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Raphael Thys
The ability to convert waste gases from steel mills into commodity chemicals could make Clostridium autoethanogenum bacteria an industrial champion.
Chemicals cost more than just money: Today, petrochemical production spews out nearly 2% of the world’s greenhouse gas emissions. Now, researchers have taken an important step to vastly reduce that footprint, by using bacteria and waste gases from steel plants, rather than petroleum, as the starting ingredient for dozens of commodity chemicals. So far, the process has been used for three commodity chemicals. But because researchers may be able to expand it to others, it could help the chemical industry escape its reliance on fossil fuels, and effectively remove carbon from the skies.
“[Harnessing] biology to utilize waste gas and produce industrial chemicals is really exciting,” says Corinne Scown, a biofuels expert with the Lawrence Berkeley National Laboratory, who was not involved with the work. “It goes after two sectors at once that are difficult to decarbonize: steel production and industrial chemicals. It tackles a hard problem.”
Humans have exploited microbes for thousands of years to make products like alcohol, cheese, and yogurt. But industrial-scale biotechnology didn’t really take off until the beginning of the 1900s, when, for example, chemist Chaim Weizmann engineered several species of Clostridium bacteria to convert starches and sugars into acetone, an essential ingredient for making cordite, a smokeless alternative to gunpowder. (Weizmann’s success was key to the Allied victory in World War I and gained him fame; a Zionist politician as well as a chemist, Weizmann eventually became Israel’s first president in 1949.) But the rise of the petroleum industry in the 1950s and ’60s gave chemical manufactures cheaper starting materials for dozens of commodity chemicals, relegating most microbes to the industrial waste bin.
But not all. Today, yeast and Escherichia coli are widely used to produce a range of commodity chemicals, including ethanol for fuel and compounds used to make pharmaceuticals and plastics. But these industrial workhorses come with their own environmental footprint, because the sugars and starches they ferment come from crops like corn and sugarcane. By 2026, corn grown to make ethanol in the United States will take up to 19% of the nation’s farmland.
Now, synthetic biologists are pushing to grow microbes on a more environmentally friendly diet, using waste gases produced from industry, garbage dumps, and crop production, says Michael Köpke, a synthetic biologist at LanzaTech, a biotech company. In the 1990s, researchers engineered Clostridium autoethanogenum (C. auto), a bacterium originally enriched from rabbit feces, to produce ethanol from hydrogen gas and carbon monoxide (CO). Since then, researchers have slowly improved the ethanol yield and commercialized the process. In June 2018, LanzaTech opened the first production plant that uses the bacterium to make ethanol from a steel mill’s waste gas (a mix of mostly CO, carbon dioxide, and hydrogen gas), which would otherwise be vented into the atmosphere. The company now uses this technology to make some 90,000 tons of ethanol per year.
Still, C. auto has never matched the versatility of E. coli or yeast. “Up to now engineering Clostridium has just been hard,” says Michael Jewett, a synthetic biologist at Northwestern University. The bacteria are slow growing and they die from exposure to oxygen. In addition, researchers have few tailored gene-editing tools to alter the microbes’ metabolism.
Not anymore. Jewett, Köpke, and their colleagues used a multistep strategy to coax C. auto into churning out acetone and isopropanol (IPA), a colorless liquid found in disinfectants and cleaners. They started with a collection of 272 sugar-eating Clostridium strains maintained by retired South African microbiologist David T. Jones. The research team broke apart the bacterial cells and mined them for enzymes that appeared to play a role in acetone biosynthesis. They sequenced the genes for 30 of these enzymes and inserted combinations of them into C. auto via gene-delivery vehicles called plasmids. Separately, the researchers engineered C. auto’s metabolism to prevent carbon and hydrogen building blocks from making other unwanted chemicals. Ultimately, Jewett, Köpke, and their colleagues generated and screened 247 genetic variants of the bacteria to find the best chemical producers. The result, they report today in Nature Biotechnology, is are C. auto strains that continuously convert steel waste gases into enough acetone and IPA to make them viable candidates for large-scale commercial production.
Jewett expects the same process could make the bacteria produce a variety of other chemicals, such as butanol, used in varnishes, and propanediol, found in cosmetics. Scown adds that the novel synthetic biology techniques could also help transform other recalcitrant microbes into industrial workhorses. “We’re just scratching the surface in terms of the [microbial] hosts that might be viable in the future,” she says.
The advance could also open the door to engineering microbes capable of feeding on other waste gases, such as those produced by municipal solid waste and agricultural debris. And if, like C. auto, these microbes use more carbon in making their products than is released into the atmosphere, industrial chemical production may someday transform from a greenhouse gas rogue into a champion.