How carbon enters the biosphere
Carlo Calfapietra
Institute of AgroAgro-Environmental and Forest Biology (IBAF)
National Research Council (CNR)
Porano (TR), Monterotondo Scalo (Roma)
[email protected]
INTERACTIONS PLANTPLANT-ATMOSPHERE
VOC
CO2
Atmospheric pollutants
• Assimilation of Carbon into organic matter – the
Calvin--Benson cycle; the importance of Rubisco
Calvin
• Photosynthetic processes and the kinetics of
gas exchange by leaves
• A quick look at the carbon losses due to
respiration and sources of variation in respiration
rates.
•How to measure C uptake by plants
•Implications of the C uptake by biosphere
Review/overview
The production of sugars from CO2 occurs in the stroma of the
chloroplast and involves a complex series of enzymeenzyme-mediated
reactions. These can be broken down into three fundamental
steps: Carboxylation (or carbon fixation), Reduction and RuBP
regeneration.
CARBOXYLATION
CO2 combines with ribulose
bisphosphate (RuBP), forming
phosphoglycerate (3PG).
THIS IS CATALYZED BY
RUBISCO: Ribulose
bisphosphate carboxylaseoxygenase
CARBOXYLATION
3-phosphoglycerate
and ribulose
bisphosphate are
both SUGARS (the
“-ose” in the name
refers to sugars).
Rubisco catayzes the carboxylation, or addition of
CO2, to the high-energy, 5-carbon acceptor
molecule, RuBP. This creates a 6-carbon sugar
that immediately dissociates into two identical 3carbon units (3-phosphoglycerate). This is called
“C3” photosynthesis because the first product of
carboxylation is a 3 carbon sugar.
RUBISCO: an amazing enzyme
• Rubisco is the most abundant enzyme on
Earth, and makes up about 60% of the
soluble protein of leaves. Rubisco is
responsible for fixing ~1011 tonnes of
CO2 each year (for reference: annual oil
consumption ~3 x 109). Rubisco provides
the only link between pools of inorganic
and organic carbon in the biosphere.
REDUCTION
To the cytosol
Phosphoglycerate (3PG) is reduced to glyceraldehyde-3-phosphate
(G3P); this requires “reducing power” which comes from NADPH.
About 1/6 of the G3P is exported from the chloroplast to the
cytosol of the cell, the rest is “recycled” back to RuBP
REGENERATION
The “high
energy”, 5carbon sugar,
RuBP, is
regenerated.
This requires
ATP.
To the cytoplasm
•
6 CO2 are fixed per glucose synthesized
(in other words, the cycle has to “crank” 6
times for one glucose molecule)
•
Most of the reactions in the Calvin cycle
are used to reduce triose sugars and
regenerate RuBP
More about the enigmatic enzyme,
Rubisco
• As well as being slow, Rubisco has
another problem. In a reaction which
seems totally wasteful, O2 competes with
CO2 for the active site of the enzyme –
this causes Rubisco to act as an oxygenase
as well as a carboxylase. Is this an accident
of evolutionary history? Did Rubisco first
evolve when the earth was anaerobic?
When Rubisco acts as an
oxygenase, there is no net carbon
fixation and the energy that was
put into the high-energy RuBP
molecule is lost
Glycolate produced by the
oxygenase reaction goes
through a very complicated
series of further steps that
consume a lot of energy and
release CO2 as a byproduct.
Eventually some of the glycollate
is recycled back to 3phosphoglycerate. This process
is called PHOTORESPIRATION.
DO NOT confuse this with
mitochondrial, or “normal”
respiration.
Net photosynthesis =
Gross photosynthesis
- photorespiration
- mitochondrial respiration
(also known as “dark respiration”)
Net carbon assimilation
The rate of net carbon assimilation is higher
in air with low oxygen levels because
photorespiration is suppressed.
Summary points of photorespiration:
1.
Photorespiration occurs when Rubisco acts as an oxygenase
instead of a carboxylase
2. The energy used to produce the RuBP acceptor in the CalvinCalvinBenson cycle is wasted
3. It takes even more energy to recycle the 2
2--carbon
glycollate back to 3 PGA
4. High light, low CO2 and high O2 promote photorespiration
5. C3 plants can lose up to 2020-40% of their newly fixed
carbon because of photorespiration!!!
Some plants have evolved another type of
photosynthetic pathway called “C4” photosynthesis.
Photorespiration does not occur in C4 plants because
they have no RUBISCO. CO2 “out competes” O2 for
the active sites on the enzyme
• We will not go into C4 metabolism in any
more detail in this class because it is
almost entirely restricted to herbaceous
plants (especially grasses). However, you
should know that C4 plants are much
more efficient with water and nitrogen
than C3 plants are, so C4 grasses tend to
be more competitive with woody plants in
hot, dry, highhigh-light conditions.
Summary of raterate-limiting steps in carbon fixation
(note: carbon “fixation” is equivalent to carbon “assimilation”)
1. CARBOXYLATION. (The maximum rate is termed Vcmax)
a. Enzyme activity (Rubisco) – depends on N, light,
temperature, activation by rubisco activase
b. CO2 supply – depends on stomatal opening (and therefore
humidity, soil water availability) and ambient CO2 and O2
levels
2. REGENERATION of RUBP. (The maximum rate is termed
Jmax). depends on the production of ATP and NADPH in
electron transport (therefore depends on light)
3. METABOLISM OF END PRODUCTS (triose
(triose--phosphates)
(termed TPU, for “triose phosphate utilization)
(If the newly produced sugar phosphates are not used or removed they will
“tie up” phosphate, making it unavailable for making new ATP)
Temperature effects on gas exchange
Reaction rate
Photorespiration +
Mitochondrial
respiration
Gross photosynthesis
Net photosynthesis
Leaf temperature
• Now, let’s look at
photosynthesis from a
gas exchange
perspective
At the same time, H2O vapor
moves out of the leaf by
diffusion (but really H2O vapor
moves both directions)
CO2 moves from the air to
the leaf to the chloroplast by
diffusion (but really CO2 moves
both directions)
Some definitions ….
