio and Aeromonas spp., can transition into a viable but nonculturable (VBNC) state.[10] 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.[11] 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.[12]
Taxonomic range
Psychrophiles include bacteria, lichens, snow algae, phytoplankton, fungi, and insects.
Among the bacteria that can tolerate extreme cold are
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.[17][18]
Some
Penicillium is a genus of fungi found in a wide range of environments including extreme cold.[22]
Among the psychrophile insects, the Grylloblattidae or ice crawlers, found on mountaintops, have optimal temperatures between 1–4 °C.[23] 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 million base pairs, is thought to be adaptive to extreme environments.[24]
Psychrotrophic bacteria
Psychrotrophic microbes are able to grow at temperatures below 7 °C (44.6 °F), 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 pathogens such as Yersinia. They provide an estimation of the product's shelf life, but also they can be found in soils,[25] in surface and deep sea waters,[26] in Antarctic ecosystems,[27] and in foods.[28]
Psychrotrophic bacteria are of particular concern to the
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.[31]
Psychrophilic microalgae
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.[18] 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.[19] 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.[20]
Psychrotrophic insects
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.[32] Chill tolerant insects succumb to freezing temperatures after prolonged exposure to mild or moderate freezing temperatures.[33] Freeze avoiding insects can survive extended periods of time at sub-freezing temperatures in a supercooled state, but die at their supercooling point.[33] Freeze tolerant insects can survive ice crystal formation within their body at sub-freezing temperatures.[33] Freeze tolerance within insects is argued to be on a continuum, with some insect species exhibiting partial (e.g., Tipula paludosa,[34] Hemideina thoracica[35]
), moderate (e.g.,
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.[40] 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.[41]
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.[42] 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.[42]
See also
- Chionophile
- Extremophile
- Halophile
- Ice algae
- Mesophile
- Osmophile
- Pathogenic microorganisms in frozen environments
- Thermophile
- Xerophile
References
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- ^ Correa-Guimaraes, A.; Martín-Gil, J.; Ramos-Sánchez, M. C.; Vallejo-Pérez, L. (2007). "Psychrotrophic bacteria isolated from Antarctic ecosystems". Department of Forestry, Agricultural and Environmental Engineering, ETSIA, Avenida de Madrid, 57, Palencia, Spain.
- ^ "Psychrotrophic Bacteria in Foods: Disease and Spoilage. – Food Trade Review". Encyclopedia.com. 1993-09-01. Retrieved 2010-09-01.
- ^ "The case of Psychrotrophic bacteria". Leon the Milkman's Blog. 2006-03-18. Archived from the original on 2011-07-13. Retrieved 2010-09-01.
- ^ Steven C. Murphy, "Shelf Life of Fluid Milk Products – Microbial Spoilage", Food Science Department, Cornell University.. Retrieved 22 November 2009.
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- ^ a b Sinclair, B. (1999). "Insect cold tolerance: How many kinds of frozen?". Eur. J. Entomol. 96: 157–164.
- ^ a b c Bale, J. (1996). "Insect cold hardiness: A matter of life and death". Eur. J. Entomol. 93: 369–382.
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Further reading
- Bej, Asim K.; Jackie Aislabie; Ronald M. Atlas (15 December 2009). Polar Microbiology: The Ecology, Biodiversity and Bioremediation Potential of Microorganisms in Extremely Cold Environments. Crc Pr Inc. ISBN 978-1420083842.
- Murata, Yoshinori; et al. (2006). "Genome-wide expression analysis of yeast response during exposure to 4C". Extremophiles. 10 (2): 117–128. S2CID 11658804.
- Mikucki, J. A.; et al. (2009). "A contemporary microbially maintained subglacial ferrous 'ocean'". Science. 324 (5925): 397–400. S2CID 44802632.
- Sandle, T.; Skinner, K. (2013). "Study of psychrophilic and psychrotolerant microorganisms isolated in cold rooms used for pharmaceutical processing". Journal of Applied Microbiology. 114 (4): 1166–1174. S2CID 26032521.