Seaweeds and Their Communities in Polar Regions - Seaweed Biology - page 263

During this time, up to 30% of its original total carbon content is depleted before
photosynthetic production begins in spring. During summer, high photosynthetic
rates are used for carbon storage and not for biomass formation (Dunton and Schell
1986 ; see also Chap. 2 by G ´ mez and Huovinen).
In the Antarctic red algal season anticipator Palmaria decipiens , photosynthetic
rates are—like the growth rates—highest in spring. There is a positive correlation
between the phycobilin content, photosynthetic capacity and efficiency, which are
highest in fall, winter, and spring. During summer, the alga reduces the photosyn-
thetic apparatus to a minimum (Luder et al. 2001a ). The presence of two
phycobilisome forms with different aggregation states has been regarded as special
advantage for a rapid acclimation to changing environmental light conditions
(L

uder et al. 2001b ; see also Chap. 1 by Hanelt and Figueroa).
The effect of darkness on physiological performance has been studied in
Palmaria decipiens and Iridaea cordata .In P. decipiens , the light harvesting
phycobilisomes and later, the chl a containing inner antennae are degraded during
long-term exposure to darkness. After 6 months, the alga has lost its ability to
photosynthesize. Following reexposure to light, pigments are rapidly synthesized
and after a week photosynthesis recovers to normal levels (L

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uder et al. 2002 ;
Weykam et al. 1997 ). In contrast, the season responder I. cordata maintains a
functional photosynthetic apparatus during dark-exposure and is therefore better
suited to grow in places with less predictable light conditions (Weykam et al. 1997 ;
see also Chap. 1 by Hanelt and Figueroa).
Overall, seasonal development and physiological performance exhibit many
similarities with temperate seaweeds. Although there is no unique mechanism
occurring only in polar species, their efficient adaptations to low light, however,
allow Arctic and Antarctic species to thrive with great success in polar waters.

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13.3.2 Radiation Climate and Depth Zonation

In polar regions, the radiation climate imposes severe constraints not only with
respect to seasonal light availability but also with respect to the irradiance level in
different water depths ultimately determining seaweed zonation. As polar algae are
mainly sublittoral, low light tolerance is a prerequisite for distribution down to great
depths. This becomes obvious when the minimum light requirement for completion
of the life history is considered, which is lower in polar seaweeds compared to
temperate, morphologically similar species. For Antarctic Desmarestiales, the
minimum annual light demand is 31 moles photons m 2 (Wiencke 1990a ) and for
Laminaria solidungula 45-49 moles m 2 (Chapman and Lindley 1980 ; Dunton
1990 ). In contrast, L. hyperborea from the North Sea requires 71 moles m 2 per
year (L uning and Dring 1979 ).

Another prerequisite for algal life in polar waters is a capacity to tolerate long
periods of darkness. Various polar seaweeds tolerate darkness for up to 18 months
(tom Dieck 1993 ; Wiencke 1990a ). Growth in the microscopic stages of Antarctic

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