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 with a glass transition 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.
Microbial activity has been measured in soils frozen below −39 °C. Among the bacteria that can tolerate extreme cold are Arthrobacter sp., Psychrobacter sp. and members of the genera Halomonas, Pseudomonas, Hyphomonas, and Sphingomonas. Another example is the Chryseobacterium greenlandensis, a psychrophile that was found in 120,000-year-old ice.
Some wingless insects (chironomid midges of the genus Diamesa) are still active down to −16 °C. The psychrotrophic pink yeast Rhodotorula glutinis causes food spoilage at temperatures as low as −18 °C. The lichens Umbilicaria antarctica and Xanthoria elegans have been recorded photosynthesizing at temperatures ranging down to −24 °C, and they can grow down to around −10 °C. Higher plants and invertebrates can survive down to around −70 °C but need temperatures of around −2 °C or higher to complete their life cycle.
Temperatures as low as −15 °C are found in pockets of very salty water (brine) surrounded by sea ice. Psychrophiles are true extremophiles because they adapt not only to low temperatures but often also to further environmental constraints. They can be contrasted with thermophiles, which thrive at unusually hot temperatures. In addition to that, distinctions between mesophilic and psychrophilic cold-shock response, including lack of repression of house-keeping protein synthesis and the presence of cold-acclimation proteins (Caps) in psychrophiles, does exist. The environments they inhabit are ubiquitous on Earth, as a large fraction of our planetary surface experiences temperatures lower than 15 °C. They are present in alpine and arctic soils, high-latitude and deep ocean waters, polar ice, glaciers, and snowfields. They are of particular interest to astrobiology, the field dedicated to the formulation of theory about the possibility of extraterrestrial life, and to geomicrobiology, the study of microbes active in geochemical processes. In experimental work at University of Alaska Fairbanks, a 1000-litre biogas digester using psychrophiles harvested from "mud from a frozen lake in Alaska" has produced 200–300 litres of methane per day, about 20–30% of the output from digesters in warmer climates.
Psychrophiles use a wide variety of metabolic pathways, including photosynthesis, chemoautotrophy (also sometimes known as lithotrophy), and heterotrophy, and form robust, diverse communities. Most psychrophiles are bacteria or archaea, and psychrophily is present in widely diverse microbial lineages within those broad groups. Some groups of psychrophilic fungi live in oxygen-poor areas under alpine snowfields. A further group of eukaryotic cold-adapted organisms are snow algae, which can cause watermelon snow. Some multicellular eukaryotes can also be metabolically active at negative temperatures, such as some conifers that can still photosynthesize when it is several degrees under 0 °C (conifers are often more cold-resistant than broadleaf trees). Psychrophiles are interesting enzymes that are very useful models in the research of proteins. Psychrophiles are characterized by lipid cell membranes chemically resistant to the stiffening caused by extreme cold, and often create protein 'antifreezes' to keep their internal space liquid and protect their DNA even in temperatures below water's freezing point. A commonly accepted hypothesis for this cold adaptation is the activity-stability-flexibility relationship, suggesting that psychrophilic enzymes increase the flexibility of their structure to compensate for the 'freezing effect' of cold habitats.
Comparison with psychrotrophs
In 1940, ZoBell and Conn stated that they have never encountered "true psychrophiles" or organisms that grow best at relatively low temperatures. 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 psychrotrophic 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. 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. 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.
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