Psychrophile

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 dairy industry.[29] 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 7 °C (44.6 °F). At freezing temperatures, growth of psychrotrophic bacteria becomes negligible or virtually stops.[30]

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]

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.[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]

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.[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., Cryptocercus punctulatus[36]), and strong freezing tolerance (e.g., Eurosta solidaginis[37] and Syrphus ribesii[38]), and other insect species exhibiting freezing tolerance with low supercooling point (e.g., Pytho deplanatus[39]).[32]

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]

• Chionophile
• Halophile
• Ice algae
• Mesophile

• 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. arXiv:1109.6589. doi:10.1007/s00792-005-0480-1. PMID 16254683. S2CID 11658804.
• Mikucki, J. A.; et al. (2009). "A contemporary microbially maintained subglacial ferrous 'ocean'". Science. 324 (5925): 397–400. Bibcode:2009Sci...324..397M. doi:10.1126/science.1167350. PMID 19372431. 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. doi:10.1111/jam.12101. PMID 23216715. S2CID 26032521.