Is CO2 Plant Food? Why you Should not try to Improve Things that Don't Need to be Improved

Humans may think that they can improve plant productivity by pumping CO2 into the atmosphere, said to be “plant food.” They forget that Gaia always knows best and the final result may be exactly the opposite of what they expect. (image by Microsoft Image Creator)

An often repeated concept in the discussion about climate change is that CO2, carbon dioxide, is not a pollutant. It is, actually, food for plants. So, if human activity increases its concentration in the atmosphere, it is a good thing because plants grow better and more, incidentally absorbing the excess CO2 and solving the global warming problem. So, we shouldn’t worry about the emissions from the combustion of fossil fuels. They are a good thing for the biosphere, which couldn’t survive without a gaseous source of carbon.

Though not completely wrong, this is one of those half-truths (at best) that fall apart when examined just a little in-depth. First of all, what we normally mean as “food” is a source of energy for a living being. In plants, it is solar light that provides energy. Plants use CO2 to build their structure, but you should call it a “nutrient.”

That’s not just a semantic problem. It has to do with how the living creatures of Earth’s biosphere function. Everything that moves (including us) does that because of the energy provided by solar lights to plants to create organic molecules through photosynthesis, a chemical reaction that can be written in this way:

CO₂ + H₂O + photons light energy → [CH₂O] + O₂,

which is a simplified view of a hugely complicated reaction that goes through a long series of steps. But the overall process is based on photons splitting water and creating reactants that then combine with CO2 to form carbohydrates (marked as CH₂O above).

Now, you probably remember having studied in high school that if you increase the concentration of a reactant (molecules on the left side of the reaction), the reaction proceeds to create more products. So, more CO2 should create more carbohydrates, which is approximately true, but it comes with strings attached. The actual CO2 splitting reaction occurs on a specific site in a large molecule (an “enzyme”) called RuBisCo. So, too many CO2 molecules will have to queue for the availability of free sites. This is another basic concept of chemistry: it says that the reaction rate saturates when it depends on a limited number of sites. Indeed, the measurements tell us that doubling the CO2 concentration in plants may lead to no more than a 10% -20% increase in the plant growth rate.

You could say that even a small increase in productivity is not a bad thing, but it has two problems: 1) it does not always occur, and 2) it is not a good thing. One problem is that plants need a host of additional nutrients that the simplified view of the photosynthesis reaction does not show. If the plant does not have sufficient - say - nitrogen, it is pretty useless to add more CO2: the productivity will not increase.

Then, even if additional CO2 speeds up growth, the problem is that the plant remains stuck with the amounts of nutrients it can gather from its root system. No wonder that the result is a dearth of minerals. Less obvious is why the resulting tissues are also low in proteins and rich in carbohydrates, but it is one of the observed results of CO2 “fertilization”. It is exactly the opposite of what we need in nutritional terms, with most people in the world eating too much carbs and too little proteins.

All that is well known and you can read a more extensive description summary in Scientific American. But there is another point that is rarely mentioned in the discussion but that I think is important. It is that CO2 is not just a reactant in the photosynthesis reaction, but also a product of respiration, the reaction that provides energy for the metabolism of living creatures. It may be written like this:

C₆H₁₂O₆ (s) + 6 O₂ (g) → 6 CO₂ (g) + 6 H₂O (l) + energy,

which is, of course, a great simplification of another extremely complex reaction that occurs in several steps. In any case, the more carbohydrates the system burns, the more energy the creature has for its metabolism. And you note that, here, the CO2 molecule is on the right side of the reaction, not on the left, as it was the case for photosynthesis. It means that, if you want the reaction to go faster, you have to do the opposite, you need to remove CO2 molecules from the reaction centers in order to leave space for the oxygen molecules to react.

You see how these two needs are in contrast with each other: plants (may) want more CO2 to make photosynthesis faster, but both plants and animals want less CO2 to make their metabolism run faster. As it is often the case for biological systems, and all those systems called “holobionts,” the metabolism of the system is optimized in terms of a compromise: too little CO2 might make photosynthesis too slow or even impossible - and in the latter case it would simply kill the biosphere. But too much CO2 would make living creatures sluggish and unable to support stupendous structures such as the human brain that need huge amounts of metabolic energy to function.

This is not just theory. It is known that even a minor increase in CO2 concentration slows down the human capacity to process information. The preindustrial CO2 concentration was about 280 ppm. We are now at more than 400 ppm. Tests have shown that 600 ppm are enough to see a measurable slowdown in people’s mental capabilities. Inside homes, now usually sealed like tombs in order to save energy, the CO2 concentration is often well over 1000-2000 ppm (and it is more if you wear a mask). These concentrations do not cause physiological harm, but may be related to the “global dumbing” effect that’s being observed nowadays.

The great holobiont we call Gaia has optimized the metabolic processes of the biosphere over millions and millions of years. Humans pretend to intervene to “improve” on something that has no need to be improved, and the result is often something worse, if not a disaster. In this case, we risk making the biosphere a little lusher than before in exchange for making it uninhabitable for creatures with large brains. Never forget that Gaia knows best.