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.
Many such organisms are bacteria, but psychrophiles also include eukaryotes such as lichens, snow algae, fungi, and wingless midges.
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.