Digesting the problem: Enzymes chew through plastic pollution

They end up everywhere on the planet – in oceans, rivers, lakes, forests, landfills, and even inside animals and humans. And by the way, plastics are also made from oil.

^{Are we doomed to drown in plastic? Not quite.}

Plastic recycling is a broken cycle

What is your answer to this question: Is using plastic a one-way street, like oil and coal?

If your answer is no, it must be because you think we can recycle plastic, unlike oil or coal. After all, plastic recycling has been discussed since the 1970s and has evolved significantly.

Despite the improvements in recycling technology plastics circularity remains a challenge.

That's because once a plastic product is recycled, the plastic loses its quality. Also, fresh plastic has to be added to the recycled plastic to create new products out of it.

More importantly, we cannot endlessly keep recycling plastic. There comes a point - usually after a few times of recycling - that the plastic becomes unrecyclable. The only solution then remains discarding it in landfills or incinerating it.

Today, less than 10% of the plastic we produce is actually recycled, making recycling seem like mere lip service to the environmental cause.

^{Plastics are used in everything. E.g. in this girl’s jacket, sneakers, sweatpants, phone, chewing gum, bag and food packaging to name a few everyday objects.}

But plastics are essential for humans. They are used in everything, from clothes and smartphones to food packaging and medical equipment. Global annual plastic production has surpassed 400 million tonnes from less than two million tonnes in the 1950s. We can't simply stop producing plastics.

So, are we doomed to drown in plastic? Not quite.

Remember that utopia we dreamed of earlier – a world with an endless circle of fossil fuel reuse, where we could burn oil, capture the gases, create fuel with it again, and reuse it?

Believe it or not, this concept is not just a fantasy. It's a reality when it comes to plastics! We can produce, use, and reuse plastics almost indefinitely.

Circularity in plastics

It's a system where fresh plastic, known as virgin plastic, is produced and turned into products. When it's time to discard these products, we convert them back into virgin plastic, which can then be used to create new products. This perpetual cycle of virgin plastic-product-virgin plastic is what we call circularity. 

Not only does this approach address plastic waste, but it also significantly reduces the need for producing new virgin plastic. This groundbreaking advancement has been made possible by enzymes – natural substances found in nature and produced by all living organisms, including microbes, animals, and plants. So far the scope is PET plastic.

Before we delve too deeply into how enzymes have helped develop a solution to tackle the plastic waste problem for certain plastic types, understanding the mechanism involved, and what the future holds, let's first take a moment to grasp the basics of plastic.

How are plastics made? 

You know how oil is like the backbone of our modern society, right? Well, it's not just for fueling our cars or keeping us warm—it's also the key ingredient in making plastic! 

But how does oil transform into the plastic we use every day? Let's break it down.  Plastics are derived from crude oil, natural gas or coal that undergo several steps in a refining and compounding process.

When e.g. crude oil comes out of the ground, it's not just one thing—it's a mix of lots of different stuff. So, it goes to a place called a refinery. There, it's heated up really hot to split it into different parts, like petrol, diesel, kerosene, and something special we need for making plastic: ethane and propane.

Ethane and propane, after they're separated from the crude oil, are converted into two important chemicals: ethylene and propylene. These are the building blocks for making certain plastics, also known as monomers. 

Once we have ethylene and propylene—they're joined together to form long chains called polymers. This process of “joining” monomers to form large polymer chains is called polymerization. Think of it like interlinking metal links to form a long chain. 

The final result, the long chains of monomers coming together to form polymers are what we call plastics.

And here's the fascinating part: depending on how we connect and arrange these monomers, and by adding other petrochemical building blocks, it is possible to create a wide variety of different types of plastic! Some, like polyethylene and polypropylene, are tough and durable, perfect for products like food containers. Others, like polystyrene, are more flexible and lightweight, making them ideal for items like foam packaging or building insulation material.
 

The word polymer originates from the Greek words "poly," meaning "many," and "meros," meaning "parts" or "units." This term aptly captures the concept of polymer - multiple monomers linking together to form a single, large chain (plastic).
 

You might be wondering—while jute bags, cotton clothes, or paper are biodegradable, why do plastics stick around for so long even though they're made from a somewhat natural product like oil? 

Why doesn’t plastic degrade in the environment like a banana peel?

Well, here's the scoop: it's because plastic is basically the new kid on the block in Earth's history!

Think about it—plastics made from oil are barely a century old. One of the most commonly used plastics – Polyethylene terephthalate (PET) – strutted onto the scene in the 1940s, and it's been causing a ruckus ever since. It is a type of plastic commonly used in the production of beverage bottles, food containers, and other packaging materials. Known for its clarity, strength, and recyclability. PET is among the most widely used types of plastic globally,

Now, you might be thinking, “And why is this a problem?”

Here's the deal: when stuff breaks down in nature—like a banana peel or a piece of paper—it's usually because of tiny powerhouses called microbes. 

