Fungi to the Rescue!

It’s a tough job, but someone’s got to do it. In this case, the “job” is the breakdown of lignin, the structural biopolymer that gives stems, bark and branches their signature woodiness. One of the most abundant terrestrial polymers on Earth, lignin surrounds valuable plant fibers and other molecules that could be converted into biofuels and other commodity chemicals — if only we could get past that rigid plant cell wall.
Fungi to the Rescue!

Fortunately, the rather laborious process already occurs in the guts of large herbivores through the actions of anaerobic microbes that cows, goats and sheep rely on to release the nutrients trapped behind the biopolymer. In a paper published in the journal Nature Microbiology, researchers in UC Santa Barbara chemical engineering and biological engineering professor Michelle O’Malleys lab prove that a group of anaerobic fungi — Neocallimastigomycetes — are up to the task. The researchers conducted this work in collaboration with colleagues at the U.S. 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 skeletal system for plants,” said O’Malley, whose research focuses on finding and accessing alternate sources of energy and chemicals from what would otherwise be considered plant waste. Additionally, she said, lignin has properties that make the plant resistant to physical degradation by enzymes and pathogens.

“Lignin is really important — it provides that hardiness and structure, but it’s equally difficult to break down for the exactly the same reason.”

“We’ve never had to extract the lignin out of there because the fungi we work with are just as happy to extract the cellulose and hemicellulose on their own,” she said. “So the fact that these fungi could grow on non-pretreated plant biomass was always a feature that was unique and unusual, and we hypothesized that they must have a way of moving the lignin around.”

To find out for sure, the O’Malley Lab conducted experiments with members of the Neocallimastigomycetes group. Tom Lankiewicz, the study’s lead author, cultivated some of these fungi on poplar, sorghum and switchgrass in an oxygen-free environment. These three types of biomass were chosen for the various ways lignin presents itself in nature, from the flexible stems and leaves of the grasses to the more rigid wood of poplar. In addition, these plants are being eyed by the DOE as potential feedstocks for biofuels and bio-based products.

The team then tracked the progress of the fungi as they went to work on the tough fibers and found that indeed, Neocallimastix californiae did break down the plants’ rigid cell walls. Using advanced imaging techniques such as nuclear magnetic resonance, they could identify specific lignin bond breakages in the absence of oxygen.

While this research proves that lignin can be broken down by fungi in oxygen-free environments, the next challenge for the researchers is to find out exactly how.

“This, of course, is not just a single lab effort,” O’Malley said. “It was made possible only because we’ve had so many collaborators that bring to the table their different kinds of expertise.”

PHOTO:  Goats, cows and sheep host anaerobic microbes that can break through tough plant cell walls to gain the nutrition behind them

Information courtesy of Sonia Fernandez, UC Santa Barbara

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