Rot is Hot to Trot: Brown Rot Fungi cries “Tora! Tora! Tora!” to biomass
As any observer of roadkill knows, time takes care of even the most complex organisms if they die by the side of the road — rots sets in, and time takes its toll, and bones are generally all that is left by the side of the road. Rot is nature’s free bioconversion technology — and we don’t know enough about rot as we should.
When it comes to fungi, their ability to break even the most recalcitrant of hardwoods down is well-documented and anyone who has spent a day in a rainforest knows it. But there’s rot and there’s rot when it comes to fungi. That is, there’s white rot and brown rot.
The older system is white rot. This is an enzyme-based attack on biomass — and if humans have been designing enzymes as a biomass conversion platform — white rot fungi is one of the reasons. We learned quite a bit about rot in the Second World War when, as Biotechnology for Beginners explains, “the cotton shirts and cartridge belts of US soldiers disintegrated at an alarming rate…rot was shouting “Tora! Tora! Tora!” in a sneak attack on the war effort, with considerable success.
Now, most microorganisms use enzymes to break down compounds, but enzymes are huge molecules and physiologically “expensive” to produce because they contain so much nitrogen, said Barry Goodell, a professor at the University of Massachusetts Amherst.
Evolution takes a step
Then, along came brown rot fungi. They’re recent arrivals in evolutionary terms, but they are taking over the world.
Brown rot fungi appear in both the northern and southern hemispheres and are some of the most common decay fungi in North America. Because they evolved relatively recently, there are fewer brown rot species compared to older white rot species.
“However, because of their efficiency in degrading wood,’ Goodell said, “brown rot fungi have come to dominate, particularly in degrading softwoods. They now dominate by recycling approximately 80 percent of the softwood biomass carbon in the world, found mostly in the great forests of the northern hemisphere.”
What’s new? Basidiomycota brown rot fungi, use a non-enzymatic, chelator-mediated biocatalysis method that is “very different than that used by any other microorganism studied,” he says. Chelators are organic compounds that bind metal ions, and in this case, they also generate “hydroxyl radicals” to break down wood and produce simple building-block chemicals.
“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 notes, “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.”
The Tyranny of Distance
Why is this important? Hydroxyl radicals are very damaging to cells, the most potent oxidizing agents known in biological systems.
The Mechanism of Destruction
Goodell says, “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.
Goodell and Jellison relate a process that begins with the fungi in the lumen – the hollow space found inside plant cells. Using their hyphae, thread-like growth filaments, the fungi then mount a biochemical attack on the wood cell components.
As Goodell explains, “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.”
Paradigm shift
Described by collaborators at Oak Ridge National Laboratory as “a paradigm shift in understanding fungal biocatalysis for biomass conversion,” the findings appear in the current issue of Biotechnology for Biofuels.
The team
Goodell and Jellison’s collaborators include scientists at the Chinese Academy of Sciences, Beijing; Pennsylvania State University; Swansea University, U.K.; University of Agricultural Science, Uppsala, Sweden; Tokyo University of Agriculture and Technology; USDA Forest Service Southern Research Station, Pineville, La., and Oak Ridge National Laboratory, Tenn. Funding was from these organizations and the U.S. Department of Energy, and USDA National Institute of Food and Agriculture.
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