PHOTOSYNTHESIS AND THE CARBON CYCLE
HA&S 220C, Billy Brazelton, 11/4/04
We have been talking different forms of energy (mechanical, kinetic, potential,
thermal, chemical) and different sources of energy (fossil fuels, crops, wind, solar). An
important part of the story we have been developing is the role of biology in the world’s
energy system. In my last lecture we saw how burning (or oxidizing) different substances
results in a release of energy we can harness to do work. Biological organisms burn fuel
sources inside their cells in order to release energy to do the work of running the
biochemical reactions that keep them alive. The basic oxidation reaction is :
C6H12O6 + 6O2 -> 6CO2 + 6H2O
Yields 686 kcal/mole of glucose
The reverse of this reaction – combing carbon dioxide and water to make sugar –
is known as photosynthesis. Photosynthesis is the process responsible for storing all the
energy we extract from fossil fuels, crops, and all of our food. We will also see that it is
part of a globally important cycle affected by our consumption of fossil fuels.
I. Photosynthesis
How is photosynthesis able to run the reaction above in the reverse
direction? Somehow it must come up with 686 kcal of energy to make each mole of
glucose. Where does that energy come from? The short answer: photons of sunlight. The
long answer: When the pigment chlorophyll inside the chloroplasts of a photosynthetic
organism (phytoplankton, trees, other plants) absorbs sunlight, it becomes energetically
‘excited’ and grabs the hydrogen atoms away from a water molecule, leaving the oxygen
atoms to escape as O2 gas.This is called ‘splitting water.’
The hydrogen atoms are then split into their component protons and electrons.
The electrons are used to reduce carbon dioxide, in a series of many steps requiring more
absorption of sunlight by chlorophyll, to glucose. When carbon dioxide receives those
electrons, the extra negative charge attracts protons from elsewhere, creating hydogen
atoms attached to the carbon atom. This process is called reduction. When those reduced
carbon dioxide molecules are combined together in a larger molecule, the result is
glucose. This ‘combing together’ of small molecules requires an input of energy, which is
provided by the ATP molecules made by the protons diffusing through the membrane of
the chloroplast. The ATP molecule is simply a high energy molecule that biology uses to
store energy for later use. In this case, the mechanical energy created by the protons
diffusing across the membrane turns a sort of molecular turbine that stores energy in
ATP. Think of it as a kind of hydroelectric dam where the protons are like water and ATP
is like the electricity created when water flows through the dam’s turbines.
You can’t expect to understand this by simply reading this. Study the powerpoint
slides I posted on the website, find a biology textbook, or contact me if you are having
trouble. It is a beautifully complex yet simple process that is easy to understand if you
can picture it in your mind well enough.
II. The carbon cycle Where does photosynthesis occur? It occurs in the phytoplankton
of the ocean and the trees and other plants on land. On land, the carbon dioxide consumed
by plants to make organic matter (known as ‘fixing carbon’) is stored for fairly long
periods of time until the plant is harvested, eaten, or otherwise dead. Trees, for example,
can live for hundreds of years and store lots of fixed carbon for long periods of time. In
the ocean, in contrast, carbon is fixed by fast-growing, quickly eaten phytoplankton. The
word ‘phytoplankton’ means any organism that undergoes photosynthesis in the water
and includes many species of algae and bacteria.
Because the carbon fixed by phytoplankton is ‘turned over’ so quickly (a term
meaning the phytoplankton get eaten often), the fixed carbon is released into the
environment creating an intricate web of carbon transfer in the ocean. The phytoplankton
are grazed (eaten) by various species of bacteria, algae, and arthropods (insects and
crustaceans). These grazers are in turn eaten by larger animals, which are eaten by larger
animals, which are eaten by big fish, which are eaten by bigger fish, which are eaten by
humans. The key concept here is that all of these organisms are continuously pooping
throughout their lives before they get eaten. If the poop is heavy enough, it sinks, and the
technical term for this process is known as the “biological carbon pump.” The feces of
animals eating phytoplankton transports the carbon contained in the phytoplankton down
to the seafloor.
As the fecal pellets (the technical term for poop) fall, the carbon contained within
is consumed by bacteria with oxygen – that is, the bacteria oxidize the carbon in poop
with O2 gas dissolved in the ocean. Almost the entire ocean contains abundant quantities
of dissolved oxygen, but in some places the local concentration of oxygen in small
microenvironments (for example, around a fecal pellet) can become quite low. Also,
when the fecal pellet falls all the way down to the sediment on the seafloor, oxygen can
become depleted quite quickly. When this happens, the bacteria oxidize the carbon with
other oxidizers than oxygen like nitrate, sulfate, and hydrogen, which become abundant
when oxygen is absent. They byproducts of these reactions include ammonia, sulfide, and
methane, to which we will return later.
That concludes that series of events, but now we need to return to the surface
where we will talk about a special kind of phyotplankton known as the coccolithophores.
They get this odd name due to their armor of calcium carbonate surrounding their cells
called coccoliths. They make their armor by precipitating carbonate (CO32-) dissolved in
the ocean onto their cell walls. When the coccolithophores are eaten, the coccoliths
remain intact in the resulting fecal pellets and sink to the seafloor.
