A recent paper in Nature Biotechnology highlights an exciting development in our continued quest for independence from fossil fuels. A group of researchers at Northwestern University and LanzaTech recently published their work outlining the carbon-negative production of industrial-scale acetone and isopropanol: two important scaffolding chemicals. The researchers used engineered bacteria to create an industrial manufacturing pipeline for each chemical while actively fixing carbon from gaseous waste.

Our dependency on fossil fuels goes well beyond cars and transportation. Manufacturing relies heavily on oil, natural gas and coal, all of which are used to make industrially important chemicals. The trillion-dollar global chemical industry is the largest energy consumer by sector and the third-largest emitter of carbon dioxide waste. Acetone, isopropanol and other chemicals rely heavily on fossil fuels, and manufacturers have long sought sustainable alternatives for producing these chemicals at an industrial scale.

An exciting alternative already exists in nature, as many microbes are known to produce chemicals important for various manufacturing processes. Unfortunately, most microbes cannot manufacture these chemicals in sufficient quantities. Even the best microbes identified to date can only produce chemicals at about 50 percent yield. By successfully optimizing a strain of bacteria, the researchers were able to make acetone and isopropanol at industrial scales through gas fermentation.

"The accelerating climate crisis, combined with rapid population growth, pose some of the most urgent challenges to humankind, all linked to the unabated release and accumulation of CO₂ across the entire biosphere," said Dr. Michael Jewett, co-senior author of the study. "By harnessing our capacity to partner with biology to make what is needed, where and when it is needed, on a sustainable and renewable basis, we can begin to take advantage of the available CO₂ to transform the bioeconomy."

Acetone and isopropanol alone have a combined global market value of $10 billion.

Both compounds are industrial solvents and chemicals used to make acrylic glass, polypropylene and other vital products that feed into global supply chains. Production of acetone and isopropanol relies on propene cracking or reforming, an energy-intensive process that creates hazardous waste and releases greenhouse gases. No green chemistry options exist for industrial-scale production of the two chemicals.

Recent years have highlighted the importance of both chemicals globally. Isopropanol is a WHO-approved disinfectant capable of killing the virus that causes COVID-19. The pandemic resulted in global shortages in isopropanol, affecting industries beyond healthcare as companies scrambled to acquire effective disinfectants.

Acetone, on the other hand, is being explored as an alternative fuel source and an additive for existing fuels. As the chemical is developed as a sustainable fuel, global production will likely need to increase, and green chemistry options will be necessary to help reduce the carbon footprint of acetone production.

Due to the global importance of both chemicals and a lack of green chemistry options, microbial production of acetone and isopropanol may be an exciting alternative for sustainable manufacturing at scale.

Biomanufacturing relies on natural fermentation processes to enable the sustainable production of industrial chemicals. Humans have long relied on microbial fermentation to produce beer, bread, soy sauce and more. Although many microbes can make industrially important chemicals in the environment, a consistent issue remains scaling this production to meet global demands.

When it comes to the production of acetone, Chaim Weizmann developed one of the first industrial fermentation processes to produce chemicals. He used bacteria to develop the ABE process in the early 20^th century, which makes acetone, butanol and ethanol in a 3:6:1 ratio. ABE produced more than 500 metric tons of chemicals per year at its peak. However, the process was eventually phased out in favor of other industrial pipelines that selectively make one product at lower costs. Unfortunately, these alternative processes rely on fossil fuels, a non-renewable resource that contributes to the production of greenhouse gases.

Since the phasing out of ABE, many researchers have tried to decouple the production of acetone and isopropanol from butane synthesis to improve the yields of the individual chemicals. Significant progress has been made in E. coli, but low yields continue to be a significant problem hindering the scaling of production. With E. coli, producing either acetone or isopropanol from sugar substrate peaks is at about 50 percent yield due to carbon dioxide production as the sugar breaks down.

An alternative to sugar fermentation is gas fermentation, in which microbes use C1 gases, such as carbon monoxide or carbon dioxide, instead of C6 sugars, such as glucose, to produce important chemicals. These C1 gases are already made in enormous quantities as by-products in industrial and agricultural processes. Capturing and recycling these gas waste products could help offset the effects of climate change while sustainably producing scaffold chemicals.

Previous work found that genetic engineering of specific types of bacteria called acetogens led to the small-scale production of more than 50 different economically important compounds using C1 gases, including isopropanol and acetone. However, the ongoing issue remains achieving the production of these chemicals at scale.

With their new research, the group at Northwestern University and LanzaTech were not starting from scratch. Last year, the researchers successfully scaled the production of ethanol using microbial gas fermentation. Now, two industrial plants produce ethanol via this gas fermentation process, resulting in an output of 90,000 metric tons per year. Their carbon-fixing ethanol is already being used to make sustainable polyethylene for L’Oréal packaging.

Building on the success of their ethanol-producing bacteria, the researchers wanted to develop a similar pipeline to produce acetone and isopropanol. Interestingly, despite a long history of ABE fermentation, the researchers found very little diversity was known about enzymes capable of synthesizing acetone. Without enzyme and strain diversity, it can be challenging for engineers to determine how species of bacteria naturally select for increased production or more efficient synthesis of specific chemicals.

So, in their quest to develop gas fermentation of acetone and isopropanol, the researchers began by optimizing the biosynthetic pathway they would use to manufacture both chemicals. Taking inspiration from natural biodiversity, the researchers analyzed genes from a collection of 272 ABE strains. They identified individual enzymes optimized for the biosynthesis of either acetone or isopropanol. They then further optimized the bacterial strain in which they would engineer the pathway by identifying and modifying biological bottlenecks hindering the production of the chemicals.

They tested their optimized bacterial strain in a 120L pilot reactor and determined they could generate about 3g/L/h for three weeks using C1 gas waste as their substrate. The fermentation bioreactor they used throughout their studies now represents interchangeable infrastructure for manufacturing ethanol, acetone and isopropanol. This is a unique divergence from traditional manufacturing infrastructure that is usually purpose-built to produce a single chemical.

With microbial production of acetone and isopropanol previously limited by a maximum yield of 50 percent, the recent paper in Nature Biotechnology highlights an exciting future for sustainable, industrial-scale manufacturing of important chemicals. Using gas waste as the feedstock for their production pipeline, the researchers decoupled manufacturing of both chemicals from the cost of sugar substrates. Traditional production of acetone and isopropanol is associated with the release of greenhouse gases, with manufacturing resulting in nearly double the weight of carbon dioxide released per kilogram of the chemical produced.

A carbon-fixing manufacturing process represents a crucial step toward a circular economy. Manufacturing facilities can now use industrial greenhouse gas waste to produce important chemicals at an industrial scale. Efficient carbon recycling can help manufacturers use existing waste to produce chemicals at impactful scales, helping to mitigate the climate crisis. Having achieved a successful track record with ethanol production, there may likely be the adoption of this pipeline for acetone and isopropanol manufacturing in the near future.