Reference: https://www.vox.com/the-highlight/2019/4/9/18274131/plastic-waste-pollution-bacteria-digestion
Scientists are trying to accelerate evolution to make plastics rot. A tiny new organism is showing them how.
Sometime between 2010 and 2015, a tiny organism with an unusual appetite made a home in an industrial site near a bottle-recycling plant in Sakai, Japan.
The site, located by a bustling port in one of the most urbanized, densely populated regions in the world, wasn’t exactly hospitable. The soil and water were contaminated with a complex unnatural substance only humans could have created: polyethylene terephthalate. Also known as PET, it’s one of the most common plastics in the world. PET is used to make soda bottles, space blankets, blister packs, food containers, magnetic tape, shirts, and dresses. It’s cheap, durable, and flexible, but it can also linger for years.
Amid this plastic wasteland, a new bacterium species, Ideonella sakaiensis, took root. Scientists prospecting for traces of life at the recycling facility named it after the city where it was found. And in 2016, they reported that the hot dog-shaped microorganism wasn’t just surviving; it was thriving.
The bacterium had figured out how to do something that humans have struggled to do for decades. Ideonella sakaiensis could break PET down and use the plastic for energy.
“It’s really cool to find a soil bacterium that’s able to both break it down and then use the building blocks as a food source, a carbon and energy source,” said Gregg Beckham, a researcher studying plastic degradation at the National Renewable Energy Laboratory who’s been trying to build on the finding in his own work.
What he and other scientists believe is that this minuscule creature may hold the key to solving a massive and rapidly growing environmental crisis. Its discovery has reignited a worldwide race to engineer and optimize nature’s tools to dissolve the overwhelming amount of plastic waste littering the land and sea.
We live in a world wrapped in plastic
From the whale that recently washed up in the Philippines with nearly 90 pounds of plastic bags in its stomach to reports that plastic has reached the darkest, deepest reaches of the ocean to the tiny fibers accumulating in the fish we eat and even our bodies, plastics just keep piling up.
Researchers estimated in 2017 that humanity has produced 8.3 billion tons of plastic since the material was invented. We’re so hooked on the stuff that annual plastic production is expected to triple by 2050.
Plastics are made from oil and natural gas, which in turn have their own impacts on the environment and the global climate. The Center for International Environmental Law reported in May that the current rate of plastic consumption will contribute 1.34 gigatons of carbon dioxide emissions per year by 2030, equivalent to almost 300 coal-fired power plants. That’s a big reason why reducing plastic use and recycling what we’ve already made is so important.
Though many types of plastic can be recycled, 91 percent of plastic is not. And even the meager amounts we do recycle are in jeopardy. China, the largest importer of recyclable plastic, began to slash its intake last year. That has forced some cities across the United States to cancel their recycling programs entirely.
At best, plastic that isn’t recycled ends up in landfills, where it can take between 500 and 1,000 years to completely degrade. At worst, it strangles rivers, lakes, beaches, and oceans, and is ingested by species, large and small, who live in them. The World Economic Forum projects that by 2050, there will be more plastic in the ocean than fish by weight.
“Just in the last couple of years, I would say, we’ve really started to pay attention to the problem as a scientific community in a much more concerted and bigger way,” said Beckham. “In the ’90s and even through the mid-2000s we knew this was a problem, but I don’t think people really understood how large the scale of the problem was.”
Heart-rending images of animals suffering from our plastic waste have helped inspire a new movement to combat plastic consumption. But bans on plastic straws and fees for plastic bags only nibble at a gargantuan, growing torrent of stubborn garbage.
Clearly, we need to do something about plastic waste. And we’re running out of time.
Scientists are increasingly scouring the natural world for a solution. For just about every material on earth — bone, wood, cartilage, shell, cotton — there is a biological mechanism to break it down. But the Ideonella sakaiensis discovery showed that organisms can figure out how to use materials that don’t exist in nature.
Now researchers around the world are rushing to coax the bacterium to do more: eat different kinds of plastic, digest them faster, and make something valuable out of the end products. The hope is to create a process — specially engineered enzymes, chemicals, and even organisms — that can devour our most persistent mess.
Eventually, this process would take place in plastic biodigesters: facilities — or maybe one day devices for your home — that use life’s tools to unmake and remake synthetic materials. But the clock is ticking to invent something that can consume plastic before it consumes us.
Why plastics take so long to break down on their own
The key problem with synthetic materials like plastics is that they contain chemical combinations that nature has never seen before. That means they don’t undergo decay, one of the most important and underappreciated functions in ecology.
Decomposition is a process that took eons to evolve alongside the detritus of life. Inside every ecosystem is a teeming zoo of microbes whose job it is to break down materials, from scavengers that eat fetid flesh to fungi that rot away the hardiest trees. This ongoing cycle of growth, breakdown, and rebuilding is critical to life as we know it, restoring nutrients to the environment so new organisms can come along and thrive.
“Plants have been around for hundreds of millions of years, and organisms that can break them down have also been around for hundreds of millions of years,” Beckham said.
PET, on the other hand, “has only been around for about 50 years.”
The challenge of digesting plastic begins at the molecular level. Most plastics are organic polymers, hence the “poly” in the names of many kinds of plastic: polyethylene, polyvinyl chloride, polypropylene. They are made up of repeating chemical units stitched together in long chains via carbon atoms.
The raw material for plastic comes largely from refining substances like crude oil, which then undergoes a variety of chemical reactions to create the starting materials for plastic.
