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Managing Microplastics Taking a magnifying glass to this tiny threat
A macroscopic view of microplastic formation Microplastics are everywhere, from the tallest mountains to the deepest depths of the ocean. Where do they come from, and how did they end up there?
Should we be worried about the microplastics in our bodies? How these tiny particles may harm our health is unclear, but scientists are alarmed by what they are finding so far
Can biodegradable polymers make microplastics? Yes, and how long they last depends on where they end up
Enzymes to make plastics disappear Scientists are giving nature a nudge and coaxing enzymes to break down plastics
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Polymers
Enzymes to make plastics disappear
Scientists are giving nature a nudge and coaxing enzymes to break down plastics
by Alexander H. Tullo
November 25, 2024
| A version of this story appeared in
Volume 102, Issue 37
Credit: Shutterstock | Firms are developing bacteria and enzymes that might one day be used to break down microplastics in compost.
Synthetic polymers have been on the planet for only about a century. But because of their strength, light weight, convenience, and low cost compared with other materials, these plastics have captured such a presence in humans’ daily lives that the world is overrun with the waste they create.
A century is a wisp of time on an evolutionary scale, however, and nature hasn’t had the chance to catch up with the new materials. Microorganisms will digest a paper bag discarded in the woods in a matter of months. In contrast, a plastic bag will persist for decades, eventually shredding and disintegrating in the wind and sun, and forming microplastics that turn up on mountaintops and even in our blood.
Better waste management and less use of plastics might slow the accumulation. Substituting biodegradable, biobased plastics for fossil fuel–based plastics could also help, but they have many years to go before attaining significant scale in the market.
But what if there were a way to break down market-dominant synthetic polymers as if they were biodegradable?
Enzymes might be a solution. Synthetic polymers don’t exist in the wild, but similar molecules, such as waxes, do. And nature has provided enzymes to break those down. Recent evidence shows that some microbes are already learning how to work on petroleum-derived polymers. In 2016, Japanese scientists made waves when they reported a bacterium that eats polyethylene terephthalate (PET) in samples taken from a recycling plant. This discovery inspired other researchers to scour the world in search of plastics-degrading microbes, and more are turning up.
Selective
Enzymes that break apart polyethylene terephthalate into monohydroxyethyl terephthalate have a much easier time
attacking the mobile fraction of the amorphous region of the polymer than they do the crystalline or
rigid fraction of the amorphous region. One strategy around this is pretreating the polymer to
minimize crystallinity.
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Credit: Adapted from Chem. Rev.
Researchers now aim to artificially speed up the evolutionary process with the microbes they’re finding. They are isolating relevant enzymes from the microbes and engineering the enzymes to increase their activity. Some want to use the enzymes to break down polymers into their original raw materials so they can be recycled; others think the enzymes would be useful in waste management to eat small polymer particles in compost heaps and wastewater treatment plants.
There aren’t many proposed solutions to the plastic waste problem, “and what’s there now is not working at all,” says Federica Bertocchini, a molecular biologist at the Spanish National Research Council (SNRL). She made headlines in 2017, when working at the Institute of Biomedicine and Biotechnology of Cantabria she serendipitously discovered that waxworms were eating holes in polyethylene bags.
“Using biological means could propose itself as a solution,” Bertocchini says. “Maybe not alone, but a solution.”
But enzymes are no magic elixir. For starters, some polymers are more receptive than others to decomposition via enzymes. In addition, factors such as polymer hydrophobicity and crystallinity pose major obstacles to enzymes doing their jobs. Even if scientists can get the enzymes to work efficiently, they still have to find out if such a strategy could release unwanted chemicals, such as plastic additives, into the environment.
Perhaps the company furthest along commercially in breaking down polymers with enzymes is Carbios. The French firm is building a recycling plant in Longlaville, France, that will be an important test case for the technology. Expected to begin production in 2026, the facility will use enzymes to convert 50,000 metric tons per year of PET—commonly used to make water bottles, food jars, and polyester fiber clothing—into its raw materials: terephthalic acid and ethylene glycol.
