Biology Reference
In-Depth Information
seaweeds is light-saturated at photon fluence rates as low as 4-12
mol photons
m
2
s
1
(Wiencke
1990a
). In young sporophytes of Antarctic Desmarestiales the
values are somewhat higher, at 15-20
m
mol photons m
2
s
1
(Wiencke and Fischer
m
1990
).
As with growth, the light demands for photosynthesis are also very low. Species
from both polar regions show a high photosynthetic efficiency (
), low respiratory
rates, low saturation points for photosynthesis (
E
k
), and low compensation points
for photosynthesis (
E
c
;G´mez et al.
2011
).
E
k
values range between 3 and 100
m
mol photons m
2
s
1
, and
E
c
values between
<
1 and 15
m
mol photons m
2
s
1
,
values usually lower than for species from temperate regions (Luning
1990
).
Generally,
E
k
values for photosynthesis are higher than the irradiances required
for saturation of growth. This represents an important ecological advantage for
coping with the strong fluctuations of incident irradiance during the open water
period. While growth is saturated at low irradiances, the photon fluence rates above
the saturation point for growth can be used for purposes other than growth, e.g., for
formation of storage compounds. For example, Antarctic seaweeds growing at
depths below 20 m are often exposed to irradiances around 80
a
mol photons
m
2
s
1
during late winter/spring (G´mez et al.
1997
) allowing considerable
carbon fixation, fuelling—aside from growth—other metabolic processes.
The light requirements for photosynthesis are an important factor for the deter-
mination of zonation patterns. If one relates the daily light course of the irradiance
to the
E
k
value, it is possible to estimate the average daily period of light saturation,
called
H
sat
. The obtained metabolic daily carbon balance is regarded as a physio-
logical indicator for the ability to live in deep waters.
Laminaria solidungula
in the
Alaskan Beaufort Sea at 70
N was for example exposed in 1986 to total
H
sat
periods
of 148 h (Dunton
1990
), corresponding to an average daily
H
sat
of 3 h. For five red
and brown algae from King George Island (Antarctica),
H
sat
determined during the
clear water period in spring decreases with depth from values close to 14 h at 10 m
to values between 7 and 12 h at 30 m depths (G
´
mez et al.
1997
). For the red algal
species
Palmaria decipiens
,
Trematocarpus antarcticus
, and
Gigartina skottsbergii
the carbon balance was between 1.7 and 2.5 mg C g FW
1
d
1
at 10 m depth and
between 0.6 and 0.8 mg C g FW
1
d
1
at 30 m depths, setting the lower depth limit
at
m
30 m. For the kelp-like brown alga
Himantothallus grandifolius
, the daily
carbon balance varied between 0.6 and 1.0 mg C g FW
1
d
1
over the studied
range of 10-30 m, indicating that this species can potentially occur even deeper,
which is actually the case. In contrast, in the brown alga
Desmarestia anceps
the
negative (!) carbon balance of
>
1.9 mg C gFW
1
d
1
limits the alga to depths of
about 30 m at this location (G´mez et al.
1997
). Based on photosynthetic
measurements, the lower depth distribution limit of the red alga
Myriogramme
manginii
at Signy Island (South Orkney Islands) has been predicted to be at approx.
23 m water depth (Brouwer
1996a
).
Polar seaweeds are not only strongly shade-adapted but can also cope with high
light conditions in summer because of their ability for dynamic photoinhibition, a
photoprotective mechanism, by which excessive energy absorbed is rendered
harmless by thermal dissipation (see Chap.
1
by Hanelt and Figueroa).
