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Cold adaptation is dependant on the adaptive changes in the cellular proteins and
lipids in response to the conditions imposed on life by such extremely low temperatures
(Russell, 1990). The following sections deal with the definition of lower growth-viable
temperature and the range of different adaptations to allow psychrophiles to grow at these
temperatures. Mindock et al. (2001) suggest two major physical or physio-chemical
threats to bacterial survival when undergoing freezing:
1. Cell lysis due to increased water volume as ice.
2. Increased salinity outside of cell leading to an osmotic gradient across the
membrane.
1.5.1 -
Physio-chemical properties of water in confined spaces
Most theories on cold adaptation generally assume that water within a bacterial
cell behaves in the same way as bulk water (Mindock et al., 2001). This however is a
fallacy. Water in confined spaces (e. g. a bacterial cell) has very different properties to
those of bulk water systems (Mindock et al., 2001). This is caused by the effect of an
ordered surface upon water. The water molecules become highly ordered inducing a
lower freezing point, which alters ionic solubilities, increases viscosity and reduces the
dielectric constants compared to those of bulk water (Etzler, 1983). The ordering effect
can extend up to 1 µm from the surface, therefore organising the majority of the water
within a cell. The ordering of water on a cell membrane can dramatically reduce the
freezing point of that water and as such acts as a form of cryo-protection. The highly
ordered water near the cell membrane also has reduced solvent properties, thereby
forcing solutes towards the centre of the cell causing an increase in solute concentration,
thereby lowering the freezing point of the centre of the cell (Mindock et al., 2001).
1.5.2 -
Lower growth-viable temperature limit of psychrophiles
Reductions in temperature tend to reduce the conformation of cellular
macromolecules and other constituents thereby determining the enzymatic reaction rate
(Russell, 1990). This is described in the Arrhenius equation (Herbert, 1986):
K=Ae
-EST
where, Ea is the activation energy, A is a constant relating to steric factors and collision
frequency, R is the universal gas constant, T is absolute temperature (K) and K is the
relation between reaction rate and temperature for a given enzyme.
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