Seaweed Responses to Ocean Acidification - Seaweed Biology - page 397

In 2005, a Royal Society Report highlighted the potential effects of OA on
marine organisms. Initial research has focused on how the lowered availability of
carbonate ions might impact marine calcifiers, particularly corals, calcareous
micro- and macroalgae, and other shell-forming invertebrates. However, because
the relative availability of CO 2 and HCO 3 ¯ will also change and algae use one or
both of these as a CO 2 source for photosynthesis, OA has the potential to affect the
rates of photosynthesis, and hence growth and productivity, of all algae.

19.3 Carbon Sources and Acquisition

In seawater, there are two sources of inorganic carbon (C i : also known as Dissolved
Inorganic Carbon, DIC, and Total Inorganic Carbon, C T ) to seaweeds, HCO 3 ¯,
which is available in high concentrations (2,200 M). Most

seaweeds are able to take up HCO 3 ¯ and/or CO 2 by active transport (termed a
carbon concentrating mechanism, CCM), while some seaweeds (mostly reds) rely
on CO 2 uptake by passive diffusion (Hurd et al. 2009 ; see also Chap. 2 by G´mez
and Huovinen and Chap. 4 by Gordillo).
The enzyme carbonic anhydrase (carbonate dehydrase) EC 4.2.1.1 (CA)
catalyses the reversible hydration of CO 2 and is widely distributed not only in
plants but also in animals and prokaryotes. CA is synthesized within the cytoplasm
but may also be found associated with the chloroplast. They are also found on
the plasmalemma, where they have access to the external medium (Giordano and
Maberly 1989 ). Although the significance of CA is not yet fully understood, it is
generally believed to play a role in carbon acquisition and photosynthesis, where
high CA activity is correlated to aquatic plant's ability to concentrate CO 2 inter-
nally in response to low CO 2 concentrations in the growth medium.
Inorganic carbon concentrating mechanisms (CCMs) cause the accumulation of
CO 2 around RUBISCO (ribulose-1,5-bisphosphate carboxylase oxygenase) in
organisms capable of oxygenic photosynthesis (e.g., all cyanobacteria, most algae
and aquatic plants, and in C 4 and crassulacean acid metabolism (CAM) of vascular
plants). In algae, CCMs are polyphyletic (more than one evolutionary origin) and
involve active transport of HCO 3 ,CO 2 , and/or H þ , or an energized biochemical
mechanism similar to C 4 and CAM plants (Raven et al. 2008 ). CCMs involve both
the active transport of HCO 3 and/or CO 2 and the catalytic conversion of HCO 3 to
CO 2 with subsequent passive diffusion of CO 2 . CCMs effectively alleviate the low
CO 2 affinity of the enzyme RUBISCO which is less than half saturated under
current dissolved CO 2 concentrations [CO 2(aq) ]. Different types of CCM are present
in different phototrophs (Table 1 in Giordano et al. 2005 ) and different models for
inorganic carbon transport and CCM are schematized in cyanobacteria (Fig. 1 in
Giordano et al. 2005 ) and eukaryotic algae (Figs. 1 and 2 in Thoms et al. 2001 , and
Moroney and Ynalvez 2007 ; Fig. 1 in Giordano et al. 2005 ). While pyrenoids in
eukaryotic algae play an important role in CCMs (Badger et al. 1998 ), their absence
does not necessarily imply the absence of a CCM (e.g., Caulerpa scalpelliformis ,
Kevekordes et al. 2006 ).

m M), and CO 2 (14 m