Describing an organism that lives and grows optimally at relatively low temperatures, usually below 15°C, and cannot grow above 20°C. Psychrophiles, which consist mainly of bacteria, algae, and fungi, are restricted to permanently cold climates. For example, the ice in polar regions often contains dense masses of algae. Such organisms are likely to be killed by even brief warming. Psychrophiles have various adaptations to enable survival at extremes of cold. These include a high content of unsaturated fatty acids in their plasma membranes, which maintain the membrane in a semifluid state. They also have enzymes that function optimally at relatively low temperatures. Psychrotolerant organisms live and grow best at higher temperatures (20–40°C), but are able to tolerate cold conditions. Compare mesophilic; thermophilic.