Thus, this is another example of phytoplankton taking carbon from the surface of
the ocean and transporting it to the seafloor through fecal pellets. Think about what this
means for the global distribution of carbon. When animals on land breathe out carbon
dioxide or burn oil and produce carbon dioxide, trees on land and phytoplankton in the
ocean take that carbon dioxide, convert it into biological molecules, which sink to the
seafloor. Now you see why scientists took to calling this the biological carbon pump.
Phytoplankton are pumping our carbon waste to the seafloor.
This sytem has resulted in a relatively stable concentration of carbon dioxide in
the atmosphere since the last ice age. The question for today, though, is: Can the
phytoplankton carbon pump keep up with modern outputs of carbon dioxide from the
burning of fossil fuels? The short answer is an obvious “no” since the concentration of
carbon dioxide in the atmosphere has been steadily increasing since the Industrial
Revolution and will continue to increase for some time even if we immediately cease all
fossil fuel burning. We will talk later in the course about what this means for global
warming and climate, but today we will ask what this means for the marine ecosystem.
First of all, the coccolithophores are not happy about this at all. The increased
carbon dioxide levels make it very difficult for coccolithophores to make their body
armor. When more carbon dioxide dissolves in the ocean, the pH of the ocean is lowered,
and this causes most of the dissolved carbon in the ocean to be converted to bicarbonate
(HCO3-) rather than carbonate (CO32-). This makes it difficult for the coccolithophores to
make their carbonate body armor, and they do not pump as much carbon to the seafloor.
So not only can the coccolithophores not keep up with our carbon waste, our carbon
waste is actually making the carbon pump worse because it poisons one of the main
carbon pumpers. Furthermore, the circulation system of the entire global ocean could be
affected by the change in chemistry resulting from the elimination of coccolithophore
carbonate precipitation. (This is an advanced concept, and we might discuss it later in the
course.)
As for the rest of the phytoplankton, you might expect that increased carbon
dioxide levels would make them very happy – we are, after all, giving them more food.
The problem is that the phytoplankton do not seem to be limited by their carbon dioxide
food – they are limited by other nutrients they need in the ocean such as nitrate and
phosphate. So giving them more carbon dioxide does not matter unless they get more
nitrate and phosphate too. If we go out to the ocean and dump a bunch of fertilizer
overboard (fertilizer is nitrate and phosphate) the result is an algal bloom that will,
indeed, export more carbon to the seafloor, but this unbalanced state of the ecosytem has
numerous negative side effects such as fostering disease and poisoning fish populations.
Also, an oceanic supply of fertilizer would be very expensive.
Another idea is to stimulate certain organisms known as nitrogen fixers which
convert the abundant N2 gas in the atmosphere into fixed nitrogen – that is, ammonia
(NH3) and nitrate (NO32-). Nitrogen fixers can increase the local concentration of nitrate,
providing more nutrients for phytoplankton. Researchers have discovered that providing
nitrogen fixers with very small amounts of iron can stimulate them to convert more N2
into fixed nitrogen. These experiments have actually been done in the ocean – scientists
throw large quantities of iron overboard and measure how the phytoplankton are affected.
The results have been that algal blooms are created by iron fertilization
experiments, but there is no evidence that these blooms result in a larger carbon pump.
Indeed, what happens is very much like what would happen if you dumped fertilizer
overboard: the phytoplankton become more numerous, but so do the bacteria. The
bacteria feed off the phytoplankton and oxidize much of the carbon fixed by the
phytoplankton back to carbon dioxide. So all of the increased productivity caused by the
nitrogen fixers does not result in more carbon being pumped to the seafloor.
Even if iron fertilization did result in increased carbon export to the seafloor, this
could result in serious imbalances in the mairne ecosystem as well. For example, the
increased oxidation rates of fecal pellets could deplete oxygen levels to dangerous levels,
killing off fish populations. This could be especially likely if the oceanic circulation
system is affected by the loss of coccolithophores.
One final idea people have proposed to cope with our carbon dioxide waste is to
directly pump the carbon to the seafloor using large man-made pipes. (The phytoplankton
won’t do it, so we’ll do it ourselves!) The hypothesis is that the carbon dioxide would be
converted into bicarbonate, and the ocean is capable of storing lots and lots of
bicarbonate, easily enough to store all of our carbon dioxide waste. This idea is being
taken very seriously by big corporations and policy-makers, and, for the most part, it
should work as expected. It is true that the ocean can store a lot of bicarbonate, and the
carbon pumped into the seafloor probably won’t affect coccolithophores for about 500
years. After a couple hundred years, however, the ocean is likely to be seriously affected
by this major change in ocean chemistry. In the short term, though, carbon dioxide levels
in the atmosphere would decrease, or, more likely, increase at a slower rate, helping to
moderate climate change. We would just have to hope that our future generations will
figure out how to save the oceans from disaster.
A final point on this topic is that on geological time scales, none of this carbon
cycling matters. Everything I have talked about takes place on time scales of months to at
most hundreds of years. The bad news is that we are likely to see the effects of these
changes in our lifetimes. The good news is that eventually the Earth will sort itself out,
re-equilibrate the carbon dioxide in the atmosphere with the ocean, and balance the
carbon cycle again. The time it takes for this to happen is the time required for oceanic
crustal plates to subduct under the continents, melt everything contained on the seafloor,
and expel the waste through volcanoes on the surface. In this way, the entire global
carbon cycle is completed, and everything balances out. It takes approximately 100
million years.