Plastic pollution is killing millions of animals every year1. Nearly 700 species have been harmed by plastics. Some of them are threatened with extinction. Most of the deaths are caused by entanglement or starvation. Large pieces of plastics can be removed from inland waters, but not smaller ones. Plastics are broken down into microplastics. They drift throughout the water column in the open ocean. They are virtually impossible to recover. Microplastics have been found in more than 100 aquatic species, including fish, shrimp and mussels that people eat. Hopefully, they pass through the digestive system and are excreted without harm. However, plastics have blocked digestive tracts or pierced organs in some people, causing death. Stomachs so packed with plastics reduce the urge to eat, causing starvation. Almost all seabird species eat plastics. Most plastic waste is not biodegraded and can only be broken down into small particles by physical processes2. Particles smaller than 150 μm can be ingested by animals, migrate through the intestinal wall and reach lymph nodes and other body organs.

Richard Thompson, a marine ecologist at the University of Plymouth in the United Kingdom, coined the word microplastics in 2004 to describe plastic particles smaller than 5 mm, after his team found them on British beaches3. They have been seen everywhere researchers looked. This includes remote places such as deep oceans, Arctic snow, Antarctic ice, shellfish, table salt, drinking water and many beverages. Microplastics also drift into the air and fall with rain. People are exposed to them by breathing and through the things we touch, eat and drink. Microplastics pollute drinking water, accumulate in the food chain, and release toxic chemicals that may cause disease, including several types of cancer.

When plastic containers are heated in a microwave oven, they shed microplastics into the food or water that they contain4. In a recent study, microplastics were found in the bloodstreams of twelve out of seventeen people5. However, the human toxicity of microplastics is poorly understood4.

Plastic trash has become so ubiquitous that the 5th United Nations Environment Assembly concluded in Nairobi with 14 resolutions6. It included an agreement to establish an Intergovernmental Negotiating Committee with the mandate to forge an international legally binding agreement to end plastic pollution. The plastics treaty is expected to address the full lifecycle of plastics, including production, design, and disposal. The estimated timeline for reaching a global agreement is two years.

Since July 1, 2022, the manufacture, import, stocking, distribution, sale, and use of single-use plastic items with low utility and high littering potential have been banned in India. Over 100 cities and municipalities in the USA have banned Expanded Polystyrene (EPS) in government facilities including New York City, Los Angeles, Seattle, San Diego and Miami Beach, Florida.

As of July 2019, 68 countries across the globe have plastic bag bans with varying degrees of enforcement. Eight states in the USA have banned single-use plastic bags. Vermont has adopted the most comprehensive plastics ban in the USA. They are regulating the use of bags, straws, drink stirrers and foam food packaging. Bangladesh became the first country in the world to ban thin plastic bags in 2002. New Zealand became the latest country to ban plastic bags in July 2019. In June 2019, Canada announced to ban on single-use plastics by 2021. More than 300 cities and towns in the USA have banned single-use plastic bags.

However, no countries restrict the manufacturing and production of plastic bags. Global production of plastics has increased considerably over the last few decades from 1.7 million metric tons per year in the 1950s to 370 million metric tons per year in 2019. Plastics that are less dense than seawater are carried from the coast into the open ocean through seawater circulation and wind. They enter ocean currents and make plastic garbage patches. The Great Pacific Garbage Patch was first discovered in 1997. It is twice the size of Texas and is located between Hawaii and California7. Four other plastic gyres have been investigated around the world8. They are the North Atlantic gyre, the South Atlantic gyre, the South pacific gyre and the Indian gyre. About eight million tons of plastic are released into the oceans every year and 51 trillion pieces of microplastics are present in them8.

However, the distribution of plastic garbage patches is not fully consistent with models of ocean circulation7. Moreover, these plastic patches only account for about 1% of the total plastics dumped into the ocean. About 99% of the plastics end up at the bottom of the ocean. Organisms cling to the plastics, causing biofouling. This increases the density of the plastic, making it sink. Barnacles and corals form macrofouling on a variety of objects. They consist mainly of calcite and aragonite, the densities of which are much more than seawater. As plastics become denser than seawater after biofouling, they sink, carrying nutrients and high biodiversity down to the bottom of the ocean. Similar to whale carcasses, these plastic drops may attract deep sea creatures, and eventually form a biota in the deep sea, along with hydrothermal vents, cold seeps, and whale falls7.

So, people are looking for solutions to this problem. The industry is using alternative materials to improve recycling and make biodegradable plastics9. However, 99% of plastics produced today are polymers made from fossil fuels. They will continue to play an important role in modern society. According to the 2020 European Bioplastics report, the total production capacity of biopolymers in the European Union is expected to reach 2.45 MegaTons (Mt) by 2024. One way to decrease waste is to use a circular economy, in which the waste from one manufacturer becomes the raw material for another. This improves production and waste management by reducing water, waste, and energy consumption. From 2006 to 2018, the amount of recycled post-consumer plastic waste doubled, reaching 32.5% (29.1 Mt), while 42.6% was used for energy production and 24.9% was put into landfills.

Many researchers are looking to nature for solutions10. They are looking for organisms that use enzymes to break down man-made plastics, such as polystyrene, polyurethane and polyesters. The search for such enzymes was successful in 2016 when Japanese researchers analyzing mud near a plastic recycling factory found a bacterium that produced two enzymes that enabled it to feed on the polyester called polyethylene terephthalate (PET). They hydrolyzed it, breaking it down into its building blocks, terephthalic acid and ethylene glycol. PET is used to make single-use plastic bottles and fibers in polyester clothing. It accounts for about one-fifth of worldwide plastic production. An international consortium has emerged. As researchers throughout the world find useful microorganisms, they can send them to laboratories such as the University of Portsmouth. There, the enzymes can be extracted, purified and their structures determined by X-ray crystallography. Then, artificial intelligence is used to design new enzymes that will be much more efficient.

