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Ask a Scientist: Why is it so hard to decompose plastic?

By Zeke Elkins

Posted Jan 7, 2019 at 2:28 PM

 

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Decomposition is a process by which organic materials like wood, animal carcasses and paper are broken down into simpler organic compounds. In order to decompose something, it is buried in soil, where bacteria can break it down. Decomposed organic material is then recycled. The broken down organic compounds provide food for plants, enrich the soil and feed other living things.

The problem with decomposing plastic is that plastic is not organic. Most plastics in use today are made of polyethylene terephthalate, or PET for short, and are nearly indestructible. It is nearly impossible to decompose PET plastics because most bacteria cannot break them down. UV light from the sun can break plastic down, but it takes a long time.

We produce 300 million tons of plastic, the weight equivalent of 50 million African elephants, each year! The amount of plastic we have thrown away is so large that there is a plastic garbage patch in the Pacific Ocean that is currently three times the size of France. Virtually all seabirds have plastic in their stomachs, and roughly 1 million sea creatures die from plastic each year.

However, there is hope. Researchers have found a bacteria that does break down PET plastic. And new, biodegradable plastics are currently in development. Hopefully some day in the future we will all use biodegradable plastics that can easily decompose. In the meantime, there are lots of ways we can reduce the amount of plastic we use, like drinking from reusable water bottles instead of single-use plastic water bottles.




The plastic bottles we throw away today will be around for hundreds of years. It’s one of the key reasons why the mounting plastic pollution problem, which is having a deadly effect on marine life, is so serious.

But scientists recently discovered a strain of bacteria that can literally eat the plastic used to make bottles, and have now improved it to make it work faster. The effects are modest – it’s not a complete solution to plastic pollution – but it does show how bacteria could help create more environmentally friendly recycling.

Plastics are complex polymers, meaning they are long, repeating chains of molecules that don’t dissolve in water. The strength of these chains makes plastic very durable and means it takes a very long time to decompose naturally. If they could be broken down into their smaller, soluble chemical units, then these building blocks could be harvested and recycled to form new plastics in a closed-loop system.

In 2016, scientists from Japan tested different bacteria from a bottle recycling plant and found that Ideonella sakaiensis 201-F6 could digest the plastic used to make single-use drinks bottles, polyethylene terephthalate (PET). It works by secreting an enzyme (a type of protein that can speed up chemical reactions) known as PETase. This splits certain chemical bonds (esters) in PET, leaving smaller molecules that the bacteria can absorb, using the carbon in them as a food source.

Although other bacterial enzymes were already known to slowly digest PET, the new enzyme had apparently evolved specifically for this job. This suggests it might be faster and more efficient and so have the potential for use in bio-recycling.






As a result, several teams have been trying to understand exactly how PETase works by studying its structure. In the past 12 months, groups from Korea, China and the UK, US and Brazil have all published work showing the structure of the enzyme at high resolution and analysing its mechanisms.

These papers show that the part of the PETase protein that performs the chemical digestion is physically tailored to bind to PET surfaces and works at 30°C, making it suitable for recycling in bio-reactors. Two of the teams also showed that by subtly changing the enzyme’s chemical properties so it interacted with PET differently made it work more quickly than the natural PETase.

Using enzymes from bacteria in bio-reactors to break down plastic for recycling is still easier said than done. The physical properties of plastics make them very difficult for enzymes to interact with.

The PET used in drinks bottles has a semi-crystalline structure, which means the plastic molecules are tightly packed and difficult for the enzyme to get to. The latest study shows that the enhanced enzyme probably worked well because the part of the molecule that is involved in the reaction is very accessible, making it easy for the enzyme to attack even the buried PET molecules.

Modest improvements

The improvements to the PETase activity were not dramatic, and we are nowhere near a solution to our plastic crisis. But this research helps us understand how this promising enzyme breaks down PET and hints at how we could make it work faster by manipulating its active parts.

It is relatively unusual to be able to engineer enzymes to work better than they have evolved through nature. Perhaps this achievement reflects the fact that the bacteria that use PETase are only recently evolved to survive on this man-made plastic. This could give scientists an exciting opportunity to overtake evolution by engineering optimised forms of PETase.

There is one worry, though. While any modified bacteria used in bioreactors are likely to be highly controlled, the fact that it evolved to degrade and consume plastic in the first place suggests this material we rely on so heavily may not be as durable as we thought.

If more bacteria began eating plastic in the wild then products and structures designed to last many years could come under threat. The plastics industry would face the serious challenge of preventing its products becoming contaminated with hungry micro-organisms.

Lessons from antibiotics teach us we are slow to outwit bacteria. But perhaps studies such as these will give us a head start.

Introduction:
Plastic is a synthetic material that is not easily degraded by bacteria. This is due to its chemical structure and composition, which makes it resistant to the enzymes produced by bacteria. While some types of plastic can eventually break down over time, the process is extremely slow and can take hundreds of years. This poses a significant environmental problem as plastic waste accumulates in landfills and oceans, causing pollution and harm to ecosystems.

Chemical Structure of Plastic:
Plastic is made up of long chains of polymers, which are large molecules composed of repeating subunits called monomers. The most common plastic polymer is polyethylene, which is composed of repeating ethylene monomers. The arrangement and bonding of these monomers give plastic its unique properties, such as durability and malleability.

Resistance to Enzymatic Degradation:
Plastic is resistant to enzymatic degradation because the chemical bonds that hold its polymer chains together are strong and stable. Bacteria produce enzymes called hydrolytic enzymes that can break down organic compounds, but they are unable to break the strong bonds present in plastic.

Lack of Recognizable Nutrients:
Bacteria primarily degrade organic compounds as a source of energy and nutrients. However, plastic does not contain easily recognizable nutrients that bacteria can utilize. Unlike natural materials such as wood or paper, which contain carbon, hydrogen, and oxygen that can be readily metabolized by bacteria, plastic lacks these recognizable nutrients. Therefore, bacteria do not recognize plastic as a suitable food source.

Limited Access to Plastic Surface:
Bacteria need to physically come into contact with the material they are degrading. However, the smooth and non-porous surface of plastic makes it difficult for bacteria to colonize and break down the material. Additionally, plastic waste is often buried in landfills or ends up in the ocean, limiting the exposure of bacteria to the plastic surface.

Conclusion:
Plastic is not easily degraded by bacteria due to its chemical structure, resistance to enzymatic degradation, lack of recognizable nutrients, and limited access to the plastic surface. This explains why plastic waste persists in the environment for long periods and poses a significant threat to ecosystems. It is crucial to reduce plastic consumption, promote recycling, and develop alternative biodegradable materials to mitigate the environmental impact of plastic waste.