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environments. Cold adapted bacteria have since been isolated from many permanently
cold environments such as Arctic and Antarctic fish, deep ocean waters. polar soils and
Antarctic marine and fresh water systems (Herbert, 1986).
One of the most important conditions imposed by low temperature is the freezing
point of water, which can fluctuate due to solute concentrations in the environment (pure
water = 0°C, sea water (38 parts per thousand (ppt) salinity) = -1.9°C) (Feller and
Gerday, 1997). The transition from fluid to crystalline state limits essential enzymatic
reactions, and ice crystal growth can cause irreparable cellular damage (Russell, 1992).
The following sections will describe the adaptive mechanisms used to survive in extreme
low temperature environments, the evolution of such mechanisms and their
biotechnological potential.
1.3 -
Psychrophiles and psychrotrophs
Microorganisms constitute the most temperature-adapted organisms on the planet
(Russell, 1990; Russell & Hamamoto, 1998). The majority of known bacterial
phenotypes demonstrate optimal growth within the 30-40°C range, however, as a group,
they show a continuum of thermal adaptation growing at temperatures from sub-zero to
boiling point (Russell, 1990). In terms of temperature response, microorganisms can be
classified into three groups, psychrophiles, mesophiles and thermophiles.
Psychrophilic microorganisms are further classified into two groups,
psychrophiles and psychrotrophs. Individuals in both groups are capable of growing at or
close to 0°C (Russell & Hamamoto, 1998). Morita (1975) defines psychrophiles as
organisms having an optimal growth temperature