Biology Reference
In-Depth Information
cold conditions prevail (Druehl
1967
). All available data indicate that growth of
seaweeds primarily depends on radiation and temperature conditions, and the more
these environmental factors approach species-specific optimum requirements, the
broader is the salinity range tolerated. In addition, if seaweeds are exposed to
desiccation during low tides, growth is completely inhibited. The physiological
strategy is to cope with and survive this stress condition by maximum reduction of
all metabolic activities.
5.2.3 Photosynthesis and Respiration
Besides growth, photosynthesis and respiration are two central physiological pro-
cesses in seaweeds which are strongly affected by salinity changes (Kirst
1990
).
Under extreme hypo- or hypersaline conditions, photosynthesis and respiration are
typically completely inhibited and in many cases confirm the growth-salinity
relationships described above. Whether salinity stress is extreme depends on the
habitat and vertical zonation of the respective seaweed species, because sublittoral
taxa are generally much more sensitive and hence stenohaline than their eulittoral,
mainly euryhaline, counterparts (Russell
1987
). A relatively salt-insensitive photo-
synthesis and respiration seem to be a prerequisite for the successful occupation of
the eulittoral habitat and may ensure long-term survival and reproduction under
large amplitudes of salinity in combination with other environmental factors
(Gessner and Schramm
1971
). A similar relation regarding photosynthesis as a
function of desiccation was reported in various seaweeds from different tidal
heights (Wiltens et al.
1978
). In addition, photosynthesis and respiration exhibit
different responses under a range of salinities as documented for a set of eulittoral
green algae from Antarctica (Karsten et al.
1991a
; see also Chap.
13
by Wiencke
and Amsler). While most investigated species showed optimum photosynthesis at
34
S
A
and decreasing rates between 7 and 17
S
A
as well as between 51 and 68
S
A
,
respiration was much less affected. Similarly, under desiccation photosynthesis of
seaweeds is much more affected than respiration (Wiltens et al.
1978
).
The photosynthetic and respiratory responses following exposure to moderate
and high changes in salinity are inconsistent among seaweeds. Frequently, a
transient stimulation of respiration and a stimulatory or inhibitory effect on photo-
synthesis have been observed (Kirst
1990
). The time required for a more or less
complete recovery is species specific and lasted for several hours to days for
seaweeds (Kirst
1990
).
Using chlorophyll fluorescence kinetics, the underlying processes leading to
salt-induced inhibition of photosynthesis were studied in various eulittoral green
and red alga (
Prasiola
,
Ulva
,
Porphyra
) (Wiltens et al.
1978
; Fork and
ยจ
quist
1981
;
Satoh et al.
1983
; Smith et al.
1986
). These investigations were aimed primarily at
measuring the effects of desiccation on photosynthesis. The two stresses (increasing
salinity and desiccation) are comparable since they result in a reduction of the
cellular water potential. During desiccation, however, cellular ionic concentrations
