Individual species has the limitation in growth temperatures. For microorganisms, the limitation is strict because they cannot control their body temperature. Microorganisms can be classified into three categories with respect to their growth temperatures: mesophiles, thermophiles and psychrophiles. Mesophiles grow optimally at the temperatures around 20-50ºC. Thermophiles can grow at 50ºC and above. Psychrophiles grow at the temperature less than 20ºC. At low temperatures, meshophiles and thermophiles hardly grow, but psychrophiles are cable of growing, owing to their intercellular mechanisms that have been adapted to the low temperatures. A reason why mesophiles and thermophiles grow slowly at low temperatures is that the enzyme catalytic rate decreases according to the rule of Arrhenius. Psychrophilic enzymes have some mechanisms by which the enzymatic activities are not reduced at low temperatures.
The fluidity of cell membrane decreases as temperature decreases. This is another reason why mesophiles cannot grow at low temperatures. The fluidity of the membrane lipids is important for the function of membrane proteins. As the temperature decreases, the membrane lipid is transformed from liquid crystal to gel state, resulting in decreased fluidity. Increased fraction of unsaturated lipid is found in psychrophilic microbes, ensuring the sufficient fluidity at low temperatures. Thus, psychrophiles have adapted to the low growth temperatures.