Psychrophile

Psychrophiles or Cryophiles (adj. cryophilic) are extremophilic organisms that are capable of growth and reproduction in cold temperatures. They can be contrasted with thermophiles, which thrive at unusually hot temperatures. 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, Arctic 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.

Psychrophiles utilize 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. 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.

Types of psychrophiles
There are generally considered to be two groups of psychrophiles: "obligate" psychrophiles, and a second group, the "facultative psychrophiles", that are sometimes referred to as psychrotrophs by food microbiologists.

Obligate psychrophiles
Obligate psychrophiles are those organisms having a growth temperature optimum of 15°C or lower and cannot grow in a climate beyond a maximum temperature of 20°C. They are largely found in icy places (such as in Antarctica) or at the freezing bottom of the ocean floor. This separation is becoming more difficult as more organisms with a fairly large growth temperature range are discovered.

Facultative psychrophiles
Facultative psychrophiles can grow at 0°C up through approximately 40°C, and exist in much larger numbers than obligate psychrophiles. They are generally not able to grow much below 0°C, though they may maintain basic functioning. They have evolved to tolerate cold, but they are not as physiologically specialized as obligate psychrophiles and are usually not found in the very coldest of environments. They are of particular significance to food microbiologists as they can grow in refrigerated environments and thereby cause food spoilage.

Examples
The following are species with psychrophilic natures
 * The genus Moritella is mostly made up of psychrophiles.
 * Leifsonia aurea found in the Antarctic.
 * Methanococcoides burtonii was isolated from Ace Lake, Antarctica.