That such an effort would involve PET shouldn’t come as a surprise. The ester linkages in the polymer provide a convenient point of attack for hydrolyzing enzymes. The same is true of the amide bonds in nylon. Polymers with only carbon-carbon bonds, such as polyethylene, are tougher to crack. “It is a chemical problem, because to break down a carbon-carbon link, you need much more energy than to break down an ester bond and the amide bond,” says Alain Marty, chief scientific officer at Carbios.
Carbios’s PET-breaking enzyme originated with a natural one that Japanese researchers discovered in a leaf-branch compost. That enzyme hydrolyzes cutin, a polymer with ester linkages found in plant leaves. The company engineered the enzyme to work faster and with a higher selectivity for PET.
Carbios also had to grapple with a novel challenge related to getting an enzyme to work with synthetic thermoplastics: crystallinity. When the polymer backbones align themselves and stack on top of each other, the enzymes have difficulty attaching to the polymer to break the bonds. “What we discovered early in the project was that the enzyme prefers amorphous PET,” Marty says. Carbios concocted a pretreatment process that melts the polymer and then cools it rapidly to fix it in the unaligned, amorphous state. Marty adds that this pretreatment also expands the polymer “like popcorn,” increasing the surface area.
Another initiative at Carbios is aimed at breaking down polylactic acid (PLA), a biobased polymer that requires relatively warm industrial composting conditions—about 60 °C—to decompose. Carbios has developed an enzyme, derived from a natural digestive enzyme protease, that the company blends with PLA.
“The idea is to introduce the enzyme inside the polymer to ensure biodegradation after use,” Marty says. Because the enzymes need to be exposed to water to start working, they are unlikely to start degrading the plastics while the objects are being used. Once that condition is met, however, the enzymes can help degrade PLA even in home-composting conditions.
The big challenge for this initiative was the blending step, Marty says. PLA melts at 170 °C, a temperature that would destroy the complex protein structures of most enzymes. To get around this problem, Carbios took as its starting point an enzyme from the thermophilic bacterium Thermus aquaticus that can survive elevated temperatures, such as those found in hot springs.
Intropic Materials, an Oakland, California–based start-up, is taking another approach to embed enzymes in PLA and other polymers to induce degradation after use. Founded in 2020 on the work its CEO, Aaron Hall, was conducting while finishing his PhD work at the University of California, Berkeley, Intropic coats enzymes with what it calls “nanoprotectants.” These protected enzymes can then be used in aqueous or solvent-based solutions for coatings and adhesives, or thermally processed into films and rigid objects.
“You’re able to get that rapid degradation that you need to meet the standards and ultimately not leave behind microplastics,” Hall says.
Another start-up, Breaking, aims to use microbes and enzymes to directly tackle the problem of microplastic pollution. The Boston firm launched this past April out of the Wyss Institute at Harvard University and gestated at Dallas-based Colossal Biosciences.
Credit: Shutterstock
Microcrobes and enzymes that break down polymers have been proposed as a solution for plastic waste in water treatment and industrial composting.
Breaking’s scientists prospected at places like polluted lakes and Superfund sites in search of microorganisms demonstrating a rudimentary ability to break down plastics. “The fact that they can degrade it means they already have the innate machinery, with all the genes and enzymes that can eat the plastic,” says Sukanya Punthambaker, a Breaking cofounder and its CEO.
The “hero organism,” as Punthambaker calls it, discovered through these efforts, was X-32. Still awaiting intellectual property protection, the company isn’t yet saying what kind of microbe X-32 is or where it came from.
Breaking initially tested the microbe with PET as the only carbon source, and it grew, which indicates that X-32 was successfully digesting the plastic. The firm then tested the bug on much tougher polymers: polyolefins such as polyethylene and polypropylene used in packaging and other applications. “We were very pleasantly surprised that it was able to grow on those as well,” Punthambaker says. Respiratory experiments determined that the polymers were being fully converted to carbon dioxide and water. Moreover, the microbes completely consumed the polymers in just 22 months.
Making polymers disappear—instead of just breaking them down into smaller parts—is what’s exciting about the technology to Ben Lamm, a director at Breaking and the CEO of Colossal. “Not only do we want to break down plastics, we don’t want to make more microplastics. That’s not helping anybody,” he says. “We really want to eradicate plastics through breaking those chemical bonds.”
Breaking’s scientists intend to isolate the enzymes in the microbe and modify them so they can work faster. If the company can get the microbes to break down polymers within 90 days, it could have a potential market in industrial composting facilities.
