It’s a tough job, but someone has to do it. In this case, the “function” is to break down lignin, the structural biopolymer that gives stems, bark and woody branches their characteristic. One of the most abundant terrestrial polymers on Earth, lignin surrounds valuable plant fibers and other molecules that can be turned into biofuels and other chemicals—if only we could get past the tough plant cell wall.
Fortunately, the somewhat laborious process is already happening in the guts of large herbivores through the actions of anaerobic microbes that cows, goats, and sheep depend on to release nutrients trapped behind the biopolymer. In a paper published in the journal nature Microbiology, chemical engineering researchers at the University of California, Santa Barbara and professor of biological engineering in Michelle O’Malley’s lab have demonstrated that a group of anaerobic fungi – Mycorrhizal fungi Neocallimastigomycetes – at the task level. They conducted this work in collaboration with colleagues at the US Department of Energy (DOE) Joint Genome Institute, Lawrence Berkeley National Laboratory, Joint Bioenergy Institute, and Great Lakes Bioenergy Research Center.
“You can think of lignin as kind of a structural system for plants,” said O’Malley, whose research focuses on finding and accessing alternative sources of energy and chemicals from what would be considered plant waste. In addition, she said, lignin has properties that make the plant resistant to physical degradation by enzymes and pathogens. “Lignin is really important — it provides that rigidity and structure, but it’s also difficult to analyze for exactly the same reason.”
For decades, it was thought that lignin could only degrade in the presence of oxygen. “It takes time, and it depends on certain chemical species — like free radicals — which as far as everyone knows can only be made with the help of oxygen,” O’Malley explained.
However, there have been hints all along that nature has more than one way of getting rid of lignin. In the realm of industrial biomass, to get to the cellulose and hemicellulose beyond lignin, plant biomass must typically undergo pre-treatment. But in O’Malley Lab’s work with anaerobic microbes, pre-treatment was never necessary.
“We never had to extract the lignin from there because the fungi we work with are equally happy to extract cellulose and hemicellulose on their own,” she said. “So the fact that these fungi can grow on untreated plant biomass has always been a unique and unusual feature, and we hypothesized that they must have a way to move lignin.”
To find out for sure, the O’Malley Lab conducted experiments with members of the neocallymastigomycetes group. Tom Lankiewicz, lead author of the study, grew some of these fungi on poplar, sorghum, and switchgrass in an oxygen-free environment. These three types of biomass were selected for the different ways in which lignin presents itself in nature, from the resilient stems and leaves of grasses to the hardier woods of poplar. In addition, the DOE views these plants as potential feedstocks for biofuels and bioproducts.
Then the team tracked the fungus’ progress as they went to work on the tough fibers and found that, Neocalimastics California The tough cell walls of plants are broken down. Using advanced imaging techniques such as nuclear magnetic resonance, they can determine the breaking of specific lignin bonds in the absence of oxygen.
“This is really a paradigm shift in terms of how people think about the fate of lignin in the absence of oxygen,” O’Malley said. “You can extend this to understanding what happens to the embryo in a compost heap, or in an anaerobic digester, or in very deep environments where oxygen is not available. It advances our understanding of what happens to biomass in these environments and changes our perception of what is possible and the chemistry of what happens there.”
While this research proves that lignin can be broken down by fungi in oxygen-free environments, the researchers’ next challenge is to figure out exactly how. Are there enzymes that mediate this process? Is this a feature of anaerobes in general? As with any interesting research, each answer leads to more questions – questions that require further research and fruitful collaboration.
“This, of course, is not just one lab effort,” O’Malley said. “It’s only been achieved because we have so many collaborators who bring to the table their different kinds of expertise.”