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apparently indicates that, while present, the LICF pathway has much less impor-
tance in the whole carbon assimilation in temperate algae (Cabello-Pasini and
Alberte
2001b
). However, results from the two contrasting polar red algae, the
endemic
Palmaria decipiens
and the widely distributed
Iridaea cordata
, indicated
that accumulation of floridean starch allows the first species to overwinter in
darkness. In the case of
Iridaea
, use of floridean starch to power growth was less
marked. In both cases, LICF accounting for up to 9% of the total carbon assimila-
tion was active during a part of the dark period (Weykam et al.
1997
), suggesting
that nonphotosynthetic carbon metabolism in Rhodophyta may have similar eco-
physiological importance in extreme environments as has been reported for large
brown algae (Wiencke et al.
2009
).
2.5.2 Thallus Anatomy and Long-Distance Transport
of Photoassimilates
Unlike vascular plants, where long-distance transport represents the exchange of
resources between genuine, highly differentiated tissues, the transport of substances
in seaweeds, called translocation, is normally a strategy to redistribute via mass flux
diverse organic compounds towards zones of high metabolic activity (L
uning et al.
1973
; Schmitz
1981
). In Laminariales and Fucales, carbon metabolism is spatially
separated in carbon “source” and “sink” regions (K
€
uppers and Kremer
1978
;
Arnold and Manley
1985
; Cabello-Pasini and Alberte
2001a
). As is shown in
Fig.
2.1
for Laminariales, photoassimilates are stored in the mature, commonly
distal regions of the algae, and then transported as mannitol and amino acids to the
meristematic region. This morpho-functional arrangement is a consequence of the
allometric growth and the action of an intercalary meristem that normally results in
tissues with different metabolic activity.
Different types of sieve elements, such as the “trumpet” cells, have been
identified in members of Laminariales and Phyllariaceae (Buggeln
1983
; Schmitz
1981
;G´mez et al.
2007
). These structures are formed by specialized, normally
vacuolated cells that are longitudinally arranged in the medulla or below the cortex
in parenchymatous and peudo-parenquimatous thalli. Cell lengths measured in
different Laminariales vary considerably and can reach several millimeters. Most
of the sieve tubes end in the so-called sieve plates, which present pores of up to
100 nm diameter permitting the connection between adjacent cells (Schmitz
1990
).
Apart from Laminariales, other groups of seaweeds have been shown to have
sieve elements or at least a translocation function has been hypothesized. This is the
case of the Antarctic genera
Ascoseira
and
Himantothallus
, whose advanced
structural organization resembles that of
Laminaria
species from the northern
Hemisphere. For example, members of the Ascoseirales are characterized by a
strap-like lamina with an intercalary basally located meristem forming new tissue
during each growth phase. Thus, the blade in this species is formed by tissues
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