Cold tolerant microorganisms exhibit distinctly different properties than representatives of mesophiles and thermphiles. Microorganism with low temperature optima are generally referred to as psycrophiles (cold loving) and the definition given by R. Mortia in 1975 become widely accepted. He based definition of cold adapted bacteria on their cardinal growth temperature, viz. lower limit, optimum and upper limit. A psycrophile can be defined as an organism whose optimal growth temperature below 2 ºC and a minimal growth temperature at 0 ºC or below. Microorganisms which grow at 0ºC or at subzero but grow optimally at 20-30ºC are referred to as psycrotolerant (also named cold tolerant or psycrotrophic). Any such classification is artificial, and individual cold-adapted micro-organism may not fit the man-made definitions. Studies from Antarctic soil bacterial isolates may have both their optimum and upper limit between 15 and 20ºC, or they may have an average optimum that is 15ºCbut an upper limit of upto 20ºC. The main difference between the two groups is the facts that psycrotolerant have a much broader growth temperature range (30 to 40ºC) than do psycrophiles (~20ºC). Psycrotolerant may grow as fast as psycrophiles at low temperatures.
The new terms 'eurypsychrophile' and 'stenopsycrophile' have been proposed to substitute psycrophile and psycrotolerant, respectively. 'Steno-' and 'eury-' are ecological terms derived from Shelford's law of tolerance that describes a narrow or wide tolerance to an environmental determinant, respectively. The term stenopsycrophile ('true psycrophile') describes a microorganism with a restricted growth temperature range that cannot tolerate higher temperature for growth. Eurypsyhrophile ('psycrotolerant' or 'psycrotrophic') describes a Microorganism that likes permanently cold environment, but that can tolerate a wide range of temperature extending into the mesophilic range (that is 'mesotolerant', not 'psychrotolerant'). The term psycrophiles is effective as a general term that describes a microorganism that grows in cold environment. It is noteworthy that the term 'trophic' pertains to a nutritional state and is not a useful term for clarifying the temperature that can tolerate by any organism.
Such psycrotrophic microorganisms are much more widely distributed than true psycrophiles, they persist in permanently cold habitats, such as in Polar Regions, at high altitudes or seasonal temperature fluctuations (e.g., area in continental climates with high summer and low winter temperatures) are favorable to psycrotrophs, which grow over a wide temperature range and have fastest growth rates above 20ºC. Diverse microorganisms have remained viable within glacial ice cores for over 120,000 years. During last two decades, a number of investigations have reported, special focus has been given to microbial life in frozen natural habitats (snow, glacial and sea ice, permafrost, ice clouds), due to the increasing interest in life on distance frozen planets (atrobiology), genetic tools to create transgenic, and a realization of the considerable biotechnological potential of these organisms.
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