Biology Reference
In-Depth Information
25% of the original carbon content of the thallus is depleted during the dark winter
period while it completes nearly 90% of its annual linear growth (Dunton and Schell
1986
). In Antarctic brown algae, due to the seasonally constant levels of nutrients, the
dynamics of synthesis and utilization of storage carbohydrates mostly depend on the
availability of light. In Antarctic Desmarestiales and Ascoseirales, depletion of
laminaran during spring and summer results in increases of mannitol, suggesting
that these compounds support requirements during lamina elongation (Drew and
Hastings
1992
;G´mez and Wiencke
1998
; Wiencke et al.
2009
).
In cold-temperate and Arctic Laminariales, the significant direct relation
between seasonal changes of amino acids to mannitol and the inverse relationship
between amino acids and laminaran content confirms that N availability regulates
remobilization of stored carbon (L
uppers and Kremer
1978
;
Cagn´ et al.
1982
). In the northern hemisphere, high ambient N supply in winter
exceeds substantially the N requirements for protein and amino acid synthesis and
free N is accumulated in surplus (Chapman and Craigie
1977
). In the case of
Antarctic algae, seasonal changes in mannitol and laminaran seem to be mainly
triggered by daylength, as seasonal carbon budget of
Himantothallus grandifolius
,
Ascoseira mirabilis
,, and
Desmarestia menziesii
is not affected by nutrients in
summer like in
Laminaria
(Drew and Hastings
1992
;G´mez and Wiencke
1998
;
G
´
mez et al.
1995b
;G
´
mez and Wiencke
1998
). As for kelps, Antarctic algae
suffer a photosynthetic carbon deficit during the growth period, i.e., carbon losses
due to anabolism and dark respiration exceed photosynthetic carbon fixation, which
may be compensated by reutilization of storage carbohydrates (G
´
mez and
Wiencke
1998
). In any case, daylength-dependent variations of storage
carbohydrates have been documented in cultured
Laminaria hyperborea
, a species
exposed generally to a severe
N
limitation in summer (Schaffelke
1995
). Further-
more, exposure to constant short day alters the seasonal growth cycle of
Laminaria
digitata
by preventing the decrease of growth rates in summer (G
´
mez and Luning
2001
), suggesting that not only nutrients but also photoperiodic responses are
crucial in carbon metabolism in this group of algae.
Figure
2.1
describes a model of synthesis, remobilization, and utilization of storage
carbohydrates during the growth phase for Laminariales. Laminaran, which is built up
mainly in distal regions of various species of kelps (K
uning et al.
1973
;K
€
€
uppers and Kremer
1978
;
€
L
uning
1979
; Cabello-Pasini and Alberte
2001a
), is degraded with release of manni-
tol, which is transported (along with various amino acids) via translocation to the
meristematic region (see below). Mannitol enters in the LICF pathway as a precursor
of 3-phosphoglycerate (PGA), which is transformed to phosphoenol pyruvate (PEP) in
the mitochondria. One molecule of mannitol generates two molecules of PEP: one can
be converted to oxaloacetate (OAA) by the enzyme PEP-CK following the biosyn-
thetic pathway in the Krebs cycle while the other is transformed to acetyl-CoA by the
successive action of pyruvate kinase (PK) and pyruvate dehydrogenase (PD) with loss
of one molecule of CO
2
, which is saved by PEP-CK (Kremer
1981
).
The glycolysis of mannitol, as well as the
14
C labeling of carboxylic diacids
(e.g., malate, aspartate) in the meristematic zones of the blade, has unequivocally
confirmed that growth in large brown algae can be supported by nonphotosynthetic,
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