Environmental Engineering Reference
In-Depth Information
fractionation equation (Yoshioka
1997
). It is shown that passive CO
2
diffu-
sion is efficient to sustain maximum growth of
Phaeodactylum tricornutum
,
which does not require active transport of inorganic carbon at [CO
2
]
aq
=
10 M
(Laws et al.
1995
). This study also shows that maximum growth rate is expected
when the CO
2
influx is equal to the growth rate (Laws et al.
1995
). In that case,
Ci
=
0 and also the growth rate (photosynthetic activity) is zero or even nega-
tive, because of the oxygenase activity of Rubisco (Yoshioka
1997
). The con-
tradiction may occur because the growth rate is not independent of Ce and Ci.
Therefore, diffusive transport of CO
2
can operate together with active transport
(Yoshioka
1997
), and CCM possibly requires an energy expenditure (Berry
1988
). However, it is difficult to identify the relative contribution of active
transport to the total CO
2
influx from the ealier fractionation equations. In the
derivation of (Eq.
5.10
), it is assumed that the resistance to CO
2
diffusion is
similar in either direction across the cell membrane, or
k
1
=
k
3
(Francois et al.
1993
). This assumption originally came from the expectation that resistance to
CO
2
diffusion through the stoma of a plant leaf would be the same in both direc-
tions (O'Leary
1981
). Aquatic phytoplankton may have a CCM with different
values for this resistance (
k
1
≠
k
3
), probably (
k
1
>
k
3
), and thus the fractionation
equation can be rewritten as:
α =
1
+ ∆
k
1
+ (∆
k
2
− ∆
k
1
)
k
3
Ci
k
1
Ce
(5.15)
which may provide some measure of the contribution of active transport. It is gen-
erally assumed that the resistances to CO
2
diffusion in both directions across the
cell membrane are the same (symmetric permeability). A fractionation equation is
required to express the decrease in fractionation with increasing contribution of
active transport (
f
), as some function
f
(Yoshioka
1997
). Basically,
f
and
k
1
≠
k
3
may have the same importance for CO
2
acquisition by phytoplankton. Therefore,
active transport of inorganic carbon by CCM may be linked (as a homologue) to
the asymmetric permeability of the cell membrane for CO
2
.
Deviation of Fractionation Equations Involving Active Transport
(
Yoshioka
1997
)
−
Various phytoplankton species can actively transport CO
2
and HCO
3
in aqueous
media (Bums and Beardall
1987
). However, they depend on two phenomena: (i)
the occurrence of internal and external carbonic anhydrase (CA), which can cata-
lyse the equilibrium between CO
2
and HCO
3
−
and can affect the determination
of the inorganic carbon species crossing the cell membrane; (ii) the difference
in inorganic carbon species can substantially vary the fractionation factor of the
substrate for photosynthesis. It is shown that fractionation between [CO
2
]
aq
and
HCO
3
−
can differ by at most 10 ‰ in both equilibrium- and CA-catalyzed reac-
tions (Deines et al.
1974
; Paneth and O'Leary
1985
). Considering these phenom-
ena, it is important to develop the fractionation equations for two cases in which
