Photosynthetic Pathways and ecological efficiencies
(Aber, chpt 6, cont.)
On Monday we talked about ecological efficiencies. Let me re-
emphasize some points:
EFFICIENCIES ARE RATIOS, USUALLY SOMETHING LIKE: PRODUCT/COST UNIT.
The efficiencies we're interested in:
production efficiency
water use efficiency
nitrogen use efficiency
light use efficiency
Will always involve trade-offs. You can see that to
1) capture lots of sunlight (high light use efficiency; which is
different from low light use efficiency, see below) you
must have lots of photosynthetic machinery to do so. Moreover, you
probably want this captured energy to go to fixed carbon (what else
are you capturing it for...) so that means that lots of carbon
dioxide must be captured. That probably means your stomata are
open, and that means you're probably losing lots of water. Hence,
as you might predict, to have high production efficiency, which is
grams of carbon dioxide fixed per gram of leaf, you probably have
low water use efficiency and probably low nitrogen use efficiency
(because you need to be loaded with enzymes to do this).
As it turns out, there has been a very interesting twist in the
story of light, carbon, water and nutrient use efficiencies with
respect to photosynthesis. This has to do with evolution
experimenting for "a better mouse trap".
Everybody has to memorize the generalized pathway for
photosynthesis, the Calvin cycle, right? Anybody remember it? I
always have to look it up...
The calvin cycle uses an enzyme, ribulose biphosphate carboxylase,
(rubisco) to shove carbon dioxide onto ribulose biphosphate (RuBP).
The problem is
1. the enzyme has a low affinity for carbon dioxide. Uptake is
therefore inefficient when carbon dioxide levels are low. Plants
therefore load up on the enzyme (using quantity instead of quality,
I guess).
2. At high leaf temperatures, low concentrations of carbon dioxide
(which occurs as it reacts with the enzymes) and in the presence of
oxygen, the enzyme actually oxidizes some of the RuBP or
intermediate products rather than fixes (adds the CO2 to) them!
This is photorespiration, which according to Ehleringer and Monson
can reduce the overall efficiency of net photosynthesis by one-
third! This defeats the purpose of the enzyme, and lead Rickefs to
ponder...
"It is not clear why plants did not develop a better enzyme for
such a crucial step."
Perhaps the plants evolved in a low oxygen, high CO2 environment,
and, in water, where super high temperatures were not encountered?
(geez, one does not need to be a rocket scientist...)
If you buy into the current story, and I do, the next evolutionary
step occurred in habitats with high temperatures, lots of oxygen,
and low CO2. The plants, over the millennia, developed the oxygen,
the high temperatures were associated with a terrestrial, perhaps
semiarid existence, and the low CO2 concentrations were experienced
around the end of the Cretaceous. To quote/paraphrase (heck, I
just kinda shortened their paragraph) Ehleringer & Monson:
"Instead of allowing the photosynthesis cycle to occur in all
photosynthetic cells, the cycle is limited to selected interior
cells, called bundle sheath cells. A layer of mesophyll cells
surrounds these sheath cells, and within the mesophyll cells is
phosphoenolpyruvate (PEP) carboxylase, an enzyme that catalyzes the
initial photosynthetic reaction. This enzyme combines with a
carbon compound to produce a four-carbon acid; hence the name C4
photosynthesis. This C4 compound is shunted to the bundle sheath,
where it's converted to a C3 compound and carbon dioxide, which is
then captured like before."
The improvement is that PEP carboxylase has a very high affinity
for its substrate and a higher v-max than Ribisco, the enzyme
associated with the C3 cycle. Carbon dioxide is therefore
concentrated in the bundle sheath cells an order of magnitude
higher than in C3 plants, and photorespiration is effectively
eliminated.
The cost of this improvement is in energy yield. Two additional ATP
are used to reduce a carbon dioxide molecule, and light use
efficiency (carbon dioxide fixed per unit of light absorbed) should
therefore be lower. In fact, this is only observed at temperatures
below 25-30 C. This is because photorespiration of C3 plants
occurs at higher temperatures.
C3 vs C4 comparisons
1. Carbon dioxide concentrations. Under higher CO2 levels, the C4
pathway loses its competitive advantage. In theory, then,
increasing CO2 concentrations of the atmosphere should be favoring
C3 plants over C4 plants.
2. Water use efficiency: C4 plants: 1 g biomass per 250-350 grams
of water, C3 plants produce 1 g biomass per 650-800 grams of water.
3. Nitrogen use efficiency. C4 plants have less Rubisco than C3
plants, and since this enzyme is loaded with nitrogen, C4 plants
can have lower nitrogen concentrations. Given that these plants
can have higher carbon fixation rates, it's not surprising to
suggest that C4 plants have higher nitrogen use efficiencies. This
becomes an important ecosystem feedback mechanism, but we won't
introduce that concept until we studied soils.
CAM plants
Heck, if the C4 pathway seems superior under many environmental
conditions, what's the next adaptation we can add? A C5
photosynthetic pathway?...well, not yet, but a modification to the
C4 pathway has occurred.
CAM photosynthesis is the same pathway as C4 photosynthesis, with
a temporal rather than spatial separation of the two enzymes
involved. In CAM plants, stomata open at night and CO2 is fixed
into a C4 acid. During the day, this material is fed to the C3
pathway, and the carbon fixed. Obviously, the advantage is the
harvesting of carbon dioxide when 1) there's no competition with
other plants for this material and 2) water losses are low.
The surprise is that CAM plants can be found in three wildly
different places: a) as succulents in deserts, b) as epiphytes
(canopy dwellers) in the tropics and subtropics, and c) in some
aquatic plants! "The common denominator in all cases is that
ambient CO2 during the day is not readily available or is available
only at extremely high costs to the plants." (Ehleringer and
Monson).
CAM plants can win the award on water use efficiency. It takes
reportedly but 80 g of H2O to make a gram of CAM plant.
1. A Global Map of C3, C4 and CAM-dominated ecosystems.
Given the apparent superiority of C4 species, it's surprising that
their dominance is somewhat limited. C4 species will dominate ONLY
in
a) Disturbed (regrowth, full-sunlight) topical systems
b) warm and arid systems (esp. warm and temperate grasslands)
CAM plants are abundant in some hot deserts. But, as mentioned
above, can occur as epiphytes or even in aquatic environments as
"minor players" in carbon fixation.
C3 plants, the primitive, inefficient beasts that they are, rule
elsewhere. In all mature forests, in cold grasslands, and
certainly in tundra, the C3s rule.
Why the emphasis? As we will see, the ability to control/use water
and nutrients as well as differences in production efficiencies
means that in systems where C3 and C4 systems can co-occur, Their
characteristics are sufficient to control other aspects of
ecosystem characteristics, such as carbon storage and nutrient
cycling. And, by controlling these characteristics, the vegetation
effectively controls plant species composition.
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