In recent years, growing attention in research has been devoted to cold-adapted microorganisms. They successfully colonize cold habitats, which compose more than 80% of the earth's biosphere and play a major role in the processes of nutrient turnover at low temperature. Polar regions are of interest since they provide diverse terrestrial and marine habitats for psychrophilic microorganisms. Temperature can influence the response of microorganisms either directly, by its effects on growth rate, enzyme activity, cell composition and nutritional requirements or indirectly by its effects on the solubility of solute molecules, ion transport and diffusion, osmotic effects on membranes, surface tension and density. With the decreases in temperature, the lag phase extends before growth phase, and the growth rate decreases thus the final cell numbers may decrease, many physiological changes occur, including a decrease in the saturation of fatty acids and inhibition of DNA, RNA and protein synthesis . Microbes are able to withstand harsh conditions, and are often submitted to rapid variations of temperature. Various environmental factors that condition the viability of microorganism growth and physiology, temperature is of particular interest since it affects the interior of the cells. According to their ability to grow at high, intermediate or low temperature, microorganisms have been divided into three broad categories viz., thermophiles, mesophiles and psychrophiles respectively. Psychrophiles, which have optimal growth temperature below 15ºC and an upper limit 20ºC and psychrotrophs (Psycrotolerant), which are able to grow at 0ºC or below and optimally at temperature around 20-25ºC.
All major taxa inhabits temperature just below 0ºC ,many microbes and cell lines can be preserved successfully at -196ºC (liquid nitrogen), but the lowest recorded temperature for active microbial community is substantially higher, at -18 ºC, more over the Antarctic bacteria have potential in bioremediation of waters following oil spills, which is a concern in cold water.
Growth of microorganisms at temperature below the optimum for growth can cause a number of physiological and morphological changes as describes by many authors. Cold adapted, or psychrophilic, organisms are able to thrive at low temperatures in permanently cold environments, which in fact characterize the greatest proportion of our planet. Psychrophiles include both prokaryotic and eukaryotic organisms and thus represent a significant proportion of the living world. These organisms produce cold-evolved enzyme that are partially able to cope with the reduction in chemical reaction rates induced by low temperatures. A general principle is that psychrophiles produces enzymes that function effectively at cold temperatures in comparison with enzymes from mesophilic or thermophiles that have little or no activity in the cold.
Bacteria are notorious for their ability to colonize different environments. Additionally the particular niche or lifestyle of many bacteria may be subjected to regular, but sudden, variations in temperature. This reasoning must apply for bacteria adjusting their activities according to seasonal variations and certainly for pathogens that circulate between the environment, vectors and hosts. Microbes respond to a decrease in temperature in a specific manner. A temperature downshift results in inhibition of cell growth and proliferation and changes in protein expression patterns and synthesis of most cellular proteins in inhibited after a decrease in temperature. However a number of proteins are found to be induced under cold shock conditions. Expression of the cold shock proteins reaches a maximum level during the phase of the cold shock adaptation, the so called cold acclimation.
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