Discovering the molecular properties of lignin could help turn trees into affordable, greener industrial chemicals

Trees are the most abundant natural resource on Earth, and scientists and engineers at North Carolina State University are making progress in finding ways to use them as sustainable, eco-friendly alternatives. environment to the production of industrial chemicals from petroleum.

Lignin, a polymer that makes trees rigid and resistant to degradation, has proven problematic. Now these NC State researchers know why. They identified the specific molecular property of lignin (its methoxy content) that determines how difficult or easy it would be to use microbial fermentation to turn trees and other plants into industrial chemicals.

The findings move us closer to making industrial chemicals from trees as an economically and environmentally sustainable alternative to petroleum-derived chemicals, said Robert Kelly, the corresponding author of a paper in the journal Scientific advances detailing the discovery.

Kelly’s group has already proven that certain extreme thermophilic bacteria, which thrive in places such as the hot springs of Yellowstone National Park, can degrade tree cellulose, but “not to a great extent,” he said. declared. “In other words, not at a level that would make economic and environmental sense for industrial chemical production.”

As Kelly explains: “It turns out there’s more than just low lignin at play.”

To get around the problem of high lignin content in trees, Kelly, director of NC State’s biotechnology program and Alcoa professor in the department of chemical and biomolecular engineering, has worked for more than 10 years with associate professor Jack Wang, chair of the department. of forest biotechnology. Program in the College of Natural Resources at NC State. Wang is also a faculty member of the NC Plant Sciences Initiative.

As reported in the newspaper Science in 2023, Wang and colleagues used CRISPR genome editing technology to create poplar trees with altered lignin content and composition. They focused on poplars because they are fast-growing, require minimal pesticide use, and grow on marginal land that is difficult to grow food crops on.

Kelly’s group found that some, but not all, of these CRISPR-edited trees worked well for microbial degradation and fermentation. Like his former Ph.D. explained student Ryan Bing, It turns out that these bacteria have different appetites for different types of plants.

“We can harness the ability of certain thermophilic bacteria from hot springs in places like Yellowstone National Park to eat plant matter and convert it into products of interest. However, these bacteria have varying appetites for different types of plants,” said Bing, who now works as a senior metabolic engineer for Capra Biosciences in Sterling, Virginia.

“The question was why? What makes one plant better than another?” he explained. “We found an answer to this question by examining how these bacteria eat plant materials of varying compositions.”

In a follow-up study, Kelly and Bing tested the extent to which a genetically modified bacteria originally isolated from the hot springs of Kamchutka, Russia, Anaerocellum bescii, destroyed poplars modified by Wang with lignin content and composition clearly different.

The researchers found that the lower the methoxy lignin content of the tree, the more degradable it was.

“This took away the mystery of why low lignin alone is not the key: the devil is in the details,” Kelly said. “Low methoxy likely makes cellulose more available to bacteria.”

Wang had bred low-lignin poplars to be better for making paper and other fibrous products, but recent research suggests that artificial poplars that not only have low lignin but also low methoxy content are best for making chemicals through microbial fermentation.

Wang’s artificial poplars grow well in greenhouses, but field test results are not yet available. Kelly’s group has already shown that low-lignin poplars can be converted into industrial chemicals, such as acetone and hydrogen gas, with favorable economic results as well as low environmental impact.

If these trees hold up in the field and “if we continue to work on our end,” Kelly said, “we will have microbes that make large amounts of chemicals from the poplars, now that we know the marker to look for: the methoxy content. “.

This gives researchers, like Wang, a specific target to produce poplar lines best suited for chemical production. Wang and colleagues recently initiated field trials of advanced lignin-modified poplars to answer this question.

Currently, it is possible to make chemicals from trees by traditional means: cutting the wood into smaller pieces, then using chemicals and enzymes to pre-treat it for further processing.

Using engineered microbes to break down lignin offers benefits including lower energy requirements and less environmental impact, Kelly said.

Enzymes can be used to break down cellulose into simple sugars, but they must continually be added to the process. Some microorganisms, on the other hand, continually produce key enzymes that make the microbial process more economical, he explained.

“They can also do a much better job than enzymes and chemicals,” Kelly added. “Not only do they break down cellulose, but they also ferment it into products, such as ethanol, all in one step.

“The high temperatures at which these bacteria grow also avoid the need to work in sterile conditions, as would be necessary with less thermophilic microorganisms to avoid contamination,” he added. “This means the process of turning trees into chemicals can work like a conventional industrial process, making it more likely to be adopted.”

Daniel Sulis, another author of the paper and a postdoctoral researcher in Wang’s lab, said environmental disasters fueled by climate change highlight the urgent need for research aimed at finding ways to reduce reliance on fossil fuels.

“A promising solution is to harness trees to meet society’s needs for chemicals, fuels and other bio-based products while preserving both the planet and human well-being,” Sulis added.

“These results not only advance the field, but also lay the foundation for additional innovations in the use of trees for sustainable bio-based applications.”