Reverse photosynthesis — sunlight can be used to produce chemicals and energy

Researchers from the University of Copenhagen have discovered a natural process that sees the energy in solar rays break down plant material, rather than build it up. Their discovery has been published in the journal Nature Communications.

“Photosynthesis by way of the sun doesn’t just allow things to grow; the same principles can be applied to break plant matter down, allowing the release of chemical substances,” explained Professor Claus Felby, who led the research. “In other words, direct sunlight drives chemical processes. The immense energy in solar light can be used so that processes can take place without additional energy inputs.”

The researchers discovered that monooxygenases — natural enzymes used in industrial biofuel production — multiply their effectiveness when exposed to sunlight. As explained by study co-author Klaus Benedikt Møllers, these enzymes use atmospheric oxygen and the sun’s rays to break down and transform carbon bonds in plants, among other things.

The result can be recreated using the following materials:

• A large sugar molecule to be oxidised, broken down from biomass such as straw and wood, for example.
• Lytic polysaccharide monooxygenase (LPMO), an enzyme found in many fungi and bacteria.
• Some chlorophyll containing green extract from leaves.

Everything is mixed in a test tube and exposed to sunlight. The sunlight is collected by the chlorophyll — the same molecule as used in photosynthesis. With the help of the LPMO, the energy of the sunlight breaks down the biomass. This means that it becomes easier to break down the larger sugar molecules into smaller constituents.

Without sunlight, it would take hours or days to achieve this transformation. With sunlight, it takes five minutes.

Described by the researchers as ‘reverse photosynthesis’, the process has the potential to break down the chemical bonds between carbon and hydrogen — a quality that may be developed to convert biogas plant-sourced methane into methanol under ambient conditions. As a raw material, methanol can be used by the petrochemicals industry and processed into fuels, materials and chemicals.

“By using the sun, we can produce biofuels and biochemicals for things like plastics faster, at lower temperatures and with enhanced energy efficiency,” said co-author David Cannella. “Some of the reactions, which currently take 24 hours, can be achieved in just 10 minutes by using the sun.”

By increasing production speed while reducing pollution, the discovery has the potential to revolutionise industry. Professor Felby said, “This is a game changer — one that could transform the industrial production of fuels and chemicals, thus serving to reduce pollution significantly.”

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