Biology Reference
In-Depth Information
Temperature limitations of growth and photosynthesis are a direct effect of
temperature sensitivity of the main cellular components, i.e., proteins and
membranes. Evolutionary adaptation to local temperatures on the cellular level
can involve one or more of the three strategies: quantitative (changing the
concentrations of enzymes and/or reactants); qualitative (using a protein variant/
isozyme with different thermal characteristics); or modulation (modifying the pro-
tein environment to minimize the impact of temperature change) (Hochachka and
Somero
2002
). The most important adjustments include changes in enzyme concen-
tration, changes in primary structure affecting the free energy of activation, and
modification to both membrane properties and intracellular milieu. These responses
are genotypic, but similar changes may be induced by acclimation on a daily or
seasonal timescale. The energetic costs of the different strategies of temperature
adaptation vary (Clarke
2003
). If protein variants/isozymes vary by only a few
amino acids, it costs an organism effectively no more ATP to make a molecule of
one variant than it does to make another. However, energetic costs become relevant
when a particular protein is required in larger amounts, or is turned over faster.
Trade-offs with other metabolic processes and changes in energy budgets need to
take place. These aspects have been analyzed in polar ectothermic marine animals
(P
ortner et al.
2005
), but remain to be investigated in polar seaweeds.
Specific knowledge of thermal physiology of proteins is limited to a very small
subset of proteins. Clear patterns of adaptive variation have been discovered in
structural and functional properties of proteins from ectothermic marine animals
adapted to different temperatures (Somero
2004
). Studies of dehydrogenase
enzymes have demonstrated that a single amino acid substitution is sufficient to
cause temperature-adaptive changes in function and stability and define species
geographic boundaries (e.g., Fields and Houseman
2004
; Dong and Somero
2009
).
Studies on temperature adaptation are missing for seaweeds and are scarce for
unicellular algae. Descolas-Gros and De Billy (
1987
) showed that the RuBisCO in
marine Antarctic diatoms effectively binds CO
2
only at low temperatures. They
describe qualitative changes, i.e., an isozyme with modified kinetic properties
which allows the maintenance and regulation of RuBisCO activity in Antarctic
diatoms at low temperatures (minimum
K
m
value at 4.5
C for Antarctic species
compared to 20
C for temperate species). In contrast, Antarctic
Chloromonas
species did not show an increase in activity at low temperatures (0-5
C) compared
to temperate species, which was counterbalanced by increasing enzyme concentra-
tion, i.e., quantitative adjustments (Devos et al.
1998
).
In addition to proteins, several physical properties of membranes are tempera-
ture sensitive, including permeability characteristics, fluidity, and membrane phase
state (for reviews, see Murata and Los
1997
; Los and Murata
2004
). All are crucial
to many membrane functions, including the activities of enzymes. A number of
studies focused on the effects of low temperature and clearly demonstrated that
membrane fluidity decreases with a decrease in temperature (Szalontai et al.
2000
;
Inaba et al.
2003
). The effects of high temperature on the physical state of
membranes have also been studied, albeit less intensively (Carrat` et al.
1996
;
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