Microbes have been honing their craft for billions of years, chomping on anything they can get their hands on to derive energy for survival, whether it is animal flesh or any kind of plant product. And they do this with the help of enzymes that they produce. 

Plastics made from oil are still alien to these microbes. They're still scratching their heads, wondering what to make of these shiny, synthetic invaders. Can they munch on them? What are they even made of? 

But wait, here's where the story takes a wild turn: Enter the bacteria Ideonella sakaiensis - the rebel of the microbial world. 

While everyone else is still trying to figure out plastics, this bacterium is like, "Challenge accepted!" It evolved to feast on plastics like a champ, pushing the boundaries of the "survival of the fittest" game more than any other living organism. 
 

Bacteria that chomp on plastic

In 2016, a team of Japanese researchers from Kyoto Institute of Technology and Keio University made a revolutionary discovery. In the samples they collected from a plastic recycling plant in Sakai City, Japan, they stumbled upon something extraordinary – Ideonella sakaiensis.

This little marvel eats plastic and survives on plastic. But what's even more remarkable is how it devours one of the most produced plastics in the world: PET, which finds common use as plastic water bottles. PET when made into threads is commonly known as “polyester” which you might know, is used for making clothes. It's tough, durable, and notoriously resistant to degradation. Yet, here's Ideonella sakaiensis, chomping away at PET like it's no big deal.

^{The discovery of this plastic-eating bacterium sent shockwaves through the scientific community. Suddenly, the seemingly insurmountable problem of plastic pollution doesn't seem so daunting anymore.} 

With the discovery of Ideonella sakaiensis, the scientific community buzzed with excitement and anticipation. It sparked a wave of public interest and media attention. 

As researchers around the world scramble to unravel the mysteries of Ideonella sakaiensis, one thing becomes clear: nature never ceases to amaze us. In the battle against plastic waste, it seems Mother Nature has a few tricks up her sleeve after all.
 

How do bacteria break down plastics? 

These enzymes, known as PETase and MHETase, proved to be potent weapons in the fight against plastic waste. 

But what do these enzymes do exactly? How do they degrade plastic? 

Imagine a big sandwich with layers of bread, meat, and cheese. If you place this sandwich near a colony of hungry ants, what do you think will happen? Within a few hours, the entire sandwich disappears!

In this scenario, the ants symbolize plastic-eating bacteria. Initially, the sandwich might appear too substantial for the ants to tackle, much like PET seems indestructible to regular bacteria.

However, ants possess powerful strong teeth that enable them to swiftly cut through the sandwich layers. Gradually, they begin to nibble away at the sandwich, breaking it down into smaller pieces. With each bite, the ants reduce the sandwich into crumbs, which they transport back to their colony. Eventually, only a pile of crumbs – if anything - remains on the ground.

As we’ve learned, plastics are created from oil by linking monomers together to form long chains known as polymers. Plastic-eating bacteria produce enzymes like PETase and MHETase, which function similarly to the powerful mandibles of ants. These enzymes work tirelessly to break down the polymers in PET plastic into individual, smaller monomers.

Just as ants survive by consuming the small, chopped-up pieces of the sandwich, plastic-eating bacteria derive energy to survive by breaking down the polymers in plastic and eating the small monomers.

Elevating the power of enzymes to the next level

Even as human beings are struggling to tackle the plastic waste crisis, Mother Nature has already come up with her own solution. Fascinating, right? 

So, inspired by nature's ingenuity, some super smart researchers decided to go on a mission to find even more of these natural solutions and make them work on a big scale to fight plastic waste. 

Then, in April 2020, a group of researchers from the University of Toulouse and Carbios, a French biotechnology firm, made headlines around the globe. 

Why? Because they reported something groundbreaking: an enzyme that can break down a whopping 90% of a plastic bottle made of PET in just 10 hours! Imagine that – turning a bottle into tiny particles in less time than it takes to binge-watch your favourite show. Otherwise, it takes hundreds of years for a single PET bottle to degrade in the environment. 

Now, why is this enzyme such a big deal? Well, picture this: you take a regular old plastic bottle, coat it with this magical enzyme, and boom! In just 10 hours, it's transformed into a bunch of tiny particles – like crumbling a cookie into bits. And those bits are the same building blocks used to make the plastic in the first place - the monomers! (Technically, the enzymes are given a helping hand in the form of a mechanical pre-treatment chopping the plastic up in smaller parts, but that shouldn’t steal their thunder!)

So, we can scoop the monomers up, polymerise them to link them into long chains in any way we want and turn them back into shiny new plastic products. How awesome is that?

But wait, there's more! 

Think about it: every single minute, a million plastic water bottles are sold worldwide. That's over 500 billion in a year! But with this enzyme, we could drastically cut down on making new plastic and give Mother Nature a well-deserved break.

