Can the microbes raised by plastic waste help us eat plastic garbage?

The problem of plastic pollution is becoming more and more serious. In addition to familiar measures such as reducing waste and recycling, scientists have also found a powerful ally for us-microbes. Some microbes have evolved the ability to "eat" some plastics, which may soon help us reduce plastic waste and build a greener circular economy.

Look around, from the mobile phone and computer you are using, to the clothes and shoes you are wearing, to the food packaging, daily necessities and household appliances in the room. Most of them contain plastic. This kind of organic polymer material has already become a part of the life of most modern people. Global plastic production has been increasing almost every year since the 1950s, reaching 367 million tons in 2020. However, most plastics are not properly disposed of after completing their mission. The study estimates that from 1950 to 2015, humans have produced 8.3 billion tons of plastic, of which 6.3 billion tons have been turned into garbage, of which only about 600m tons (9 per cent) have been recycled, the rest have been incinerated, and most of them have been buried or entered the natural environment.

Incineration of plastic waste produces toxic pollutants, and plastics exposed to natural weathering are difficult to degrade like other wastes-from polymers to monomers. Plastic waste all over the world has killed a large number of birds, fish and many other animals, and they continue to release additives and become microplastics to spread to every corner of the world.

As plastics have irreplaceable value in many ways, we cannot ban the production of plastics across the board. For example, plastic bottles are much lighter than glass bottles, so transporting plastic bottles requires less energy and emits less greenhouse gases. But we need to revolutionize the way we deal with plastic waste, and microbes may be able to do us a big favor.

Diners attracted by a feast

PET (polyethylene terephthalate) is a common material for beverage bottles and synthetic fibers. It is a long-chain molecule formed by the polymerization of ethylene glycol and terephthalic acid. Since this substance does not exist in nature, most microbes do not have the ability to decompose this kind of plastic. In the natural environment, the degradation of this compound may take hundreds of years.

The reason why Osaka fungus can "break PET into pieces" is that it can produce two unique enzymes. PET enzyme decomposes long-chain PET molecules into smaller monohydroxyethyl terephthalates (MHET), while MHET enzyme can further decompose MHET molecules into ethylene glycol and terephthalic acid. In other words, Aspergillus Osaka can completely reverse the process of PET formation.

In fact, the discovery of microbial degradation of plastics was made in the early 1990s at the latest. These findings may be less interesting because these microbes can only "eat" plastics with less strong chemical structures. But by 2000, scientists had discovered enzymes that could deal with harder plastics. By around 2015, scientists had discovered a large number of plastic-degrading enzymes.

So why did the discovery of Osaka fungus cause such a stir? John McGeehan, a professor of structural biology at the University of Portsmouth in the UK, said: "what makes this microbe unique is that it can use plastic as the only source of food. This is actually quite surprising and shows the role of evolutionary pressure to some extent. If you are the first bacteria in the garbage to take an interest in plastic, you have an unlimited source of food all of a sudden. "

In other words, the previously discovered enzymes did not evolve for the degradation of plastics, they just evolved the ability to break down strong long-chain molecules in organisms, which is an incidental function. By contrast, the enzymes of Aspergillus Osaka evolved specifically for the degradation of plastics.

There is evidence that microbes around the world are evolving the ability to degrade plastics. A study published in 2020 found a soil bacterium that can feed on polyurethane, which is toxic to most bacteria. A study in 2021 found that in areas with severe plastic pollution, microbes are more likely to contain enzymes that can degrade plastics.

Circular economy of plastics

In 2018, McGehan and his colleagues did further research on Aspergillus Osaka. They described the three-dimensional structure of the PET enzyme formed by Aspergillus Osaka to reveal how it works. To understand the evolution of the enzyme, they fine-tuned its structure. Unexpectedly, this makes it more efficient! Obviously, there is room for improvement in this enzyme.

McGehan continues to try to improve the PET enzyme and other similar enzymes so that they can degrade plastic waste on an industrial scale. In 2020, McGehan's team reported that they linked PET and MHET to form a "super enzyme" that degrades PET six times faster than the two enzymes work alone. At the same time, other research teams have improved the enzyme to varying degrees.

These enzymes can decompose plastics at the molecular level, and the decomposed products can recreate high-quality plastics. In contrast, other recycling methods will lead to a gradual decline in the quality of plastic, until the final product can no longer be recycled and can only be landfill or incinerated. So at least in theory, using enzymes to recycle plastics is a real circular economy.

Some groups are trying to commercialize such plastic recycling technologies. In September 2021, Carbios, a French biotechnology company, opened a pilot plant in Claremont Ferrand, where it will test a PET recovery system. They used an enzyme found in composting and modified it to work faster and at higher temperatures.

In a study published in July 2021, McGehan and colleagues estimated the cost of reprocessing PET with enzymes and found that it could compete with conventional PET manufacturing methods based on fossil fuels. "I think in the next five years, we will see demonstration factories everywhere." McGehan said.

However, there is a limit to the role of enzymes. Lars Blank, an applied microbiologist at the University of Aachen in Germany, points out that enzymes work best when plastics are heated and softened. This means that putting enzymes into the natural environment is of little use, because they really work only in reactors where the temperature is controllable.

We need to change the way plastics are made and used to make them easier to recycle. For example, avoid designs that use a variety of plastics or blend plastics with other materials because they will be difficult to recycle.

The environment filled with plastics has prompted microbes to evolve enzymes that break down plastics, and advanced science and technology have made them more powerful. However, enzymes can be involved in only one part of the process of controlling plastic pollution. Make plastic products that are easier to recycle, reduce unnecessary use, and separate and recycle after use. These more critical steps have to be completed by ourselves.