By JiaJia Fu
From water bottles to spacecraft parts, plastic is a versatile, durable, and cost effective material that has saturated daily life. However, the convenience plastic brings to our lives comes at a heavy price. Discarded plastic, often used only once, breaks down into microplastics that embed themselves in human tissue, infiltrates the most remote regions of the environment, and remains in the environment for millennia. Public alarm and awareness about the severity and scale of the plastic crisis is increasing, but we still have no means of actually eliminating plastic waste. Recently-discovered enzymes that digest plastic present a novel, urgently needed mechanism to address this global crisis. Plastic-degrading enzymes are isolated from plastic-eating bacteria, which are enzymes that allow these bacteria to digest and consume plastic. Coupled with biological enhancements, plastic-eating enzymes such as PETase may provide an innovative, genuine strategy to manage a pressing and progressively worsening crisis.5
Plastic cannot be efficiently broken down because of its unique synthetic polymer structure - it is made of repeated, tightly linked carbon bonds, derived from petroleum and crude oil substrates. Most decomposers cannot degrade plastic as synthetic polymers which make up plastic have only been mass produced in the past century and could not evolve quickly enough to digest its complex molecular structure. However, plastic-eating bacteria and other organisms are very unique from a genomics and evolutionary context as they demonstrate the emergence of a new behavior that did not exist before.
The first bacteria was discovered in 1975 from the waste water drainage ponds of a Japanese nylon plant. This species of bacteria did not exist before the mass production of nylon and relied on “nylonase” enzymes to survive on nylon byproducts. However, current research is predominantly focused on PET-eating bacteria, which consumes PET or polyethylene terephthalate, one of the most common types of plastic in disposable products like water bottles and plastic bags. The first strain of PET-eating bacteria was isolated in 2016 from a plant in Sakai City, Japan. Named Ideonella sakaiensis, the gram-negative rod-shaped bacteria uses two enzymes in conjunction to digest PET - PETase and MHETase. The mechanism behind all plastic-digesting enzymes is the severing of the chemical bonds, or esters, in a synthetic polymer like PET. Severing esters breaks down the complex molecule into simpler monomers that this organism can then digest. The enzyme PETase binds to PET and breaks the bonds linking the repeating monomers, mono(2-hydroxyethyl) terephthalic acid, or MHET, and trace amounts of an analogous compound BHET. MHETase then converts the MHET into its two monomers, TPA and ethylene glycol (EG), which are then digestible by I. sakaiensis and serve as an energy source. A colony of I. sakaiensis can completely degrade a low-grade plastic water bottle in six weeks. Higher-grade PET products would require heating and cooling to weaken it before bacteria could start eating.7
Only 9% of all plastics produced are recycled due to the high cost of chemically breaking down plastic relative to its cheap and efficient production costs.1 As recycling is expensive and resource consuming, the majority of plastic goes to landfills or the environment. Proposed experimental solutions for plastic waste only target the aftermath - when plastic enters the environment and becomes pollution. Other than attempting to reduce plastic production and its entering the environment (physical measures like nets to collect plastic in the ocean), humans have not found a way of actually eliminating the waste itself until now. Although the wild type enzymes are not efficient enough to be deployed on an industrial scale at the moment, biotechnology techniques such as recombinant DNA and enzymatic engineering can improve enzyme degradation efficiency enough for wide-scale applications. Researchers from the University of Portsmouth chemically linked the PETase and MHETase enzymes to produce a 2-enzyme system that functions at room temperature, and is 6 times more efficient than the wild enzyme. Through analyzing the crystal structure of the enzyme and identifying key amino acids, a team from French company Carbios isolated a mutant enzyme 10,000 times more efficient at PET breaking than natural LLC. The team found that it could break down 90% of 200 grams of PET in 10 hours. They converted 90% of that same plastic back to its starting materials, and resynthesized PET out of the terephthalate and ethylene glycol composites - the recycled PET was just as strong as conventional plastic. The success of their trials has led to scaling up the technology and plans to open a demonstration plant next year.
Thanks to genomics and biotechnology, the potential of a unique biological mechanism can be fully realized and even begin to be applied on a massive scale. Plastic-eating enzymes could bring about a future with a reliable industrial plastic waste management system, ocean plastic and microplastic eliminating serums, or even at home, plastic waste consuming compost bins. The potential impacts of utilizing plastic-eating bacteria are undeniable, but there are certain potential risks - dumping plastic eating enzymes into the ocean and hoping they will solve ocean pollution would not be a prudent decision. There’s no way to predict how an engineered plastic-eating enzyme would react with a new ecosystem like the ocean, or a landfill. Extensive research to first achieve a functional PET digestion threshold in plastic eating enzymes, as well as fully understanding how they could interact and methods, would allow us to implement them on a wide scale. Although there are concerns and risks involved with developing such technology, plastic-eating bacterial enzymes may lead the efforts to combat the environmental disaster the world is facing. Through utilizing genomics, cutting edge biology, and nature, we are one step closer to solving the plastic pollution crisis and creating a healthier society, planet, and future.
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