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een well established (Russell & Nichols, 1999), however, not all marine psychrophiles
have PUFAs. It is therefore doubtful that this is a response to exposure to constant low
temperature (-1.9°C). It seems more likely that, because the proportion of PUFAs in
membrane lipids change with variations in temperature and may be maximal at the lowest
temperature (Hamamoto, et al., 1994; Russell & Nichols, 1999), they are part of a
homoviscous adaptive response to maintain the correct membrane fluidity phase (Russell
& Nichols, 1999). It appears that the increase in double bond formation with the PUFAs
causes a `hairpin' effect which reduces the effective acyl chain length (Keough & Kariel,
1987) providing a high degree of packing order whilst preventing the formation of an
hexagonal-II phase packing order (non-bilayer phase, which would inhibit membrane
permeability and kill the cell) and allowing sufficient molecular motion to lower the
liquid-crystalline phase transition temperature (Russell & Nichols, 1999).
Another thermal adaptive mechanism for microbial lipids is the isomeric distribution
of acyl chains between the sn-1 and sn-2 positions of the phospholipids which can alter
their gel-to-liquid-crystalline phase transition temperature by up to 7°C (Russell, 1990).
This combined with a high level of unsaturation can reduce the phase transition
temperature to well below 0°C. An example of this adaptive ability can be seen in the
psychrophilic bacterium, Psychrobacter urativorans (isolated from the Vestfold Hills,
Antarctica), which shows the sn-1 /sn-2 distribution of fatty acyl chains (permanently not
just in response to temperature change) which provides it with a maintained lower-
melting-point phospholipid state even after a sudden reduction of temperature
(McGibbon & Russell, 1983).
1.5.4.1 -
Biochemical mechanisms of fatty acid induction
Adaptations of the membrane lipid composition are important for enabling them
to compete successfully within their ecological niche (Russell & Hamamoto, 1998). The
ability to alter fatty acyl composition and the speed with which it can be achieved, are
related to the biomechanism. Short term changes (aerobic desaturation) and long term
changes (anaerobic desaturation, chain length and branching) can all be understood by
looking at the biomechanism (Russell, 1984).
The anaerobic desaturation pathway involves the insertion of a double bond using
fatty acid synthetase producing both saturated and unsaturated acids (Harwood and
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