Plastic-Eating Microbes to the Rescue?

Nature breaks everything down—eventually. It’s time to accelerate the process.

The next time you walk through a pristine forest, among moss-hung branches and many-hued mushrooms on fallen trees, think of plastic. Think of the last plastic water bottle you drained and then tossed away, theoretically to be recycled but more likely to end up in a landfill and possibly in the ocean. Because a natural tool that’s poised to help us truly, completely recycle some of the heaps of plastic we produce is growing right there among those trees.

That natural tool is cutinase, an enzyme produced by bacteria and fungi, including forest mushrooms. Cutinase breaks down cutin, the water-repellant coating on plants, like the waxy shine on a leaf or the gloss on an apple. It turns out that the chemical bonds holding together the long polymer chains in cutin are similar to the polymer bonds in a common type of plastic. So a handful of microbes that normally munch plants have adapted to consume plastic bottles.

The problem is that they’re incredibly slow. Biology will eventually, over eons, find a way to degrade plastics efficiently. It eventually does for almost every material. But plastics are so new to the Earth that naturally occurring cutinases can only barely nibble them. And, with 6 billion metric tons of plastic clogging our lands and oceans, and 300 million new tons of plastic being added to the deluge each year, we don’t have eons to spare.

The solution would be for scientists to fast-forward the process, to engineer enzymes or microbes that can chop plastic polymers into their monomer bits on a vast scale. That would enable us to recycle plastic far more easily and effectively, reducing the amount that ends up in the waste stream and potentially in the water. It could also help us create new kinds of plastic that break down quickly even when they do land in the trash. 

Researchers around the world are working on that now, searching for ways to make the process quicker and cleaner, before we drown in our own sea of plastic trash. It would be the ultimate proof of the power of synthetic biology. Yet the challenges involved in realizing that vision are a reminder of the complexity of hacking nature.

Dirty secret

In 2016, international headlines heralded the discovery of bacteria near a plastic bottle recycling plant in Japan that were naturally digesting PET plastic, which is used to make beverage bottles and is the kind marked as recycling type No. 1. Some of the news reports were so enthusiastic that the casual reader might have thought this was the beginning of the end of plastic pollution. 

But the fanfare was overblown. 

“The dirty secret … is that enzyme is really crappy,” says Gregg Beckham, senior research fellow and group leader at the National Renewable Energy Laboratory (NREL) in Golden, Colorado. In this case, “crappy” means “slow.”

When it comes to plastic-eating organisms or enzymes, the reality, as in so much of synthetic biology, is still promising but not quite as imminent as the hype makes it sound. 

In general, scientists are improving their ability to engineer microorganisms to do a host of different jobs. We can envision a future of expert microbial workhorses, sustainably producing materials with characteristics that surpass what humans can create alone. Enzyme engineering is already enabling us to make new kinds of drugs and chemicals.

“It changes the game,” says Richard Gross, a professor of biological and chemical sciences at Rensselaer Polytechnic University, who has engineered a cutinase from leaf and branch compost to break down a thin plastic film in two days. “There is an exponential increase in capabilities in biotechnological processes. You can equate it to what’s going on in electronics.”

Even so, it’s not obvious how to economically scale up these processes.

The enzymes from the famous bacteria identified by the Japanese scientists took six weeks to break down a thin sheet of plastic. British researchers then accelerated that by about 20 percent. Other Japanese researchers recently managed to engineer a cutinase to degrade 97 percent of plastic in 30 hours, though that was only a very thin plastic film.

That’s a long way from digesting a whole landfill. Plus, the technologies tested so far have been used only in contained spaces under controlled conditions. And that’s not likely to change in the foreseeable future. 

Then there’s the matter of byproducts. Plastics are laced with dyes, additives, and catalysts used in production, and there’s still very little known about what to do with that potentially toxic stuff once you’ve broken up the main polymers. “The sexy science is: ‘I’m going to make an enzyme that breaks down plastics,’” says NREL’s Beckham. “But the make-or-break thing from an economic perspective is what you do with all the stuff after you break it down.”

If we used whole microbes, not just enzymes, to digest plastics, those microbes could be engineered to mop up harmful byproducts along with plastic. But that research has much further to go to get the bacteria working at viable speed.

What biotechnology is even less likely to do is disintegrate the 5 trillion pieces of plastic debris poisoning our oceans. The release of all those loose monomers and byproducts into the water could cause a cascade of unintended consequences, with the chemicals potentially even working their way into the human food web, says Douglas Rader, chief oceans scientist at the Environmental Defense Fund. Plus, much of the ocean’s plastic is not at the surface but near the bottom. As Rader says, “I don’t think anybody’s going to be spraying the garbage patch from a ship and going home anytime soon.”