Because plastics are manufactured from the ground up, we can control every step of the process to tweak them to suit just about any given need. Plastics can be stiff or flexible, opaque or transparent, durable or fragile. They can be molded in just about any shape or threaded into fibers. So it’s not too surprising that plastics are now in everything from firearms to fabric to food containers.
However, decomposers like bacteria and fungi usually don’t have the biological hardware to break long plastic polymers back down into their individual building blocks. Imagine demolishing a house brick by brick. It’s relatively simple if you already have a hammer and chisel. But if the structure is made of other materials — steel, aluminum, glass, concrete — you’ll need more specialized tools, especially if you want to preserve and reuse the materials as you take it apart.
And like a modern house, plastics can degrade somewhat on their own in the heat, sunlight, and rainfall, but it takes a long time. In the process, toxic chemicals can be released, and ultimately some element of the structure will remain.
So even though plastics can fracture into smaller pieces, at a molecular level they endure for a long time. That’s why microplastics can end up in tiny forms of sea life, which are then eaten by larger animals, and then eaten by us. It’s also why plastic bags, toys, and diapers will likely linger for centuries.
Yet even in an environment so thoroughly reshaped by humanity, Ideonella sakaiensis showed the world that life, uh, finds a way. In the desolate, plastic-littered ground with little else to eat, the bacterium quickly acquired a taste for our waste. The question now is whether we can turn that taste into a ravenous appetite.
Nature can work fast, but we need to make it work faster
I visited the National Renewable Energy Laboratory in Golden, Colorado, last year to learn about the latest developments on the cutting edge of clean energy from one of the federal government’s premier research facilities. It’s a sprawling 327-acre campus on gently rolling green hills with mountains in the distance. In 2018, the lab spent $410 million on research in areas including wind power, fuel cells, vehicles, and energy efficiency.
Biofuels were on my agenda, but the scientists working on making gasoline and jet fuel from plants and algae were more eager to talk to me about their work on the plastic waste crisis. Scientists there said they felt pride in telling friends, family, and even strangers in ride shares who were horrified by the scale of the problem that they were creating a solution.
“It’s one of the more exciting things, maybe the most exciting thing we’re doing right now,” said Bryon Donohoe, a senior scientist at the lab. “Everywhere we look, in the press and everywhere, it seems like people are awakening to the reality of what plastics recycling has been, what we’re really doing when we chuck all that stuff in the bin.”
Beckham, an avid diver, said he saw the amount of plastic waste swirling in the water increasing over the years on trips to Southeast Asia. “This is a very personal thing for me.” he said. “This is a much bigger problem than it was 15 years ago. Literally, you can see it’s a much bigger problem.”
And the youngest researchers at the lab are just as driven. “We have students who come here and they work seven days a week on this,” Beckham said. “They are so pumped about this problem.”
The eight staff scientists tackling plastic waste found that a lot of their expertise in turning plants into energy translated to digesting plastic. At the heart of the work is understanding how Ideonella sakaiensis, a bacterium that evolved in mere decades, figured out how to digest plastic on its own and whether we can speed it up. “That discovery is amazing. It’s really cool,” Beckham said. “But the huge caveat is that it’s very slow.”
Ideonella sakaiensis doesn’t share our sense of urgency when it comes to the plastic waste problem. Under precisely controlled laboratory conditions, it took a colony about six weeks to completely degrade a sample PET sheet. It can take weeks, even months, for the bacterium to disassemble a PET bottle or food container in the real world. That’s far faster than plastic would degrade on its own, but it’s not fast enough for, say, a plastic digester we might build one day. At that pace, it would be hard to justify the cost of building a facility to break down PET, and it would scarcely keep up with the speed at which our plastic waste is piling up.
How do you eat away at a mountain of plastic? With trillions of tiny bites.
To get to the scale and speed needed to make this bacterial digestion technique worthwhile, the National Renewable Energy Laboratory’s scientists are deploying a variety of tactics.
One approach is adaptive laboratory evolution. Here, researchers cultivate bacteria in miniature test tubes and measure how well they digest a tiny 6-millimeter disc of a given plastic. With confocal microscopes, they can even watch this in real time. The best bacteria are selected and cultivated, and from the next generation the best plastic eaters are picked, and so on.
Another technique is directed evolution. This effectively speeds up the evolutionary process of a biological mechanism in a laboratory with the goal of performing a specific task: in this case, digesting plastic. The pioneers of directed evolution won the Nobel Prize in chemistry last year, having deployed it to make medicines and biofuels.
In this case, the scientists are targeting PET-digesting enzymes, known as PETases. These are relatively large, complicated proteins. They act as catalysts, speeding up a chemical reaction that would otherwise occur very slowly. They’re also very picky about their working conditions — too hot, too cold, too acidic, too basic, too salty, not salty enough, and the enzyme might not function at all.
Enzymes also tend to be very specific to a single chemical reaction, like copying DNA or breaking apart sugars. Since they are so complex and focused on a specific job, it’s hard to coax an enzyme into doing something different without breaking it.
But it’s not impossible. Researchers can deliberately induce mutations in enzymes and track how well they work. Most will fail, but a handful will work better. Scientists can then analyze the winners to see what makes them tick. By closely measuring their shapes and monitoring their work, scientists hope to figure out how to nudge enzymes to work faster, or to keep them working in more challenging environments.
And with supercomputers, scientists are also designing their own enzymes using the ones they’ve found in nature as templates. They can calculate what a durable, fast-acting, plastic-breaking enzyme would look like. Eventually, they hope to build it from the bottom up.
From there, scientists can formulate an enzyme combination that can break plastics apart quickly and cheaply. Or they can engineer bacteria or algae to produce a newly designed enzyme and digest a synthetic material on their own.