Another major benefit arising out of this innovation is the ability to reuse polyester clothes. More than 50 million tonnes of polyester clothes (made from PET) end up in dustbins globally. These clothes have become a major environmental nuisance. In fact, polyester clothing can be broken down into monomers in the same way as plastic bottles, enabling enzymatic recycling and circularity for polyester clothing.
 

Enzymes can degrade more than just PET plastic

PET isn't the only plastic bowing down to nature's mighty enzymes. In a stunning revelation, German researchers unveiled a remarkable bacterium capable of not only surviving but thriving on a different plastic frontier: Polyurethane. Yes, you heard that right – nature's microbial warriors have set their sights on yet another synthetic adversary.

And if that wasn't impressive enough, brace yourself for this: the humble wax moth, often overlooked as mere fishing bait, emerges as an unlikely hero in the war on plastics. Scientists from Spain and the UK were astounded to discover that this unassuming worm boasts enzymes capable of breaking down polyethylene bags, those notorious symbols of environmental degradation. 

It's almost as if nature, fed up with the onslaught of plastics suffocating its ecosystems, has unleashed its own secret weapon – a silent revolution against the plastic plague.

These discoveries illuminate a broader truth: biology’s ingenious solutions know no bounds. From PET bottles to polythene bags, and beyond, synthetic plastics face an unprecedented onslaught from biology’s relentless enzymes, signaling a remarkable shift in the battle against plastic pollution.

While PET recycling has been a +10-year journey to get to industrial scale with a well -defined enzyme, our wax worm friends still represent the music of the future. They could hold a long-term potential but are currently far away from being an immediately implementable solution.
 

Embracing the transformative prowess of enzymes

In the battle against plastic waste crisis to protect nature, our powerful ally comes from the most unexpected of places: nature itself. Nature has always had its own way of dealing with waste, whether it's fallen leaves getting decomposed by microbes into nutrient-rich soil or animal carcasses being eaten by scavengers.

Plastic, however, presented nature with a unique challenge. Unlike organic materials, plastics are synthetically produced polymers engineered to be durable and resistant to degradation.

The answer to our plastic problem has been lurking in various corners of our planet, just waiting to be uncovered by diligent researchers. And now, armed with this newfound knowledge, we're on the brink of a revolutionary breakthrough.

Enzymes, drawn from the natural world, hold the key to unlocking the full potential of plastic circularity – a seismic shift in our sustainability playbook.

Unlike conventional recycling methods that often degrade plastic quality with each cycle and end them in dumpsters eventually, circularity presents a holistic solution. 

By dismantling plastics into their primal components – the monomers – enzymes empower us to establish a closed-loop system. This perpetual dance of production, utilization, and regeneration minimizes the demand for virgin plastic production, thus dramatically curbing our ecological footprint.

While there is a lot of research going on, there is an urgent need for out-of-the-lab solutions. At the forefront of this revolution stands the company Carbios, which is harnessing the power of enzymes to introduce circularity in plastics on an industrial scale. They are building a plant in France to convert 50,000 tons of PET waste per year into reusable monomers with the help of their groundbreaking enzyme. That is as much as two billion PET bottles. 

As we embrace the transformative prowess of enzymes and circularity, we embark on a collective journey towards a world where plastic pollution is but a distant memory. And yes, much work remains including building an infrastructure to collect the plastic, but the enzymes are hungry for changing our relationship with plastic.
 

_{^{Fun facts:}}
Did you take a bite of your credit card today?

We’re eating microplastics everyday:
Microplastics are extremely small plastic pieces – often invisible to the human eye – that contaminate the environment. An interesting study by researchers from the University of Newcastle, Australia concluded in a study in 2019 that humans might be consuming as much as 5 grams of plastic every day from eating food and drinking beverages containing microplastics. That is equal to a credit card a day!

Biodegradable plastic:
Researchers across the world are working on a new kind of plastic that can be composted like any organic product like food or plant waste. This new kind of plastic is called Polylactic acid and is made from plant materials such as corn starch and sugarcane.

Metal-eating and gold-pooping bacteria:
Meet Cupriavidus metallidurans, the bacteria with a golden touch! It lives in soils contaminated with toxic metals. This remarkable microbe has evolved unique processes to deal with situations where there is excess copper or gold in the soil – which is toxic. When faced with excess copper, it activates an enzyme called CupA to pump out the surplus copper and stay healthy. But when gold is around, it cleverly switches to producing an enzyme called CopA, transforming toxic copper-gold compounds into harmless gold nuggets, just nanometers in size. Nature truly has some amazing tricks up its sleeve!

Even so, as with all complex worldwide challenges, we cannot solve the plastics problem by means of one thing only; in this case, biosolutions encompassing the transformative prowess of enzymes and circularity. We need more research to really understand the biology and develop industrial-scale solutions, but perhaps more importantly, we need waste management practices and an infrastructure to collect the plastic. And above all, we need to use less - and use alternatives - where possible and relevant.

Only then can we truly embark on a collective journey towards a future world where plastic pollution is but a distant memory.