Environmental Engineering Reference
In-Depth Information
Rubisco. At steady state, or d[CO
2
]
in
/dt
=
0, the overall fractionation factor (
α
)
can be written as
α=
1
+∆
K
1
+ (∆
K
2
− ∆
K
1
)
CI
CE
(5.10)
where Ce and Ci are the CO
2
concentrations in air and at the carboxylation site,
respectively, and
Δ
k
i
=
α
i
−
1. In the equation (O'Leary
1981
), subscripts for
efflux and carboxylation steps are 2 and 3, respectively, and
E
i
=
1
+
Δ
k
i
:
α=
E
1
(
E
3
/
E
2
+
k
3
/
k
2
)(
1
+
k
3
k
2
)
(5.11)
When
a
=
Δ
k
1
,
b
=
Δ
k
2
and CO
2
concentrations in air and intercellular leaf
spaces are denoted in partial pressure
p
a
and
p
i
, respectively, then (Eq.
5.10
) can
be modified into Farquhar's equation:
∆ = α −
1
=
a
+ (
b
−
a
)
p
i
p
a
(5.12)
On the other hand, the fractionation equation for passive diffusion-phytoplank-
ton photosynthesis is substantially similar to that of land C
3
plants (Eq.
5.10
). The
CO
2
diffusion must be considered in the aqueous phase and Ce denotes the CO
2
concentration in bulk solution or [CO
2
]
aq
. The term 'CO
2
demand'
=
'Ce - Ci' has
been introduced into the new model (Rau et al.
1992
). The relationship between
the
δ
13
C value of POM and [CO
2
]
aq
can be determined using the fractionation
equation that includes the (Ce-Ci) term:
1
−
CE
−
CI
CE
(5.13)
ε
P
= ε
1
+
(ε
2
− ε
1
)
where (Ce - Ci)
=
7-8
μ
M in southwestern Indian Ocean. When (Ce - Ci) is con-
stant, the (Eq.
5.10
) at infinite Ce can be expressed as:
(5.14)
α =
1
+ ∆
k
2
This implies that the overall fractionation can reach a maximum value, which
corresponds to that of Rubisco (
α
=
1.027-1.029, or
Δ
k
2
=
0.027 - 0.029) at
high Ce (Roeske and O'Leary
1984
; Farquhar and Richards
1984
). Furthermore,
(Ce - Ci) may increase with increasing Ce as found in a culture study of
Skeletonema costatum
and
Emiliania huxley
, which introduces the possibility of
β
-carboxylation at high Ce (Hinga et al.
1994
). Interestingly, the activity of the
PEPCKase of
S.costatum
can increase to >50 % of Rubisco activity at the end of
growth (Descolas-Gros and Fontugne
1985
,
1990
).
The low fractionation observed at high Ce is possibly due to
β
-carboxylation
(Goericke and Fry
1994
), particularly in the case of PEPCKase-mediating
β
-carboxylation. The latter shows similar discrimination against
13
CO
2
as
that of Rubisco (Arnelle and O'Leary
1992
). Active transport by CCM may
contribute to a fractionation at high Ce, which is lower than that given by the
