What is "psychrophile"

n. An organism that can live and thrive at temperatures much lower than normal; a form of extremophile.

Psychrophiles or cryophiles (adj. cryophilic) are extremophilic organisms that are capable of growth and reproduction in cold temperatures, ranging from −20 °C to +10 °C. 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. Additionally, recent research has discovered novel groups of psychrophilic fungi living 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 metabollicaly active at negative temperatures, such as some conifers that can still photosynthetize when it is several degrees under 0 °C (conifers are known to be 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.

Examples 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 years old ice.