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Jagannadham, 2001). A decrease in temperature can cause formation of deleteriously
strong lipid-lipid
interactions (Russell & Hamamoto, 1998) causing rigidity through loss
of liquid-crystalline
phase which leads to loss of membrane transport mechanisms
(impairing ion and solute regulation) and can cause membrane rupture.
The most important response of membrane lipids to temperature change are
alterations in the fatty acyl composition (Russell, 1984,1990). A decrease in growth
temperature can cause one or a combination of the following responses:
9 Increase in unsaturation
" Decrease in average chain length
" Increase in methyl branching
" Increase in the ratio of anteiso branching relative to iso branching. (Russell &
Hamamoto, 1998).
Unsaturation of the fatty acids is the most common adaptation to cold stress (Rotert et
al., 1993; Russell & Hamamoto, 1998; Russell & Nichols, 1999, Mindock et al., 2001),
but changes in chain length frequently occur. Changes in methyl branching are found
only in bacteria and mostly in Gram positive genera. As a rule Gram negative genera
employ unsaturation and acyl chain length alteration in cold adaptation (there are
relatively few Gram negative genera containing branched fatty acids), whereas Gram
positive genera tend to alter the amount and type (anteiso/iso) of methyl branching whilst
also utilising changes in chain length and unsaturation.
The majority of bacteria do not contain polyunsaturated fatty acids (PUFAs; Rotert et
al., 1993; Russell & Hamamoto, 1998; Russell & Nichols, 1999) therefore the above
changes relate to monounsaturated fatty acids. Until 1977 it was considered that nonphotosynthetic
bacteria did not produce PUFAs (for a review of the history of discovery
of polyunsaturated fatty acids refer to Russell & Nichols, 1999). However, since 1977
there have been numerous reports of PUFA in a range of eubacterial species (Johns &
Perry, 1977; Delong & Yayanos, 1985; Wirsen et al., 1987; Temara et al.,
1991; Intrigo,
1992). PUFAs are mainly found in psychrophilic bacteria, examples of which show an
increase in PUFA with a decrease in temperature (Russell, 1990). Some examples of the
complexity of adaptive mechanisms shown by Antarctic psychrophilic bacteria is
provided by Nichols et al (1997). The presence of PUFAs in Antarctic psychrophiles has
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