Cutinases also are not suited to digest all the other plastics—the ones that are numbered 2 through 7 for recycling purposes and are used to make everything from milk jugs to plumbing pipes to grocery bags. Those plastics are held together by completely different bonds, “and in many cases those bonds are a hell of a lot stronger and way more difficult to break down,” Beckham says. He and other researchers agree that biology alone is not likely to be able to degrade most of those plastics. It will take a combination of chemical and biological processes to do that.

But the possibility of optimizing enzymes to switch from plants to plastic, as the researchers who accelerated the bottle-eating bacteria from Japan have begun to do, is nonetheless a breakthrough that has both biotechnologists and environmentalists excited. 

In the case of PET, it should soon be possible to break down old beverage bottles into their original ingredients and use them to create new, good-as-virgin plastic over and over again. That’s a lot better than current recycling options, which involve mechanically chopping bottles into tiny bits and melting them down to produce subpar plastic that’s typically only good for one more use.

Some of the most advanced research is happening at a French company called Carbios, which claims its enzymatic process for breaking down PET plastic can degrade 90 percent of the material in 10 hours. Carbios then purifies the monomers (leaving the byproducts as waste) and remakes them into bottles comparable to those made with virgin plastic. In addition to packaging, Carbios says it can degrade and remake PET fiber products, such as carpets and clothing.

“No one wants their product packaging to be at the center of a dead bird skeleton on the beach.”

Jenny Ahlen, Environmental Defense Fund

“The idea is to turn the waste into money and then to organize a market on this and then to stop plastic pollution,” says Carbios’s chief scientific officer, Alain Marty.

Marty says Carbios is currently re-creating plastic at pilot scale; it can process 200 kilograms of used plastic at once. It’s also partnering with an engineering firm to build a demonstration plant that will process 20 times that much, which would be just short of full commercial production. Last spring, the company announced a plan with L’Oreal, Nestlé Waters, PepsiCo, and Suntory Beverage & Food Europe to make high-quality recycled plastics commercially available within four years.

Both Beckham and Gross call that prospect plausible. “They’re probably going to make it real,” says Beckham. “I’m very optimistic that companies like Carbios, with contributions from government and scientists around the world, could [make that process] viable.”

When manufacturers do make biologically recycled bottles, they’re likely to hit on a big business opportunity. “Companies would be very excited to use materials that they could make sustainability claims around,” says Jenny Ahlen, director of business at the Environmental Defense Fund. She works with Walmart and other major retailers to lessen the environmental impact of their supply chains. Consumers are increasingly concerned about plastic trash in the oceans, Ahlen says—especially since a 2015 video of a sea turtle with a plastic straw stuck in its nose went viral, garnering 38 million views.

“With the ocean plastic zeitgeist happening,” Ahlen explains, “no one wants their product packaging to be at the center of a dead bird skeleton on the beach. I think that’s highly motivating.”

Upcycling

The researchers’ and environmentalists’ biggest concern about the hype on plastic-eating bacteria is that it could make consumers complacent. Even the best biotechnology will only make a dent in the plastic waste stream. Achieving large-scale changes will still require us to reduce and reuse like mad, and to collect our plastic waste better. And there’s no putting all the plastic trash that’s choking waterways and animals back in the proverbial bottle.

“[For] the plastic which is now in nature, I think it is too late,” says Carbios’s Marty. 

What biology can do is enable us to design new kinds of plastics that degrade far more easily. Even plastics built from biological ingredients, such as corn instead of petroleum, can still take centuries to break down in the environment.

This video from Carbios explains the company’s process.

Carbios, for example, is developing a method to make a kind of PLA, a plant-based plastic, that’s programmed for decomposition. Carbios would introduce an enzyme into the PLA as it’s manufactured that makes it break down in the presence of water (such as in compost, or in a river). Obviously, you couldn’t sell Coca-Cola in it, but Marty says the degradable PLA could hold solid products or work in multilayer packaging. The company plans to start selling its enzyme to plastic manufacturers in 2020.

Beckham’s lab is also working on ways to redesign petroleum-based plastics. It’s possible to incorporate a biologically based material into a PET bottle to make it biodegradable without affecting its mechanical and thermal properties. “We’re really excited about designing water bottles that, if they escape to the environment, won’t last 400 years, but will last five years or less,” he says. 

It’s even possible that plastic could fuel synthetic biology itself. Some researchers are trying to engineer microbes that can eat the monomers from broken-down PET and then churn out various sorts of useful products. The result would be not just samecycling—making plastic from plastic—but upcycling: making something better than a water bottle.

“The bottom line is we are not yet near the point where a silver bullet exists to take down the plastic in our world,” says Rader, the oceans expert. “But maybe we can see the glint of the material that might make up the bullet.”

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