Plastic is one of the most popular and useful materials of modern times. We now use about 20 times more plastic than we did 50 years ago. Of the 50 kg of plastic that is discarded annually per person, just 14% is recycled.
First produced in the 1950s, plastics today consist of polyethylene terephthalate (PET) which is imperishable. Plastic bags take 10-20 years to break down, while PET bottles could take 450 years. These mainly single-use plastics end up in landfills and water bodies. Plastics often contain additives (chemicals) that make them stronger, more flexible, and durable, which enter organisms in the food chain from tiny species like plankton through to whales. Plastic debris is lightweight, so they float and toss around and break down into microplastics which get into the flora and fauna. These are preyed upon by organisms at higher trophic levels.
Over the past few years, worms, bacteria, and enzymes that break down polymers have made headlines. The idea that biodepolymerization research may succeed where humans have so far failed is tantalizing. Most reported cases of enzyme degrading plastic are incomplete and slow. Making these processes faster and more efficient is not trivial. And even if scientists manage to improve polymer biodegradation technologies, they must also bring down the cost to compete in a marketplace that includes chemical recycling methods and virgin monomers.
Knowing some polymer chemistry can help to break apart polyesters such as PET is relatively easy because these plastics contain the ester bonds that enzymes can degrade in nature. But plastics with an all-carbon backbone, such as the polyethylene in shopping bags or the polystyrene in takeout food containers, are more challenging. News about breaking down those kinds of plastics tends to be greeted with more excitement, but it also merits more scepticism.
Any enzyme that can break down plastic will be judged by whether it can degrade real-world plastic waste and at what efficiency. The breakthrough that builds on plastic-eating bugs was first discovered by Japanese researchers in 2016. They tested sludge from a recycling plant and discovered a microbe that could completely break down films of PET to carbon dioxide and water - a feat that was a step above partial degradations reported previously. From that microbe, the scientists plucked two enzymes that degraded PET to its monomers of terephthalic acid and ethylene glycol. Even if optimization goes smoothly, there’s still the matter of cost. Engineered enzymes tend to be more expensive than commodity chemicals, and scaling up a biotechnological process takes yet more investment.
The advances in microbial biotechnology are creating exciting possibilities to design a novel pathway to known biodegradable polymers, but also pathways to novel biodegradable polymers, which address the start and end of life of materials.
Recently, researchers have developed mutant enzymes to break down the PET polymer into Terephthalic acid (TA). This is a novel biosynthetic pathway in the laboratory using bacterium 'E. coli' to convert plastic-derived monomer TA directly into the value-added molecule vanillin using a single engineered microorganism.
E. coli has previously been used by many researchers for the biosynthesis of vanillin from glucose. This research is published in the journal Green Chemistry this month. This is the first example of using a biological system to recycle plastic waste into an important industrial chemical, and it has very exciting implications for the circular economy. Vanillin is used widely in food mainly in bakery products, cosmetics industries, and is an important bulk chemical used to make pharmaceuticals, cleaning products, and herbicides. Global demand is growing far exceeding the supply from natural vanilla beans. More than 80% of vanillin is currently synthesized from chemicals derived from fossil fuels.
With the demand for vanillin, scientists have switched to synthetically producing vanillin. For the new study, researchers used a novel method to convert plastic waste into vanillin, as a way to both supply vanillin and cut down plastic waste on Earth. This work demonstrates the first biological recycling of post-consumer plastic waste into vanillin using an engineered microorganism.
The researchers raised the temperature of a microbial broth to 37°C for a day, using the same conditions used for brewing beer. This converted 79% of the TA into vanillin. TA and vanillin have very similar chemical compositions and the engineered bacteria only need to make minor changes to the number of hydrogen and oxygen that are bonded to the same carbon backbone.
As the next step, the scientists will further train the bacteria to enhance the process to convert larger amounts of plastic. Other valuable molecules could also be brewed from TA, such as some used in perfumes. Let’s hope that biodepolymerization will one day solve the problem of the staggering pile of plastics in the environment.
