How do we get the carbon dioxide that was released into the air by burning fossil fuels out of there? There are quite a few suggestions on how to manage such “negative emissions”. However, there is already a natural process that does this: photosynthesis. This metabolic process in algae and green plants sucks around 400 gigatons of CO₂ from the earth's atmosphere every year. Photosynthetic organisms have been active for two to three billion years, and their exhaust gas, oxygen, made the development of multicellular higher life possible in the first place. "The global total output of photosynthesis corresponds to the equivalent of around 215 terawatts," says Axel Kleidon from the Max Planck Institute for Biogeochemistry in Jena.“That exceeds all non-biological processes in the atmosphere by a factor of more than four.” By way of comparison: In 2018, mankind's energy needs were covered by almost 19 terawatts over the course of the year.
As impressive as that is, on closer inspection the efficiency of photosynthesis is surprisingly puny.
“Even in the highly productive Amazon rainforests, photosynthesis is only around three watts per square meter,” explains Kleidon.
The sun provides 165 watts per square meter of radiant power in equatorial regions, and in our latitudes it is still 100 watts.
This means that the plants of the rainforest can only use around 1.8 percent of the solar energy flow.
They need about half of this to keep their metabolism going, with the other half they bind CO₂.
Photovoltaics are ten times more efficient
In the tropical rainforest, a little more than one kilogram of carbon is fixed per square meter and year. Photosynthesis in the ocean is only half as efficient as on land, mainly due to the limited supply of nutrients. But even well-fertilized agricultural areas hardly achieve more than three percent efficiency. Compared to the efficiency of around twenty percent with which today's commercial photovoltaic systems convert the energy of sunlight into electricity, photosynthesis is astonishingly weak. Why? So, for once, has evolution failed to find the cleverest solution?
There are different answers to this question, depending on whether you approach it from a microscopic perspective or from a macroscopic one like Axel Kleidon. In addition to detailed computer simulations, he also uses analytical models from the physics toolbox. He was able to show that the decisive hurdle for greater efficiency in photosynthesis does not lie in its molecular mechanism, but in the gas exchange with the environment.
Kleidon argues primarily with thermodynamics, the theory of the conversion of forms of energy.
It can thus be calculated that the binding of the carbon by the photosynthetic apparatus can basically use around 19 percent of the energy in the captured solar radiation and thus come close to the current photovoltaics - even though plants only use the long-wave, red part of the solar color spectrum.
Light waves of other colors leave the vegetable dyes, the chlorophylls, behind, which is why plants look green.
The plant as a heat engine
Photosynthesis should now be able to bind around 670 grams of carbon in plant material with one kilogram of water, if one draws up a balance sheet of the chemical reactions involved.
In fact, it is only around two grams of carbon per kilogram of water consumed.
The exact value varies somewhat depending on the climatic zone, plant species and variant of the photosynthesis process.
But the order of magnitude of this “water use efficiency” applies in principle.
That put Kleidon on the right track: The actual bottleneck for efficiency is at the very end, in the transpiration of the plants through the stomata of the leaves.