Researchers have developed a synthetic pathway to convert carbon dioxide in the air into organic compounds like glucose. The pathway, reported in the 18 November issue of Science, might someday be used to equip living plants to more efficiently sequester climate-warming greenhouse gas from the air.
"A significant challenge for the future is increasing carbon dioxide concentrations in the atmosphere," said senior author Tobias Erb, a research group leader at the Max Planck Institute for Terrestrial Microbiology in Marburg, Germany. "That's usually looked at as a problem, but at the same time, it's also a chance, because carbon is a source from which we can make useful products. Our motivation is to find new ways to remove carbon dioxide from the air and convert it into useful materials."
Although natural photosynthesis plays a vital role in absorbing and "fixing" carbon dioxide emitted from fossil fuel use, it has not prevented the net increase of this gas in the atmosphere since the Industrial Revolution. This is in part because a main plant enzyme involved in the process is relatively slow.
Other, more efficient enzymes do exist. "Knowing this, we saw a chance to improve existing biology with synthetic biology," Erb said.
To reinvent carbon dioxide fixation using such enzymes, Erb and colleagues carefully selected 17 enzymatic compounds from nine organisms — including bacteria, archaea, plants, and humans — bringing them together in a single, collaborative pathway. "Finding all the enzymes for each individual step was a tough business," said Erb. "It took us more than two years to screen, test and engineer about five dozen enzyme candidates and variants to identify the final set of 17 that we used to construct our cycle."
He and his colleagues used a series of optimization techniques, including a redesign of individual enzymes involved, to meticulously refine and improve their cycle.
In a laboratory experiment they demonstrated that the pathway could capture carbon dioxide at a rate faster than the natural Calvin cycle in plants, in which carbon dioxide enters the leaf and sugar is synthesized.
"With our cycle," Erb said, "we are not only comparable to plants, if not faster, in carbon dioxide fixation speed, we are also cheaper in terms of energy required."
Apart from one day boosting plants' synthetic capabilities, this pathway might be used in systems that recapture carbon dioxide to turn it into cattle feed.
"To make cattle feed now, the petrochemical industry relies mainly on building blocks generated from fossil fuels," said Erb. "Our vision is to create pathways to these building blocks from atmospheric carbon dioxide. This is something that chemistry still cannot do."
Erb explained why it has been so difficult to date for scientists to create an efficient synthetic carbon dioxide fixation cycle like the one they demonstrated in the new study.
"Many approaches in synthetic carbon dioxide fixation have been driven by top-down 'trial-and-error' approaches through direct implementation of new routes into the complex background of living organisms," Erb said. "Our idea was to build synthetic carbon dioxide-fixation from bottom-up in a controlled environment to learn how the individual components worked together, and which parts needed to be improved, and then transplant that pathway into living organisms."
"Looking ahead," Erb added, "my dream is to come up with designer catalysts in the lab that we can use … to synthesize any desired product or chemical building block from carbon dioxide."