Trees represent one of the most abundant natural resources on Earth’s land masses, and scientists and engineers at North Carolina State University are making significant progress in exploring their potential as sustainable and environmentally friendly alternatives for producing industrial chemicals. Lignin, the polymer responsible for the rigidity and resistance to degradation in trees, has presented challenges.
Now, researchers at NC State have pinpointed the specific molecular property of lignin – its methoxy content – that determines the feasibility of using microbial fermentation to convert trees and other plants into industrial chemicals.
The breakthrough brings us one step closer to producing industrial chemicals from trees in an economically and environmentally sustainable manner, as an alternative to petroleum-derived chemicals, according to Robert Kelly, the corresponding author of a paper in the journal Science Advances detailing the discovery.
Kelly’s team had previously demonstrated that certain extreme thermophilic bacteria, found in environments like Yellowstone National Park’s hot springs, can break down the cellulose in trees, although not to a significant extent, he explained.
“In other words, not at the level that would make economic and environmental sense for producing industrial chemicals.” As Kelly explained, “It turns out that there’s more than just low lignin at play.”
To address the challenge of high lignin content in trees, Kelly, the director of NC State’s Biotechnology Program and Alcoa Professor in the Department of Chemical and Biomolecular Engineering has collaborated with Associate Professor Jack Wang, heading the Forest Biotechnology Program in NC State’s College of Natural Resources, for over a decade.
Wang, who is also a faculty member with the N.C. Plant Sciences Initiative and his team utilized CRISPR genome editing technology to engineer poplar trees with modified lignin content and composition. The choice of poplar trees is strategic due to their fast growth, minimal pesticide requirements, and ability to thrive on marginal lands unsuitable for food crops.
Kelly’s team discovered that while some CRISPR-edited trees were suitable for microbial degradation and fermentation, not all of them exhibited the desired traits. According to Kelly’s former Ph.D. student, Ryan Bing, this discrepancy is attributed to variations in the bacteria’s preferences 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 the plant matter and convert it to 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 the next?” he explained. “We found an answer to this by looking at how these bacteria eat plant matter of various compositions.”
In a subsequent investigation, Kelly and Bing conducted experiments to assess the efficacy of a genetically modified bacterium, Anaerocellum bescii, originally obtained from hot springs in Kamchatka, Russia. They examined its ability to break down Wang’s engineered poplar trees, which exhibited varying lignin contents and compositions.
The researchers observed a correlation between the degradability of the trees and their lignin methoxy content, indicating that lower methoxy content resulted in increased degradability.
“This cleared up the mystery of why lower lignin alone is not the key – the devil was in the details,” Kelly said. “Low methoxy content likely makes the cellulose more available to the bacteria.”
Wang developed low-lignin poplars to enhance their suitability for papermaking and other fiber products. However, recent research indicates that poplars engineered with not only low lignin but also low methoxy content are most effective for producing chemicals through microbial fermentation.
While Wang’s engineered poplars thrive in the greenhouse, field test results are pending. Kelly’s team has previously demonstrated that low-lignin poplar trees can be converted into industrial chemicals, such as acetone and hydrogen gas, yielding favorable economic outcomes and minimal environmental impact.
If these trees hold up in the field and “if we keep working on our end,” Kelly said, “we will have microbes that make large amounts of chemicals from poplar trees, now that we know the marker to look for – the methoxy content.”
Researchers like Wang now have a specific target for creating poplar lines tailored for chemical production. Wang and colleagues have recently started field trials of advanced lignin-modified poplar trees to tackle this issue.
Currently, traditional methods involve chopping wood into smaller pieces and using chemicals and enzymes to pretreat it for further processing to make chemicals from trees.
Using engineered microbes to break down lignin offers several advantages, including lower energy requirements and reduced environmental impact, according to Kelly.
While enzymes can be used to break down cellulose into simple sugars, they need to be continually added to the process. On the other hand, certain microorganisms continuously produce the essential enzymes, making the microbial process more cost-effective, he explained.
“They also can do a much better job than enzymes and chemicals,” Kelly added. “They not only break down the cellulose but also ferment it to products, such as ethanol – all in one step.
“The high temperatures that these bacteria grow at also avoid the need to work under sterile conditions, as you would need to do with less thermophilic microorganisms to avoid contamination,” he added. “This means that the process for turning trees into chemicals can operate like a conventional industrial process, making it more likely to be adopted.”
Daniel Sulis, a co-author of the Science Advances paper and a postdoctoral researcher in Wang’s lab, emphasized that the increasing environmental catastrophes driven by climate change underscore the critical necessity for conducting research to discover alternative solutions that reduce our reliance on fossil fuels.
“One promising solution lies in harnessing trees to meet society’s needs for chemicals, fuels, and other bio-based products while safeguarding both the planet and human well-being,” Sulis added.
“These findings not only move the field forward but also lay the groundwork for further innovations in using trees for sustainable bio-based applications.”
Journal reference:
- Ryan G. Bing, Daniel B. Sulis, Morgan J. Carey, Mohamad J. H. Manesh, Kathryne C. Ford, Christopher T. Straub, Tunyaboon Laemthong, Benjamin H. Alexander, Daniel J. Willard, Xiao Jiang, Chenmin Yang, Jack P. Wang, Michael W. W., Robert M. Kelly. Beyond low lignin: Identifying the primary barrier to plant biomass conversion by fermentative bacteria. Science Advances, 2024; DOI: 10.1126/sciadv.adq4941
