Biology Reference
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differing in age and developmental stage (G
´
mez et al.
1995a
;G
´
mez et al.
1996
).
Histological studies have revealed the presence of medullar structures denominated
“conducting channels”. Apparently, putative translocation could occur only in
young plants, as early “conducting channels” are metabolically active, possess
plasmodesmata, and contain relatively few physodes (Clayton and Ashburner
1990
). Long-distance transport of substances has also been documented in members
of the brown algal orders Scytosiphonales (Guimaraes et al.
1986
), Desmarestiales
(Moe and Silva
1981
; Wiencke and Clayton
1990
), and Fucales (Moss
1983
). In
Rhodophyta, evidence for translocation of photoassimilates using
14
C labeling has
been obtained in
Polysiphonia
sp. (Wetherbee
1979
),
Delesseria sanguinea
(Hartman and Eschrich
1969
), and
Gracilaria cornea
(Gonen et al.
1996
). Although
the structures and probably the mechanisms of translocation in red algae are
different compared to brown algae, a relationship between carbon fixation and
translocation has been clearly demonstrated in
Gracilaria
(Gonen et al.
1996
).
2.5.3 Patterns of Carbon Allocation
Large and complex seaweeds show a differential allocation of carbon fixation
products along the thallus. Various brown algal genera such as
Sargassum
(Gorham
and Lewey
1984
),
Macrocystis
(Wheeler and North
1981
; Gerard
1982
),
Lessonia
(Percival et al.
1983
; Westermeier and G
´
mez
1996
),
Durvillaea
(Cheshire and
Hallam
1985
; Lawrence
1986
;G
´
mez and Westermeier
1995
), and
Desmarestia
(Carlberg et al.
1978
) show longitudinal variation in organic composition. Primar-
ily, changes in carbon allocation can be directly caused by differential capacity for
carbon uptake among parts of thallus. Using
13
C/
12
C ratios (
13
C), it was possible
to identify active HCO
3
uptake sites along the thallus of Antarctic seaweeds
correlated to growth activity (Wiencke and Fischer
1990
,
1992
). For example,
d
d
16.8% (indicating
13
C enrichment) were measured
in new blade regions of
Ascoseira mirabilis
during high irradiances and summer
daylength (G´mez
1997
). Apparently, enhanced carboxylation rates during high
light compensate for the energy costs of active HCO
3
incorporation by decreasing
the C supply via diffusive CO
2
entry, and thus the heavier C isotope is preferentially
assimilated (K
13
C values between
12 and
ubler and Raven
1994
; Raven et al.
1995
). On the other hand,
changes in light use and carbon fixation efficiency along with increasing thallus
size and age affect the carbon uptake and allocation. In cultures of
Desmarestia
menziesii,
€
13
C values
d
>
29
%
were found in small algae, but with increasing size,
13
C signatures increased accordingly (
d
32
%
) (G´mez
1997
).
Hydrodynamic processes regulate also the allocation of photoassimilated carbon
in the thallus. In many large brown algae, carbon (normally in the form of structural
carbohydrates) is preferentially allocated in the basal structures, which are biome-
chanically designed to attach algae to the substrate and to withstand drag forces
from water movement (Hurd
2000
). In the fucoid
Durvillaea antarctica,
characterized by large and floating laminar blades, 85% of the total energy contents