(note that this leaf has
stomata only on the
“abaxial” or bottom side.
Some leaves also have
stomata on the adaxial,
or upper surface. Leaves
with stomata on both
sides are called
“amphistomatous”)
Ci = internal CO2 concentration. This
value can be measured (indirectly) with
common gas exchange instruments
Ca = external CO2 concentration
CO2 diffuses into leaves, moving “down” a
concentration gradient
The CO2
concentration at
the site of
fixation
approaches “zero”
Ca = 360-370 ppm?
Typical
CO2
concentration
of a C3 plant at
midday is about
270--300 ppm
270
Ci is a “substrate” for carboxylation
(RuBP is the other substrate)
• Ci increases when leaf conductance
(“gs”) increases relative to rubisco
activity.
• Ci decreases when leaf conductance
decreases relative to rubisco activity.
Diffusion
Ambient [Co2] (Ca)
H2O
Boundary layer
Stoma
Ca=360ppm
Internal [CO2] (Ci)
Ci=260ppm
Assimilation by RuBP Carboxylase
The diffusive movement of CO2 into and out of a
leaf can be described by Fick’s Law:
Net flux = ∆ concentration * conductance
[xo] =
concentration
of “x” on the
“outside” of
“barrier”
Net flux of
“x” = Fx
(a membrane or barrier with a
“conductance” to substance “x” = gx)
Fx = ([xo] – [xi]) * gx
[xi] =
concentration
of “x” on the
“inside” of the
“barrier”
Total leaf resistance to CO2 is made up of (at least) two separate
resistances, the stomatal resistance (rs) and the boundary layer
resistance (rb). Diffusive resistances “sum” just like electrical
resistances, so rtotal = rs + rb
Applying Fick’s Law to carbon
assimilation:
Net C assimilation = (ca-ci) * leaf
conductance
Or: A = (ca-ci) * gleaf
We can apply these ideas about kinetics to
carbon assimilation, with Ci as a substrate, to
develop an “A/Ci” curve
40
35
These are actual data for
Douglas-fir
Net A , µ mol/m2/s
30
25
20
15
10
5
0
-5
0
50
100
150
Ci, Pa
200
250
But the situation is a little more complicated, because there
are THREE raterate-limiting reactions that affect overall net C
assimilation. At any value of Ci, the measured point will be
determined by the
Rubisco limitation
(Wc)
Model Fit to Measured Data
35
30
Measured
N et A , µ mol/m2/s
25
Wc-limited
20
RUBP regeneration
(Wj)
Wj-limited
15
Wp-limited
10
TPU (Wp)
Γ
5
Compensation point (Γ)
0
0
50
100
150
-5
Ci, Pa
Ca
200
250
Respiration and Plant Carbon
Balance
On a wholewhole-plant basis,
respiration consumes from 30%
to 70% of total fixed carbon
Leaves account for about half of
the total
Respiration is often subdivided into Growth,
Maintenance and sometimes Transport costs
Growth respiration:
respiration: ( “construction respiration”)
– a “fixed cost” that depends on the tissues or
biochemicals that are synthesized. Often
described in terms of “glucose equivalents”
Maintenance respiration:
respiration: The cost of maintaining
existing tissues and functions, (Protein turnover
is the largest cost of maintenance respiration)
Transport respiration:
respiration: The cost of moving
materials (e.g. ions, nutrients) across membranes.
The amount of
photosynthate
consumed in
respiration varies with
tissue type and with
environmental
conditions.
When plants are
nutrient stressed,
respiration rates in
roots increase
dramatically. This is
due to increased
transport costs,
increased enzyme
activity, increased
exudation, and
increased allocation to
nitrogen fixing bacteria
and mycorrhizae
Q10: the multiplicative
change in respiration
over a 10 degree C
change in
temperature
Mitochondrial
Respiration (like
photorespiration)
increases rapidly
with temperature.
Can this lead to
reduced growth at
high temperatures?
Maybe, but most
likely only in extreme
cases. Respiration
“generally”
acclimates to
changes in
temperature.
LCP p. 119
A new book on
photosynthesis is in press
Ecophysiology of photosynthesis
in terrestrial higher plants.
Cambridge University Press
Cap. XV. Calfapietra C, Bernacchi C, Centritto M, Sharkey TD.
Photosynthetic responses to increased CO2 and air pollutants.
MEASUREMENTS OF CO2 and POLLUTANT UPTAKE BY VEGETATION
Eddy flux sites
Carbon Storage & Sequestration
Carbon Storage carbon stored
in plant tissues (roots, stem and
branches)
Carbon Sequestration carbon
annually removed from trees and
from soil
Important to increase our knowledge on soil
carbon sequestration capacity
CO2 accumulation rate in the
atmosphere
4.1 GtC / year
Atmosphere
Biosphere
7.6
Fossil fuels
1.5
Land use
change
2.8
Terrestrial
Sinks
2.2
Ocean
Sink
Courtesy of G. Matteucci
THANKS FOR YOUR ATTENTION!
[email protected]