Although industrial chemicals can break down plastics, natural enzymes provide a greener approach, requiring less energy and also targeting specific plastics that are mixed with trash10. A company in France is already building a demonstration factory that will use enzymes to turn plastic trash into raw material for new bottles. In 2014, just 19% of all plastic was recycled. Meanwhile, plastic production is expected to grow 70% by 2050, to almost 600 million tons per year. At the same time, the Department of Energy in the USA is funding research into ways of recycling plastics using heat, light and electricity.

Bacteria in the gut of an insect called the mealworm (Tenebrio molitor) are able to degrade polystyrene, polyurethane, polyethylene, polypropylene, polylactic acid, polyvinyl chloride and vulcanized rubber11. The beetle Tribolium confusum can degrade both polyethylene and polystyrene. The wax moth Galleria mellonella can break down polyethylene. The land snail Achatina fulica can degrade polystyrene.

Some fungi can break down plastics and eliminate many pollutants8. Some of them have enzymes that break down the natural polymer called lignin. The enzymes are secreted by white-rot fungi such as Bjerkandera adusta, which can break down nylon. The brown-rot fungus Gloeophyllum trabeum can break down polystyrene sulfonate. Also, soil samples from the rhizosphere of the mangrove Avicennia marina were able to break down polyethylene. The most efficient fungi were Aspergillus terreus and Aspergillus sydowii. Some researchers were able to isolate 12 strains of fungi from plastic debris from a shoreline in Switzerland that can break down polyurethane and polyethylene.

These organisms can be grown in the laboratory in cell cultures. This provides researchers with enough of their DNA to enable them to determine the sequences of nucleotides that make up genes. In the culture-dependent method, microorganisms expressing the desired enzyme are first enriched and isolated under proper cultivation conditions12. This is followed by taxonomical classification of genus and species and identification of potentially useful enzymes. However, the culture-dependent method seriously limits the scope of finding new enzymes because that can degrade plastic. Less than 1% of the total microorganisms on the planet have been cultured. Many are not stable outside their natural environment (such as the human gut) and can’t be grown in cell cultures. The best way to identify such microorganisms is by sequencing as much of the DNA in an environmental sample, even though it mostly contains pieces of genomes and not complete genomes from intact cells. This is called metagenomics. It is the study of the structure and function of sequences of nucleotides that are isolated and analyzed from all the organisms in a bulk sample, such as those on human skin, in the soil, trash or water. Many genes that encode enzymes capable of breaking down a variety of plastic materials have been identified from many environmental metagenomic samples.

In addition, researchers are looking at the proteome, or collection of all proteins that are expressed in samples12. Proteomics has been useful in finding new enzymes from a broad repertoire of microbial sources for biotechnological applications.

Protein engineering has produced a new enzyme that can degrade plastics better than the enzymes that occur in nature13. Researchers started by looking at the enzymes that exist in bacteria that live in compost containing leaves and branches of dead plants. They contained enzymes that can break down the naturally occurring polymer called cutin. The researchers changed some crucial amino acids and produced an enzyme that is stable at higher temperatures and has increased enzyme activity on PET.

So, scientists and engineers are developing new enzymes that can break down plastics into monomers that can be used to recycle plastics in a circular economy that is environmentally friendly.

Notes

1 Parker, P. The world's plastic pollution crisis explained. National Geographic, 7 June 2019.
2 Yuan, Z. et al. Human health concerns regarding microplastics in the aquatic environment - from marine to food systems. Science of the Total Environment, Vol. 823, article 153730, 2022.
3 Yuan, Z. et al. Human health concerns regarding microplastics in the aquatic environment - from marine to food systems. Science of the Total Environment, Vol. 823, article 153730, 2022.
4 Lim, X.Z. Microplastics are everywhere, but are they harmful? Nature, Vol. 593, pp. 22-24, 2021.
5 Leslie, H.A. et al. Discovery and quantification of plastic particle pollution in human blood. Environment International Vol. 163, article 107199, 2022.
6 Ansah, K.B. Brief on global plastics treaty. Towards a global plastics treaty: perspectives on key considerations for negotiators, governments, businesses, and all stakeholders in the plastics ecosystem. United Nations. 30 Sept., 2022.
7 Li, X. and Sun, W. The formation of deep sea plastic biotas. Science Bulletin. Vol. 67, pp. 674-675, 2022.
8 Delangiz, N. et al. Can polymer-degrading microorganisms solve the bottleneck of plastics’ environmental challenges? Chemosphere. Vol. 294, article 133709. 2022.
9 Beghetto, V. et al. Recent advances in plastic packaging recycling: a mini-review. Materials. Vol. 14, article 4782, 2021.
10 Cornwall, W. The plastic eaters. Science. Vol. 373, pp. 37-39, 2021.
11 Bulak, P. et al. Biodegradation of different types of plastics by Tenebrio molitor insect. Polymers. Vol. 13, article 3508, 2021.
12 Zhu, B. et al. Enzyme discovery and engineering for sustainable plastic recycling. Trends in Biotechnology. Vol. 40, pp. 22-28, 2022.
13 Li, Z. et al. Structural insight and engineering of a plastic degrading hydrolase Ple629. Biochemical and Biophysical Research Communications. Vol. 626, pp. 100-106, 2022.