Punthambaker explains that organic waste collected from homes and institutions currently goes to landfills instead of compositing facilities if it is contaminated with plastic. But Breaking’s microbes or enzymes—whichever is best suited for the job—would be added to the compost mixture to decompose any plastic litter. “It’s a better final product for farmers because microplastics are contaminating the soil and crop production,” Punthambaker says.
Another potential use is water treatment, where Breaking’s microbes might be able to combat microplastics originating from nylon and polyester fibers that end up in sewage.
The water treatment application is also a focus of Julie M. Goddard, a professor of food science at Cornell University. “Our interest is in how we can engineer an enzyme to perform specifically in the complex conditions of wastewater treatment, which are variable and often extreme with pH values, salts, lipids, and stuff that can get in the way of enzymatic digestion or enzymatic activity,” she says.
Goddard’s team engineered a library of mutant versions of the naturally occurring enzyme PETase, which was derived from Ideonella sakaiensis, the same strain of bacteria that was the subject of the groundbreaking 2016 paper from Japan. Her group tested how well the enzymes would break down PET fibers in simulated sewage sludge. The activity of one of the mutants was 17 times as high as the wild-type enzyme, “which is huge,” Goddard says.
Goddard is testing the enzymes’ performance on different kinds of polyesters, as well as ways to recapture the enzymes after use. “I would like to see some of the science, whether it is from my lab or any of the other great research going on with PETase, be something that can have an actionable change,” she says.
Since Bertocchini at SNRL discovered waxworms producing holes in plastic bags, she has founded a company, Plastic Entropy, to further develop the technology. The firm’s scientists have determined that enzymes in the worm’s saliva are responsible for the plastic degradation effect.
They have isolated four enzymes from the worms and found that three of them, phenol oxidases, were oxidizing the hydrocarbon chains in the polymer into alcohols in the range of C4–C20. The company is now able to get the enzymes to break down plastics in a “few hours,” Bertocchini says.
A potential use for Plastic Entropy’s enzymes is recycling. For instance, the enzymes could peel off polyethylene linings from multilayer packages so that materials like paper, aluminum, or PET can be recycled more easily.
Not only do we want to break down plastics, we don’t want to make more microplastics. That’s not helping anybody. We really want to eradicate plastics through breaking those chemical bonds.
Ben Lamm, director, Breaking, and
CEO, Colossal Biosciences
But Bertocchini is cautious about using the enzymes cavalierly—say, by sprinkling them in a lake or on a landfill in an attempt to remediate microplastics, even if the enzymes could be made to work in that way. “Whenever you break down synthetic plastics, you release additives,” she says, adding that the enzymes should be used only under controlled conditions.
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Tanja Narancic, assistant professor of microbiology and biomedical science at University College Dublin, agrees that researchers should proceed with caution because of possible unintended consequences. She studies processes that take fossil fuel–based plastic that has been depolymerized via enzymes and then convert those reaction products through fermentation to a new material, such as the biobased polymer polyhydroxylakanoate.
“I think that enzymatic depolymerization is something we can work with, but we have to be aware of and we have to fully know what’s present in the material that you’re starting with, which is very hard,” Narancic says.
Narancic is also concerned about the effect enzymatic depolymerization may have on ecology. For instance, some bacteria that feed on the monomers or other products of enzymatic breakdown of plastics might grow faster than microorganisms that don’t. “That can cause a shift in microbial communities and a shift in plant communities and so on,” she says. “So it is very important, whatever you’re doing, that it’s in a controlled, closed environment.”
Cornell’s Goddard points out that overcoming challenges is an important part of technology development. “We know that microplastics are a problem,” she says. “I think that addressing that known concern is important, even if there may be other things that we have to figure out in addition.”
Lamm at Colossal is under no illusion that Breaking’s enzymes, or enzymes in general, will be the only answer to the microplastics problem. “You need a tapestry of technologies and solutions to tackle this,” he says. “We’re not so bullish to say that Breaking is the be-all-and-end-all silver bullet to the crisis. We hope we’re one of a thousand companies that exist.”
Chemical & Engineering News
ISSN 0009-2347
Copyright © 2024 American Chemical Society
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