13C/12C
fractionation during carbon fixation by plants
Fractionation occurs during different phases of CO2 uptake and
fixation:
•Diffusion of ambient CO2 through the leaf stomata: 12CO2 diffuses
faster resulting in an enrichment of 12C in the cell
•Enzymatic carboxylation: Fixation of CO2 onto ribulose biphosphate
by the RUBISCO enzyme
PAR, hν
CO2 + H20 → CH20 + O2
•Further biosynthesis of cellulose, proteins, lipids ..
1
2
Excretion
CO2
εout
εin
Metabolic intermediates
ε7
ε2 ε1
δ1 2
1 ε4
ε3
δ2
ε8
ε5
δ3
ε6
3
ε9
δ6
δ5
δ4
Biosynthetic products
1, 2, 3 = branch points
13C/12C
fractionation in biosynthetic products will depend also on the
carbon budgets (i.e. fractional yields) at the branch points.
3
Model for 13C/12C fractionation during C-fixation by C3 plant
Important assumption: CO2 diffuses freely in and out of the cell
CO2
leakage
CO2
Ce
δe
Φout
εt
Φin
εt
Branch point
δo
CO2
Ci
δi
Φfix
Rubisco
εf
CH2O
δf
Calvin cycle
εt = discrimination during diffusion
εf = discrimination during C-fixation
RUBISCO (= Ribulose biphosphate carboxylase oxygenase) prefers
12C over 13C → δo > δe
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Φfix/Φin = f, the fractional yield of the product
for f = 1 → all incoming CO2 is fixed
(1-f) = unused CO2 fraction, diffusing out of the cell
δo = δi + f εf
δf = δi – (1-f) εf = δo - εf
δo = δf + εf
for f → 0 ⇒ δo → δi
δf → δi - εf
for f → 1 ⇒ δo → δi + εf
δf → δi
assuming fluxes are proportional to the concentrations:
→ Φin = k1.Ce ; Φout = k-1.Ci
for k1 = k-1, it follows that: Φout/Φin = Ci/Ce and Φfix/Φin = 1 – Ci/Ce
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C-fixation in an open system
δi + εf
δo
ε
δi
δf = δo - εf
δi - εf
0
0.5
1.0
f = Φfix/Φin
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Write mass balance:
Φin (δe - εt) = Φout (δo - εt) + Φfix (δo - εf)
(δe - εt) = Φout/Φin (δo - εt) + Φfix/Φin (δo - εf)
substitute
Φout/Φin by Ci/Ce and Φfix/Φin by 1 – Ci/Ce
→ δe - εt = (Ci/Ce) (δf + εf - εt) + (1- Ci/Ce)(δf)
→ δe – δf = εp = εt + (Ci/Ce) (εf - εt)
with εp = the overall isotope effect, i.e. the isotopic difference
between fixed carbon and CO2
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Values of εp van vary between εt and εf depending on Ci/Ce, with
Ci ≤ Ce
Since εt for CO2 diffusion is 0.7‰ (at 25°C) and εf for CO2
fixation by RUBISCO is about 27‰ it follows that:
0.7‰ ≤
= εt
for Ci/Ce → 0
εp
≤ 27‰
= εf
for Ci/Ce → 1
8
Relationship between εp and cell growth rate:
Since
Φfix = Φin - Φout
= k1.Ce – k-1.Ci
→ Φfix/Ce = k1 – k-1 . Ci/Ce ; taking k1 = k-1
→ Φfix/(k . Ce) = 1 – Ci/Ce and Ci/Ce = 1 - Φfix/k.Ce
substituting Ci/Ce in εp = εt + (Ci/Ce) (εf - εt)
gives:
εp = εt + (1 - Φfix/k.Ce) (εf - εt) with Φfix ≤ k . Ce
for increasing carboxylation rate (Φfix) ↑ → εp ↓
for Ce ↑ → εp ↑
9
For a given Ce and k, εp is a linear function of Φfix :
εp
25
εf
20
15
10
5
increasing membrane permeability
εt
increasing Φfix
Conclusions:
•The faster cells grow the smaller the discrimination against 13C
•The larger the external carbon concentration (Ce) the greater the
discrimination against 13C
10
Solubility of a gas is temperature dependent; lower Temp → higher aqueous
conc of the gas or the lower Temp, the higher Ce
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12
C4 Carbon fixation pathway
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Uptake of
HCO3- instead
of CO2:
role of PEPCase
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Vertical profiles of δ13 C in the western basin of the Atlantic Ocean
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19
Polluted rivers and rivers
with a large input of
terrestrial organic matter
will exhibit large CO2
verpressure relative to the
atmospheric CO2 pressure.
These systems are net
heterotrophic (respiration >
primary production
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23
24
25
Note: ε ≅ δA - δB
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27
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Uptake of 13C depleted anthropogenic CO2 by the ocean: can we see it ?
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