Harnessing the hidden powers of plants could help fix the climate

Luis Mendo

Photosynthesis is the source for much of the world around us. It produces wood for our buildings, fibres for our clothes, food to sustain life and much more, including medicine, dyes, rubber and, of course, fossil fuels. Indeed, our current society burns in one year what photosynthesis took one million years to make, leading to the enormous amounts of carbon now present in our atmosphere. In 2021 we will move towards harnessing this extraordinarily powerful process – artificially.
Photosynthesis is a two-step process, taking place first in light and then in dark. In daylight, a leaf uses the energy in photons from sunlight to separate water into its elemental components of oxygen and hydrogen. The oxygen is released into the atmosphere; the hydrogen is stored as a component of a molecule called nicotinamide adenine dinucleotide phosphate (NADPH). In the dark, the leaf absorbs CO2 from the air and combines it with the solar-produced hydrogen to provide it with chemical energy in the form of sugars.

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To replicate these two stages, my colleagues and I at Harvard University have created two devices, which we have called the Artificial Leaf and the Bionic Leaf. The Artificial Leaf is a silicon-based solar cell with different catalytic materials bonded on to each of its sides. Immersed in water, it uses the energy of sunlight to break down the water into oxygen and hydrogen. The Bionic Leaf extends this idea by incorporating a bacterium, Ralstonia eutropha, which absorbs carbon dioxide from air and combines it with the hydrogen produced by the Artificial Leaf to make liquid fuels.
In 2020, by replacing Ralstonia eutropha with another bacterium, Xanthobacter autotrophicus, we were able to develop a device that combines nitrogen from air and the hydrogen from the Artificial Leaf component to make fertiliser. The result is that, by using only sunlight, air and water, we have been able to manufacture renewable fuels and the building blocks for food production.
Artificial photosynthesis is ten times more efficient than natural photosynthesis. In 2021 we will begin to explore practical ways to scale up the technology. In the short term, this will lead to a new, decentralised infrastructure of energy, food and manufacturing that is carbon-free. Society is already using hydrogen, for example, to fuel vehicles.
In the longer term, we will see the manufacturing of plastics, pharmaceutical drugs and chemicals driven by the Sun. And, when we finally send humans to Mars, we will be able to use the process to break down the water in the astronauts’ urine and combine it with the carbon dioxide they exhale to produce synthetically engineered drugs, vitamins, food, medicines and more.

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The world is urgently seeking ways to produce what it needs without using fossil fuels. In 2021, we will move a step closer by harnessing a powerful natural process that was under our noses all along.
Daniel Nocera is Patterson Rockwood professor of energy in the department of chemistry and chemical biology, and director of the Nocera Lab at Harvard

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