Polishing off Plastic Pollution?

Fanciful illustration of plastic-eating microbes

Illustration by Tim Ellis

“There’s a great future in plastics.” The iconic line, advice to a fresh-from-college ingenué played by Dustin Hoffman in the 1967 film The Graduate, was intended for laughs. But in 2023, it highlights a grim reality.

The world’s economies generate roughly 400 million tons of plastic waste each year, and according to a 2022 report from the Organisation for Economic Co-operation and Development, only nine percent of that waste is recycled. Some is incinerated, but the bulk of plastic trash ends up in landfills or the environment. And when that discarded plastic gets battered by sunlight or ocean waves it can release microplastics, tiny particles that are increasingly finding their way into air, food, water, and turning up in tests of human blood.

Even the small proportion of plastic that is recycled presents a range of challenges, say the experts. The molecular structure of plastic includes unreactive carbon chains that render it extremely durable and difficult to break down. (Depending on the type, plastic can take decades or even hundreds or years to fully decay.) The mechanical recycling process, which involves shredding and melting plastic waste into pellets for reuse, is labor intensive and expensive, and chemical recycling releases toxic emissions.

Enter a group of Harvard scientists who are looking to nature and the lab for solutions.

During the past two years, researchers at the Wyss Institute for Biologically Inspired Engineering in the laboratories of Folkman professor of vascular biology Don Ingber and Winthrop professor of genetics George Church have been working to identify organisms that naturally degrade plastic. Next, they plan to enhance the organisms’ capabilities via genetic modification.

“Recycling is basically a myth. Nothing really gets recycled—it ends up in the landfills and oceans and leads to toxic gases being released,” said Sukanya Punthambaker, one of the project’s lead researchers and a postdoctoral fellow studying synthetic biology in Church’s lab. “Several hundred million tons of plastic are generated each year, and it’s not going to go anywhere…in our lifetimes unless we develop a strong intervention.”

The search for plastic-metabolizing microbes spanned ponds, landfills, waste sites, and even the cells of certain animals, where the researchers tested for “plastivores,” organisms that have evolved to gorge on the synthetic material.

“We go back into nature and see what it has given us,” said Vaskar Gnyawali, a postdoctoral fellow in Ingber’s lab working on biosensors and microengineering. Gnyawali notes that the natural approach to breaking down plastics is more environmentally friendly than chemical recycling solutions. Microbial degradation is also safer than shredding, which generates microplastics that affect “every part of the ecosystem.” Naturally occurring microbes that consume plastic, he explains, instead produce water, carbon dioxide, and organic material known as biomass.

The hitch is that nature’s plastic feeders don’t consume their food quickly. To overcome that obstacle, once they’ve identified their hungry microbes, the researchers will attempt to give their microbes’ appetites a boost. “We try to evolve them,” said Gnyawali. “Make them faster, optimize them.”

The work, based on genetic engineering and synthetic biology, involves both reengineering the microbes’ core genetic material so they become voracious eaters, and directly manipulating the enzymes they use to break down plastics. “We hope to both create modified organisms that will degrade plastic more quickly and pass their feeding efficiency on to their progeny as they replicate, and altered enzymes with more voracious appetites that can be released separately,” said Punthambaker.

Their experiments build on the efforts of a group of Japanese researchers who in 2016 identified a new species of bacteria known as Ideonella sakaiensis that—with the help of two separate enzymes—can eat polyethylene terephthalate (PET), a thermoplastic polymer commonly found in bottles and packaging.

An enzyme is a biological catalyst, typically a protein, that expedites a cell’s chemical reaction. The enzymes found in plastic-eating microbes work by breaking the polymer chains of plastic into smaller parts, says Punthambaker. One way to make the process more efficient, she notes, “is by modifying the enzyme’s amino acids so that it can grasp or attack the chain more effectively.”

Artificial intelligence (AI) algorithms willvastly increase the power of the researchers’ molecular analyses, rapidly generating predictions about changes to an enzyme’s molecular structure that could potentially speed up its ability to degrade plastic. The researchers will then evaluate the enzyme variants and their different properties in the lab.

Still a work in progress, their plan is to eventually make predictions for hundreds or thousands of mutated enzymes using AI and then screen them to see which ones work best.

A longstanding obstacle to traditional recycling is the fact that multiple kinds of plastics are discarded together. The Harvard team has identified a promising bacterial strain capable of breaking down more than one type of plastic at a time, a discovery that could be useful when dealing with landfills crammed with mixed plastics.

Ingber, who is alsoWyss professor of bioinspired engineering at Harvard’s Paulson School of Engineering and Applied Sciences,knows tackling the world’s pressing plastic problem will require a range of different approaches, and he’s confident that this example of applied research is one of them. “If we can actually go beyond having to isolate a single plastic before you even start the process of degrading it and instead use these microbes or enzymes on complex plastic mixtures found in waste handling systems, it could have a major positive effect,” he says. “There’s no doubt about it.”

The group aims to introduce its super-charged bacteria and enzymes into bioreactors, oceans, and landfills—where they are expected to significantly reduce the amount of plastic waste—in the next three to five years pending regulatory review. To allay fears about the potential for modified microbes to go rogue, the researchers say they can engineer the altered organisms to self-destruct. “One way of keeping them contained is to have a kill switch,” says Punthambaker, so that they will “die after a certain point.”

The more minds attacking the problem the better, say the scientists. “There are other companies and universities that are also doing this work, but we need more people to get involved because plastic waste is a serious global issue,”says Punthambaker—“and it’s only getting worse.”

Read more articles by Colleen Walsh

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