Craig L Moyer, Western Washington University, Bellingham, Washington, USA Richard Y Morita, Oregon State University, Corvallis, Oregon, USA
Based in part on the previous version of this Encyclopedia of Life Sciences (ELS) article, Psychrophiles and Psychrotrophs by Richard Y Morita.
. Introduction . The Cold Environment .Physiological Adaptations . Enzymes . Membrane Structure and Function . Biodiversity
Psychrophiles are extremophilic bacteria or archaea which are cold-loving, having an optimal temperature for growth at about 158C or lower, a maximal temperature for growth at about 208C and a minimal temperature for growth at 08C or lower. Psychrotrophs are cold-tolerant bacteria or archaea that have the abilityto grow at low temperatures, but have optimal and maximal growth temperatures above 15 and 208C, respectively. The Earth has extensive cold ecosystems that do not reach temperatures greater than 58C (e.g. worldwide deep oceans, polar surface, polar ice-cap regions, permafrost). Psychrophiles and psychrotrophs are cold-loving extremophiles adroitly adapted to these environmental conditions, andare often capable of enduring extended periods of cryobiosis.
Psychrophiles are cold-loving bacteria or archaea, whereas cryophiles are cold-loving higher biological forms (e.g. polar ﬁsh). Owing to precedence, the term has been retained. Morita (1975) deﬁned psychrophiles as organisms having an optimal temperature for growth at about 158Cor lower, a maximal temperature for growth at about 208C and a minimal temperature for growth at 08C or lower. The term, psychrotroph (also termed psychrotolerant), was retained to denote organisms that have the ability to grow at low temperatures, but have their optimal and maximal growth temperatures above 15 and 208C, respectively. The reason why the maximal growth temperature was set at 208Cwas simply because laboratory temperature in the USA is around 21–228C, which is not considered cold. Although it is recognized that there is a continuum of cardinal temperatures among the various thermal groups, the above deﬁnition is a useful one because it has relevance in terms of their respective ecological distributions, as psychrophiles are limited to permanently cold environments (Barossand Morita, 1978). The food and dairy microbiologists and, indeed, most microbiologists have accepted the foregoing deﬁnition. Psychrophiles were ﬁrst reported in 1884, but most of the early literature actually dealt with psychrotrophic bacteria and not with true psychrophiles. Since investigators were not working with extreme cold-loving bacteria, there was much debate and, as a result, many termswere coined to designate psychrophiles. These terms were cryophile, rhigophile, psychrorobe, thermophobic bacteria, Glaciale Bakterien, facultative psychrophile, psychrocartericus, psychrotrophic and psychrotolerant (Morita, 1975). This
proliferation of terms also resulted from the fact that no true cold-loving bacteria existed in the various culture collections. Ingraham (1962) wrote, ‘Otherauthors have felt that the term psychrophile should be reserved for bacteria whose growth temperature optima are below 208C if and when such organisms are found’. Because of this situation the research on true psychrophiles was neglected, especially when compared to the amount of research on thermophiles. The ﬁrst true psychrophiles, employing the foregoing deﬁnition, to be described taxonomicallyin the literature were Vibrio (Moritella gen. nov.) marinus (marina comb. nov.) MP-1 and Vibrio (Colwellia gen. nov.) psychroerythrus (psychrerythraea comb. nov.) in 1964 and 1972, respectively. Currently, the Arctic sea ice bacterium Psychromonas ingrahamii has demonstrated the lowest growth temperature (2128C with a generation time of 240 h) of any organism authenticated by a growth curve...