601
Photosynthesis in Nature: A New Look
Rubisco. At steady state, or d[CO 2 ] in /dt = 0, the overall fractionation factor (α)
can be written as
where Ce and Ci are the CO 2 concentrations in air and at the carboxylation site,
respectively, and Δk 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 + Δk i :
When a = Δk 1 , b = Δk 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:
On the other hand, the fractionation equation for passive diffusion-phytoplankton 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:
where (Ce – Ci) = 7–8 μM in southwestern Indian Ocean. When (Ce – Ci) is constant, the (Eq. 5.10) at infinite Ce can be expressed as:
This implies that the overall fractionation can reach a maximum value, which
corresponds to that of Rubisco (α = 1.027–1.029, or Δk 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
(5.10)
α= 1+∆k 1 + (∆k 2 − ∆k 1 )
Ci
Ce
(5.11)
α=E 1 (E 3 /E 2 + k 3 /k 2 )(1 +
k 3
k 2
)
(5.12)
∆ = α − 1 = a + (b − a)
p i
p a
(5.13)
ε p = ε 1 +
1 −
Ce − Ci
Ce
(ε 2 − ε 1 )
(5.14)
α = 1 + ∆k 2
Photosynthesis in Nature: A New Look
Rubisco. At steady state, or d[CO 2 ] in /dt = 0, the overall fractionation factor (α)
can be written as
where Ce and Ci are the CO 2 concentrations in air and at the carboxylation site,
respectively, and Δk 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 + Δk i :
When a = Δk 1 , b = Δk 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:
On the other hand, the fractionation equation for passive diffusion-phytoplankton 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:
where (Ce – Ci) = 7–8 μM in southwestern Indian Ocean. When (Ce – Ci) is constant, the (Eq. 5.10) at infinite Ce can be expressed as:
This implies that the overall fractionation can reach a maximum value, which
corresponds to that of Rubisco (α = 1.027–1.029, or Δk 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
(5.10)
α= 1+∆k 1 + (∆k 2 − ∆k 1 )
Ci
Ce
(5.11)
α=E 1 (E 3 /E 2 + k 3 /k 2 )(1 +
k 3
k 2
)
(5.12)
∆ = α − 1 = a + (b − a)
p i
p a
(5.13)
ε p = ε 1 +
1 −
Ce − Ci
Ce
(ε 2 − ε 1 )
(5.14)
α = 1 + ∆k 2