Biomass is a vital source of energy used by people and myriad other organisms. Two decades ago, microbiologist Barry Goodell discovered that some microbes have a special mechanism for the breakdown and recycling of wood biomass. This unique system was called “brown rot fungi,” and while three types have now been classified, the exact details of the process have been a mystery. New work by Goodell and colleagues has determined that this bioconversion actually occurs without the help of enzymes, which are the usual drivers of chemical reactions. The work has been reported in Biotechnology for Biofuels.
The investigators learned that Basidiomycota brown rot fungi use a biocatalysis process that is mediated by a chelator, making it "very different than that used by any other microorganism studied," said Goodell. Chelators are a class of organic compounds that can attach to metal ions. In this instance, hydroxyl radicals are generated that can break the wood down into simple chemicals. Collaborators at Oak Ridge National Laboratory describe these efforts as "a paradigm shift in understanding fungal biocatalysis for biomass conversion."
“Our research on fungal bioconversion systems looks at a novel mechanism that has potential use in bio-refineries to 'deconstruct' woody biomass for conversion into platform chemicals for biopolymers or energy products,” said Goodell.
Brown rot fungi have evolved relatively recently and are one of the most prevalent decay fungi in North America. Because of their recent evolution, there are fewer species compared to white rot fungal species. "However, because of their efficiency in degrading wood, brown rot fungi have come to dominate, particularly in degrading softwoods," Goodell explained. These brown rot fungal species now dominate, doing around 80 percent of the softwood biomass carbon recycling in the world.
"Scientists used to think that these fungi would make holes in the cell wall that would let in the big enzymes," he noted. "But as we explain here, that is not how it works." Enzymes are very large molecules and would use a lot of energy, making them impractical.
"The fungi we study use a non-enzymatic, catalytic chelator-mediated Fenton system instead, a very simple process that makes use of hydrogen peroxide, also generated by the fungal system, and iron found in the environment," Goodell noted.
Learn more about wood decay from this video featuring Isabel Munck, a Forest Pathologist with the USDA Forest Service.
The researchers think that the efficiency of the fungus is derived from the chelator-mediated Fenton system, unlike white rot, which uses enzymes.
"This group of brown rot fungi figured out how to generate hydroxyl radicals at a distance, that is, away from the fungus, to keep them away so the radicals won't damage themselves while breaking down wood," explained Goodell. Hydroxyl radicals are powerful oxidizers, making them very damaging to cells.
"These fungi do produce a limited number of enzymes, but they come into play after the non-enzymatic action conversion by the fungi using chelators. The chelators are secondary metabolites, whose function is not easily followed using 'omics' techniques such as genomics. Using many advanced techniques though, we saw that some very small, low-molecular-weight compounds were working their way into the cell wall. This new paper describes how,” Goodell continued.
Goodell and Jellison describe how the fungi use hyphae, growth filaments, to break down the wood. "This group of fungi evolved a way to break down the wood substrate by first diffusing chelators into the cell wall. The fungus makes the chelator and produces hydrogen peroxide from oxygen, and together they start to digest the cell wall into the sugar found in the basic building block of wood, glucose, which the fungus can use as food. This is how these fungi are eating the wood,” explained Goodell.