Biology Reference
In-Depth Information
A number of seaweeds lack a CCM and rely solely on CO
2
diffusion from the
external environment (Raven et al.
2005
). In order to carry out net C assimilation
under current CO
2
and O
2
levels, RUBISCO of the organism should have high
selectivity factor (
S
rel
) that defines the relative rates of carboxylase and oxygenase
reactions to diffusive CO
2
supply (Giordano et al.
2005
). Macroalgae lacking
CCMs are often found inhabiting fast-flow environments, to minimize the diffusion
boundary-layer thickness (see Sect. 19.7.1), or in deep water with low light levels
where a smaller CO
2
flux is required to satisfy cellular assimilatory requirements
(Sherlock and Raven
2001
; Raven et al.
2002a
,
b
; Hepburn et al.
2011
). Oxygenic
photolithotrophs have a diversity of incompletely understood mechanisms of inor-
ganic carbon acquisition that require further studies. Raven (
2010
) recommended
four areas of research where more studies are needed: (1) Diffusive CO
2
entry,
where a number of algae are, in various respects, intermediate between diffusive
CO
2
entry and occurrence of a CCM, (2) the nature and role of cyanelles (plastids)
in organic carbon assimilation, (3) the energetics of CCM in
Chlamydomonas
reinhardtii
, the eukaryotic alga with the best understood CCM, and (4) the occur-
rence of C
4
-like metabolism in the CCMs of marine diatoms.
Methods used to determine whether an alga is a HCO
3
or CO
2
user include:
(1) pH drift experiments, with HCO
3
-using algae being capable of raising pH of
the surrounding medium to values in excess of those attained by species using only
CO
2
, (2) pH dependence of half saturation constants,
K
0.5
(HCO
3
)or
K
0.5
(CO
2
),
(3) HCO
3
utilization pathway inhibitors, (4) isotope disequilibrium, and (5)
membrane-inlet mass spectroscopy (Giordano et al.
2005
). Aspects of inorganic
carbon acquisition, metabolism, and carbon isotope discrimination by marine
macroalgae are discussed by Kremer (
1981
), Johnston (
1991
), Maberly et al.
(
1992
), and Raven et al. (
2002b
).
Applying one or a combination of these various techniques shows that most
algae examined can take up both HCO
3
and CO
2
. However, there is some
evidence that some algae can take up only CO
2
while other species can only
actively transport HCO
3
. Using the natural abundance of
13
C/
12
C ratios (as
13
C values lower than -30
‰
rely on
diffusive CO
2
supply to RUBISCO (Maberly et al.
1992
) while organisms with
d
13
C) as a proxy, marine macroalgae with
d
d
13
CofCO
2
in seawater) must
involve HCO
3
use (Raven et al.
2002b
). In combination with pH drift experiments,
HCO
3
-using macroalgae can shift seawater pH to 9.0 or higher (Maberly
1990
)
and have corresponding
13
C higher than -10
(a value more positive than
d
‰
13
C values less negative than
d
30
. Conversely, CO
2
-
‰
13
C values more negative than
using species have
d
30
and were unable to raise
‰
pH above a critical value of pH 9 (Maberly et al.
1992
).
The expression
13
C value of the algal sample (
13
C
alga
)
D
which represents the
d
d
13
C of the source of inorganic carbon used in photosynthesis
relative to the
d
13
Cco
2
), is also indicative of certain photosynthetic pathways. Previous work
strongly suggests that
(
d
D
values in excess of 20
are associated with diffusive CO
2
‰
entry and C
3
pathway of CO
2
fixation while
are consistent
with HCO
3
use and CO
2
uptake by a nondiffusive mechanism (Raven et al.
1995
and references therein). A correlation was found between the two proxies for
D
values below 20
‰
