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activity has experimentally been demonstrated in different seaweeds (Haglund et al.
1992
; Mercado and Niell
1999
). CA activities can vary considerably depending on
various environmental factors (e.g., temperature, UV radiation) and at different
temporal scales (Flores-Moya et al.
1998
;G
ยด
mez et al.
1998a
,
b
;Chooetal.
2005
).
Various surveys carried out in North Atlantic (Giordano and Maberly
1989
), Mediter-
ranean (Mercado et al.
1998
,
2009
), southern Chile (Huovinen et al.
2007
), and the
Arctic (Gordillo et al.
2006
) have confirmed that the CA-based inorganic carbon
acquisition is broadly extended in seaweeds, suggesting that this metabolic ability is
advantageous in coping with changes in the availability of ambient CO
2
.Other
mechanisms include the nondiffusive incorporation of HCO
3
via a specific trans-
porter or proton pump, e.g., ATPase, an OH
/HCO
3
antiport system, which have
been reported, e.g., in the brown alga
Laminaria digitata
(Klenell et al.
2002
), and the
green alga
Cladophora
(Choo et al.
2005
), or an anion exchanger at the plasmalemma
as has been postulated for
Ulva
sp. (Drechsler et al.
1994
). Many aspects dealing with
the nature of the transporter or the unbalance in the electrochemical potential across
membranes remain unknown, but apparently its operation does not preclude the action
of any intracellular CA (Raven and Lucas
1985
).
Unlike terrestrial C3 plants that base their inorganic acquisition on diffusive CO
2
entry, the majority of seaweeds exhibit functional CCMs (Raven
2010
). For example,
the model of carbon acquisition/assimilation in the Chlorophyta
Ulva
sp. is based on
the capacity of this alga to convert HCO
3
into CO
2
via external CA and also to
transport actively HCO
3
through the plasmalemma (Beer
1996
). If one considers
that at normal atmospheric CO
2
level (350 ppm, chloroplast flux of CO
2
of about
2.8 mM s
1
), the uncatalyzed rate of interconversion of CO
2
to HCO
3
is 10,000
times slower than the biological flux via CO
2
fixation by RUBISCO, then the action
of CA is necessary in seaweeds (Badger and Price
1994
). In the case of some
intertidal algae such as
Fucus
, CCMs have also been proposed to serve as an inhibitor
of the oxygenase activity of RUBISCO (photorespiration) during emersion periods
(Kawamitsu and Boyer
1999
) (see below). Overall, the ecological significance of
these mechanisms in seaweeds, as well as their prevalence in relation to phylogeny
and biogeography, has been proposed (Surif and Raven
1990
;Raven
1991
; Mercado
et al.
2009
). An increasingly relevant issue is the unpredictable effect of present and
future global change-driven increases in CO
2
concentration, which probably have
impact on carbon acquisition patterns of seaweeds (Raven et al.
2002
;Hurd
2000
;
Mercado et al.
2009
,seeChap.
19
by Roleda and Hurd).
2.3 Photosynthetic Carbon Fixation
2.3.1 Calvin-Benson Cycle and RUBISCO
The process of fixation of CO
2
into ribulose 1,5-bisphosphate (RuBP) to form triose
phosphate is denominated photosynthetic carbon reduction cycle or Calvin-Benson
cycle and occurs in the chloroplasts of seaweeds. The whole cycle consists of three
