Bacteria have inspired artificial enzymes that can convert carbon dioxide into hydrocarbons, which could be used to make fuel or plastics.
Azotobacter vinelandii is a harmless soil bacterium found everywhere on the planet while Methanosarcina acetivorans survives in marine sediments. They both contain nitrogenase enzymes which have been shown to turn carbon dioxide into hydrocarbons – including methane, propane, butane and ethylene.
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±·´Ç·ÉĚýÂ at the University of California at Irvine, and her team, have developed synthetic versions of these naturally-occurring enzymes. When they bubbled carbon dioxide through water containing colonies of bacteria with this enzyme, or the artificial one, both versions converted the gas into a mixture of hydrocarbons.
“Some, such as methane, propane and butane, could be used as fuel sources and others, such as ethylene, could be used to generate polymers such as polyethylene and other plastics,” says Hu.
Currently, these hydrocarbons – many used to make plastics – are manufactured through an energy-hungry, industrial-scale procedure. At temperatures up to 300°C and pressures tens of times higher than atmospheric pressure, it scrunches carbon monoxide and hydrogen gases together to create the liquid hydrocarbons.
By contrast, the nitrogenase enzymes make the hydrocarbons at room temperature and without extra pressure. The artificial version worked even better than the natural versions, producing yields up to fourfold higher.
One step conversion
Already, companies around the world are commercialising processes for turning carbon dioxide into useful products, including plastics, condoms and concrete. But the mild reaction conditions needed for the enzymes to work could make them an attractive rival process.
Hu says that there are other natural enzymes that break down carbon dioxide, but they don’t do it alone or in one go. “Ours is the first enzyme that is capable of a one-step conversion of carbon dioxide into hydrocarbons,” she says. “Also, ours is the most simplified enzymic catalyst yet discovered that performs such a difficult reaction under everyday conditions.”
Although the work is currently only at lab scale, and the yields much too low to be practically useful, the team is confident the process can be scaled up. One approach would be to genetically engineer bacteria so they produce much more of the enzyme than normal. Another is to refine the artificial versions so they work much more efficiently.
One unexpected implication from this discovery is the possibility these enzymes may have played a hitherto unknown role helping life to establish on early Earth.
The hydrocarbons they created might have helped kick-start life on Earth by turning the primordial atmosphere’s carbon dioxide and monoxide gases into simple hydrocarbons that could be consumed by primitive life-forms.
“It may have made carbon bio-available for simple, early life forms on Earth,” says Hu.
Nature Catalysis
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