Biology Reference
In-Depth Information
Complementing these culture-independent studies, the last few years
have seen an increase in genomic sequences of cultured isolates. The study
of individual genomes facilitates the characterisation of physiological adap-
tations to the specific polar conditions. Nevertheless, in contrast with the
large diversity observed with molecular techniques, the phylogenetic
breadth of the taxa with at least one representative genome sequence is lim-
ited to a few genera (
Fig. 8.1
).
In view of the high degree of temporal and spatial variability observed in
polar environments, which positively correlates with changes in microbial
community structure and function, there is a pressing need for increasing
culturing efforts and single-cell genomic analysis targeted at under-
represented phyla. These should be integrated within the larger framework
of global organismal biogeography and ocean models.
3. THE ROLE OF TEMPERATURE IN EVOLUTIONARY
ADAPTATIONS
The bulk of the Earth's biosphere is cold (e.g. 90% of the ocean is
below 5
C), sustaining a broad diversity of microbial life. Evolution under
extreme conditions has been marked by a suite of adaptations (evolutionary
gains) including the development of proteins that function optimally in the
cold. A commonly accepted view for protein cold adaptation is the
activity/stability/flexibility relationships. Although active sites are generally
highly conserved among homologous proteins, adaptive changes may
occur at recognition site(s). These alterations in the strength of subunit
interactions may affect thermal stability and energy changes associated with
conformational transitions due to ligand binding (
D'Amico, Collins, Marx,
Feller, & Gerday, 2006
).
Comparative genome analysis indicates that the cold-adapted lifestyle is
generally conferred by a collection of changes in the overall genome content
and composition. The flexible structures of enzymes from cold-adapted bac-
teria compensate for the environment's low kinetic energy.
In cold environments, challenges to cellular function and structural
integrity include low rates of transcription, translation and cell division,
inappropriate protein folding and cold denaturation, as well as intracellular
ice formation (
D'Amico et al., 2006
). The ability of an organism to survive
and grow in the cold is dependent on a number of adaptive strategies
(
Table 8.1
) to maintain vital cellular functions at cold temperatures
(
Rodrigues & Tiedje, 2008
). These strategies include the synthesis of
