One day, amid a pile of discarded plastic bottles, a bacterium learned to do something that had evaded the best scientists, confounded the efforts of conservationists and had been among the most urgent goals of environmentalists: it digested plastic.
Now a group of scientists has taken what that bacterium does and improved it to make a “super-enzyme” that could, they hope, help to solve the world’s plastic problems.
The approach combines the two plastic-munching enzymes developed by that bacterium into one, to degrade PET (polyethylene terephthalate) plastic at a speed the researchers hope might make full-scale recycling plants viable. John McGeehan, from the University of Portsmouth, said: “It’s starting to become commercially viable, which is a lot faster than I would have expected a couple of years ago.”
The discovery in 2016 that a species of bacterium in a Japanese recycling plant had learned to digest plastic caused a sensation — and spawned a global effort to work out how it achieved it. It should not have been that surprising.
The reason plastics are such a problem, and take so long to degrade, is not because of something intrinsic to the material. Instead, it is because they are new. Before plastics arrived, no creature on Earth had seen anything with their chemistry, so why would any have learned to eat it?
But evolution is a machine for exploiting biological niches, and the first bacteria that could break the bonds of plastics to extract their energy had a huge advantage.
The discovery in 2016 that a species of bacterium in a Japanese recycling plant had learned to digest plastic caused a sensation.
By studying how those Japanese bacteria did it, researchers established the involvement of two enzymes, working together, called PETase and MHETase. First the PETase breaks the plastic into soluble chunks, then the MHETase degrades it into simpler chemicals still.
What if they could be partnered even more efficiently? “We thought it would be a good idea to try and link these two enzymes together to create a super enzyme,” said Professor McGeehan, whose work is published in Proceedings of the National Academy of Sciences. They achieved this by splicing the DNA that made each into one long segment and when they did they found a six-fold speed improvement compared with PETase alone. This could have commercial as well as environmental potential.
Plastic is a polymer, made by joining together repeating chains of hydrogen, carbon and other elements. Although some plastic claims to be recyclable, each time it is recycled it loses quality and can only be used in lesser-value products. The enzymes offer something different: breaking down the polymers to building blocks, which can then be recombined into true virgin plastic, without using oil.
“If you compare making a plastic bottle with enzyme monomers to digging up fossil fuels and transporting them all over the place, there’s a 70 percent energy saving,” Professor McGeehan said.
Mark Lorch, from the University of Hull, was not involved in the research but said that it gave him hope “that nature will, eventually, be able to clean up our mess.”
Tom Whipple is the author of several books, most recently Get Ahead in Physics
