We often look to the smallest lifeforms, to help solving the biggest problems. Microbes help to make food and beverages, cure diseases, treat waste and even clean up pollution. Bacteria and Yeast can also convert plant sugars into biofuels and chemicals which is traditionally derived from fossil fuels — a main factor of most plans to slow climate change.
Presently the University of Wisconsin–Madison researchers have engineered bacteria which can produce two chemical products at the same time from underutilized plant fiber. And unlike humans, these multitasking microbes can do both things equally well.
Tim Donohue, UW–Madison professor of bacteriology and director of the Great Lakes Bioenergy Research Center says, according to his knowledge, it’s one of the first times you can make two valuable products simultaneously in one microbe.
The finding which was, detailed in a paper in the December issue of the journal Applied and Environmental Microbiology, could help to make biofuels more supportable and commercially viable.
In principle, the approach reduces the net greenhouse gas emissions and improves the economics,” Donohue says. The amount of energy and greenhouse gas which you need to make two products in one pot is going to be less than running two pots to make one product in each pot.
Every molecule counts
The quest to replace fossil fuels with sustainable substitutes hinges on extracting the most possible value from renewable biomass. Just as with petrochemicals, every molecule counts. Low-volume, high-value products will help to keep fuel more affordable.
One of the biggest barriers is a part of the plant cell wall called lignin. Lignin is the world’s most abundant source of renewable aromatic carbons, but its irregular structure makes it extremely difficult to break apart into useful components.
That’s the reason why scientists with GLBRC have studied a bacterium named Novosphingobium aromaticivorans, which is sometimes referred to as simply Novo, can digest many components of lignin and is quite easy to genetically modify.
In 2019, researchers engineered a strain of Novo which can produce an important ingredient of plastics like nylon and polyurethane known as PDC. Recently, a team in Donohue’s lab discovered another modification which allows Novo to make a different plastic ingredient called ccMA. But they didn’t stop there.
Ben Hall, a recent doctoral graduate said that, we’re not going to solve our carbon emissions problem by only producing two products.
Donohue’s team used genomic modeling to come up with a list of potential products which could be made from biomass aromatics. Near the top of the list was zeaxanthin, one of a group of organic pigments known as carotenoids.
Carotenoids, which give carrots, pumpkins, salmon and even flamingos their distinctive hues, are used as nutritional supplements, pharmaceuticals and cosmetics have a cumulative market value worth billions of dollars a year.
Researchers knew that Novo had the genes to produce another carotenoid with less market value. Based on the bacteria’s genome sequence, they suspected zeaxanthin is a steppingstone to that less valuable carotenoid in the process which cells use to make complex molecules. It was just a matter of altering the right genes to stop the digestive assembly line at the more valuable product.
By deleting or adding selected genes, they engineered strains which produced zeaxanthin as well as other valuable carotenoids — beta-carotene, lycopene and astaxanthin, when grown on an aromatic compound, and usually can be found in lignin.
Next, the team showed that the engineered bacteria could produce the same carotenoids from a liquor made from ground and treated sorghum stems, a solution which contains a mixture of aromatics which many industrial bacteria can’t digest.
One pot, two products
Hall then wondered what would happen if he joined the genetic changes needed to make PDC and a carotenoid in the same microbe.
The resulting strains produced both PDC and the target carotenoid with no noticeable loss to either yield. Even better, the bacteria collected carotenoids within their cells, which must be separated from the solution that contains the PDC, which they secreted.
Hall says, we’re already separating the cells from the media, now we would have a product coming out of both.
The next steps include testing whether engineered strains can simultaneously produce carotenoids and ccMA, which Donohue assumes that they will, and to engineer strains to improve yields in industrial conditions.
While there are lucrative markets for each of these products, Donohue and Hall say the real value of the discovery is the ability to add many functions to this biological platform.
He also added that, to him, it’s both the strategy and the products, and since we’ve done this, I think it opens the door to see if we can create other microbial chassis which make two products.
