A few genetic modifications can induce a strain of soil bacteria to convert a renewable material, lignin, into plastics. The best part? Lignin is so cheap and plentiful we don’t even bother trying to use it right now.
Woody plants show great promise as a potential replacement for petroleum in various uses — such as fuel, plastics, and chemical production. They contain a lot of sugars, which can be used for those applications, but they’re kept out of reach behind the cellulose in their cellular walls.
Those walls are so durable and hard (read: ‘expensive’) to break down industrially, that we generally don’t really bother extracting the materials.
Mister bacteria, break down this wall
A team of researchers at the Great Lakes Bioenergy Research Center (based at the University of Wisconsin-Madison-based and funded by the Department of Energy) hopes that a bacteria species can point the way to woody-plants-based replacements for petroleum. Their plan is to take this microscopic critter, tweak its genome around a bit, and unleash it on the plants’ cells — where it will transform all the lignin, a polymer that ties cellulose to the sugars, into something we can actually use.
Lignin is actually super abundant. It’s the second-most abundant type of aromatic compound (those ‘rings’ you see in organic chemistry) on the planet after petroleum, the team explains. However, isn’t very valuable right now. That’s actually an understatement. Lignin today is so cheap that paper mills — which have been in the business of stripping lignin from wood for centuries — can’t even bother trying to sell the stuff; they just dispose of it in huge boilers.
“They say you can make anything from lignin except money,” says Miguel Perez, a UW-Madison graduate student in civil and environmental engineering and the paper’s first author.
The bacteria in question is Novosphingobium aromaticivorans. It was first isolated in soils that were previously contaminated by petroleum products. And, in this environment where most other organisms find it hard to eek out a living, N. aromaticivorans was thriving. Its name aromaticivorans means ‘aromatic-eater’ as a nod to its unique adaptations.
Lignin is a large molecule that’s very difficult to break down into smaller pieces. But N. aromaticivorans already had a natural appetite for lignin-like products when discovered — in fact, it’s the only known organism so far that can digest many parts of the lignin molecule and excrete smaller aromatic compounds.
“Other microbes tried before may be able to digest a few types of aromatics found in lignin,” Perez says. “When we met this microbe, it was already good at degrading a wide range of compounds. That makes this microbe very promising.”
During this process, N. aromaticivorans produces 2-pyrone-4,6-dicarboxylic acid or PDC. The team engineered the bacteria by removing three genes in its genome, further stabilizing the digestion process and coaxing it reducing all its meal into PDC. In the end, what they obtained was an organism which could be fed any part of the lignin molecule and produce PDC.
“There’s no industrial process for doing that, because PDC is so difficult to make by existing routes,” says Daniel Noguera, the study’s corresponding author. “But if we’re making biofuels from cellulose and producing lignin — something we used to just burn — and we can efficiently turn the lignin into PDC, that potentially changes the market for industrial use of this compound.”
“The compound performs the same or better than the most common petroleum-based additive to PET polymers — like plastic bottles and synthetic fibers — which are the most common polymers being produced in the world,” Perez adds.
PDC is also biodegradable and doesn’t leach any by-products while it degrades.
For now, the engineered variation on N. aromaticivorans can turn at least 59% of lignin’s potentially useful compounds into PDC. The team suggests that a greater efficiency is possible through further genetic manipulation of the microbe. They’re currently at work implementing such changes and “might create a new industry,” Noguera says.
The paper “Funneling aromatic products of chemically depolymerized lignin into 2-pyrone-4-6-dicarboxylic acid with Novosphingobium aromaticivorans” has been published in the journal Green Chemistry.
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