Biology Reference
In-Depth Information
grows between 0.3 and 105
S
A
(Jacob et al.
1991
). In contrast to the generally broad
salinity tolerance of upper-shore seaweeds, those from the sublittoral usually
exhibit the narrowest tolerance limits (Russell
1987
). Salinities commonly encoun-
tered in areas of abundance are most favorable for growth (Bird et al.
1979
; Bird
and McLachlan
1986
). This is well reflected in the growth pattern of the green alga
Ulva pertusa
from an eelgrass bed in a semi-protected bay at the southwest coast
of Korea (Choi et al.
2010
).
Ulva pertusa
exhibited optimum growth at 20
S
A
,
a situation encountered in the field during the rainy season when this species often
forms blooms in eelgrass beds. Growth under hyposaline conditions, i.e., in aquatic
systems such as estuaries or the eastern part of the Baltic Sea, may be governed
by the availability of certain inorganic ions which increase the lower tolerance
limits of seaweeds. In this context, the presence of Ca
2
þ
plays an essential role in
cell signaling, as structural component of seaweed cell walls and membranes, and
as cation to balance organic anions in the plant vacuole (Kauss
1987
; Tazawa et al.
1987
; Verret et al.
2010
). Whole seaweed thalli have been reported to exhibit
differential salinity tolerances. Particularly young apical growing parts of species
of the genera
Cladophora
,
Ceramium
,
Phycodrys
, and
Plumaria
are more sensitive
to hyposaline conditions than older, basal parts (Russell
1987
). Kirst (
1990
)
speculated that this observation is a secondary effect of Ca
2
þ
availability, as
particularly fast-growing cells depend on this cation, for example, for cell wall
formation.
If not only growth but also survival is considered, most seaweeds show a
remarkable physiological potential. While the red alga
Porphyra umbilicalis
exhibits optimum growth between 7 and 52
S
A
, it survived without cell division
even in sixfold fully marine salinities for 2 weeks (Wiencke and Lauchli
1980
).
Similar observations have been reported in the studies of elongation growth in the
siphonous green alga
Valonia macrophysa
(Gutknecht et al.
1978
). Near the limits
of salinity tolerance, growth of most studied seaweeds is typically strongly reduced
or even completely inhibited in order to funnel all available metabolic energy into
the process of osmotic adjustment, which guarantees survival under fluctuating
salinities. Besides these energetic considerations, high inorganic ion concentrations
under salt stress conditions exert negative, i.e., inhibiting effects on seaweed
growth. This is reflected in conspicuous changes in size and morphology of
seaweeds under long-term salt stress (Russell
1987
).
If additional environmental factors that greatly affect the growth rate of
seaweeds as well are included in the investigation of the growth-salinity relation-
ship (e.g., radiation (UV), temperature, etc.), the emerging picture becomes very
complex. Therefore, only a few macroalgal species have been investigated in this
respect, such as the green algae
Cladophora glomerata
and
C. rupestris
(Thomas
et al.
1988
), the red alga
Polysiphonia lanosa
(Reed
1983
), or the kelp species
Laminaria groenlandica
and S
accharina latissima
(Druehl
1967
). Low salinity
may be compromised by temperature as shown for North Pacific
L. groenlandica
which cannot tolerate the combination of low salinity and high temperature
conditions encountered in areas subjected to snow-melt runoff, whereas
S. latissima
can. Both species, however, do well in areas subjected to winter rain runoff where
