Biology Reference
In-Depth Information
Fig. 3.3
Electron micrographs of cross-sections of
Valonia utricularis
cells, showing chloroplast
ultrastructure of the cells. Scale bars measure 1
m. (
a
) Warm-temperate, Mediterranean isolate
grown at optimal temperature (25
C), (
b
) warm-temperate, Mediterranean isolate grown at slight
cold stress (15
C), (
c
) tropical, Indian Ocean isolate grown at severe cold stress (18
C). Cells were
fixed in situ with 6.0% (w/v) glutaraldehyde in 0.1 M Na-cacodylate buffer (pH 7.2) overnight at
0
C, washed three times with 0.1 M Na-cacodylate buffer (pH 7.2), and postfixed (15 min at room
temperature) with 1.5% (w/v) KMnO
4
. Cells were washed with distilled water and stained in 1.0%
(w/v) uranyl acetate overnight. Cells were then dehydrated in an upgrade series of ethanol and
embedded in Epon 812 (for more details, see Eggert
2002
)
m
All these types of damage cause reduced photosynthesis and carbon assimilation
and ultimately cell dysfunction and cell death.
However, many seaweeds, particularly from temperate regions, can evolve
greater resistance to temperature stress via increased tolerance or activation of
recovery mechanisms. Both types can serve to extend their temperature range
for survival during acute temperature stress. For instance, Davison (
1987
) reports
an acquired high-temperature tolerance of photosynthesis in sporophytes of
Saccharina latissima
grown at 10-20
C compared to specimens grown at 0-5
C
due to changes in the thermal stability of the photosystem II electron transport
system. Similarly,
Chondrus crispus
grown at summer seawater temperatures
(20
C) maintains constant rates of light-saturated photosynthesis at 30
C for 9 h,
while photosynthesis of 5
C-grown algae declined rapidly within 10 min following
exposure to 30
C(Kubler and Davison
1993
). Cold-acclimated (15
C)
Valonia
utricularis
exhibited a faster recovery kinetics from chilling-induced photoin-
hibition than specimens grown at 25
C (Eggert et al.
2003b
), which was related
to a faster recovery from chronic photoinhibition.
Increased thermal tolerance involves a large suite of processes that modify the
cellular metabolism. Again, cellular and molecular investigations in seaweed spe-
cies are scarce, but the following examples cover a wide range of processes: (1) Heat
shock proteins are accumulated as they function as molecular chaperones that
protect cellular proteins from protein mis-folding and degradation by environmental
stress, including cold and heat stress (Sorensen et al.
2003
). Henkel et al. (
2009
)
suggest a high thermal tolerance of the invasive kelp species
Undaria pinnatifida
in
the northeast Pacific (California) due to a high expression of the hsp70 gene which
encodes heat shock protein 70. (2) The unsaturation of fatty acids increases mem-
brane fluidity and such an increase is necessary if cells are to tolerate cold stress and
to survive low temperatures (Murata and Los
1997
). Increasing proportions of poly-
unsaturated fatty acids and, consequently, a higher degree of unsaturation in
Caulerpa racemosa
from the northern Adriatic coincided with a sharp decrease in
