Biology Reference
In-Depth Information
conceived as fundamental adaptations for optimal allocation of energy in changing
environments (G
ยด
mez et al.
2007
).
In terms of its ecological importance, the photosynthetic transformation of
inorganic carbon into organic molecules carried out by benthic seaweeds accounts
for an important fraction of the coastal primary production and biomass. Further-
more, the fate of seaweed-derived organic material is central for the higher trophic
levels and geochemical processes of the coastal ecosystems (Mann
1973
).
2.2
Inorganic Carbon Acquisition
Seaweeds must use CO
2
dissolved in seawater as inorganic carbon source. In the
surface water the different species of inorganic carbon (CO
2
, HCO
3
, and CO
3
2
)
are in equilibrium in the so-called carbonate buffer system (see also Chap.
19
by
Roleda and Hurd). For example, at partial pressure of 365
atm, pH 8.1, 25
C, and
m
mol kg
1
, while HCO
3
salinity of 35 psu, the CO
2
concentration is close to 10.4
m
and CO
3
2
have values of 1,818 and 272
mol kg
1
, respectively. Thus, only a
small fraction (~0.5%) is in the form of CO
2
(Zeebe and Wolf-Gladrow
2001
).
Diffusion of CO
2
in water (0.16
m
10
4
cm
2
s
1
) is four orders of magnitude
lower than in air (0.16 cm
2
s
1
), which has important consequences for photosyn-
thesis (Badger and Spalding
2000
). In the case of aquatic organisms, the entry of
CO
2
into the cell is normally limited by the diffusion boundary layer, whose
thickness, and hence its resistance, depends on the form and volume of the alga
as well as the speed of the water flow around it. For example, for an aqueous phase
system with a diffusion coefficient of 1.5
10
5
m
2
s
1
and a boundary layer of
m, a maximum flux of CO
2
close to 2.6 m
2
s
1
can be estimated (Falkowski
and Raven
1997
). Due to these constraints the sole diffusive entry of CO
2
does not
support photosynthetic demands and thus algae can suffer carbon limitation. This
situation has been documented for some subtidal red algae that apparently rely on
CO
2
diffusion as the only mechanism of inorganic carbon uptake (Raven and
Beardall
1981
; Maberly
1990
). The majority of seaweeds, however, have developed
the capacity to concentrate CO
2
in order to guarantee an adequate supply to
RUBISCO. One of the most efficient carbon concentrating mechanism (CCM) is
the active transport not only of CO
2
but also HCO
3
, which accounts for up to 95%
of the total dissolved inorganic carbon in seawater (reviewed by Raven
2010
; see
is converted to CO
2
in the vicinity of RUBISCO (Badger and Price
1994
). Similar
to strict CO
2
users, there is depth dependence in the ability of seaweeds to use
HCO
3
with a tendency of higher affinity for HCO
3
in seaweeds occupying
the upper littoral zones compared to their counterparts from deeper locations
(Sand-Jensen and Gordon
1984
; Mercado et al.
1998
; Murru and Sandgren
2004
).
The principal way by which algae utilize HCO
3
is through the enzyme carbonic
anhydrase (CA). This enzyme catalyzes the interconversion between HCO
3
and CO
2
in distinct sites outside or inside the cell (Badger and Price
1994
; Badger
2003
)andits
15
m
