Biology Reference
In-Depth Information
Steneck and Watling
1982
). It is now well established that thallus morphology
defines much of the carbon physiology of large brown and red algae as well as some
siphonal Chlorophytes.
2.5.1 The Role of Storage Carbohydrates
Storage carbohydrates of seaweeds are normally formed in the chloroplasts. Starch,
a common compound of Chlorophyta and plants, is an insoluble storage polysac-
charide constituted by units of ADP-glucose, which is synthesized from the triose
phosphate generated in the Calvin-Benson cycle. Alternatively, triose phosphate
can be exported via an antiport system to the cytosol to form sucrose. A significant
part of the pool of sucrose is recycled to RuBP and thus the formation of starch or
sucrose in the cell is a highly regulated process, closely synchronized with
the carbon requirements of the Calvin-Benson cycle (Nelson and Cox
2004
). The
D
-glucose monomers are linked to form branched polymers of starch composed of
two types of chains:
-1,6-
D
-glucans
(amylopectin). In Rhodophyta, carbohydrates synthesized from carbon fixation are
stored as floridean starch, which is characterized by
a
1,4-
D
-glucans (amylose) and/or additional
a
a
1,4-
D
and
a
1,6-glucans. The
brown algae have storage laminaran (
b
-
D
-glucopyranose), a combination of soluble
and insoluble chains of the type
-1,6-
D
-glucans. Seaweeds contain also
important amounts of low-molecular-weight compounds such as sucrose (green
algae), mannitol (which can form part of laminaran chains of brown algae), and
floridoside (red algae). These compounds are not only reserve products but also
have a series of intracellular functions (e.g., osmolytes; see also Chap.
5
by
Karsten) or are also precursors of cell wall polysaccharides (reviewed by Craigie
1974
;W
ยจ
iwer et al.
2008
).
Seasonal variation in major organic compounds, especially carbohydrates, is well
known since 60 years and primarily based on studies of large brown algae, especially
Laminariales and Fucales (Black
1950
; Haug and Jensen
1954
; Jensen and Haug
1956
) and some Rhodophyta (Dawes et al.
1974
). Although these changes were
related to gradients of environmental variations (salinity, temperature, light, etc.),
the importance of these compounds in life strategy, morpho-functional processes, and
stress tolerance mechanisms was addressed later. Only after the classic works by
Chapman and Craigie (
1977
,
1978
), the relationship between nutrient availability,
growth, photosynthesis, and organic composition in Laminariales could be compre-
hensively understood. In these seaweeds, degradation of storage carbohydrates, which
are built up in summer (when net photosynthetic C assimilation occurs), supplies the
energy requirements for growth during high nutrient availability in winter-early spring
(Hatcher et al.
1977
). In species such as
Saccharina latissima
, mannitol and laminaran
vary from total absence in winter (4.5 and 0% DW, respectively) to high values close
to 26% DW in summer (Black
1950
). This strategy is extreme in the Arctic species
Laminaria solidungula
, which grow only in darkness in winter powered by the
carbohydrates (laminaran/mannitol) synthesized during the previous season. About
b
-1,3 and
b
