THURSDAY, 6 JUNE 2024
SINCE THE 1950S, global production of plastic has increased from 1.5 million tons per year to over 359 million tons in 2018, projected to double by 2050. Approximately 91% of plastic waste is not recycled, ending up in landfills and oceans. Only a small fraction (9%) is recycled (even then, incompletely) due to a lack of infrastructure and education. The low value of many types of plastic makes them unrecyclable by default; instead, they persist in the environment as microplastics (less than 5mm in size) found in drinking water, seafood and even in the air we breathe. Studies suggest that microplastics can cause inflammation, harm the immune system and potentially cause cancer. Plastic pollution in oceans can be linked to the deaths of more than a million seabirds and 100,000 marine animals each year. Animals can mistake plastic for food and ingest it, causing intestinal blockages which can lead to starvation and death. Especially unsettling is the recent discovery of fishing net remnants and other types of plastic in sedimentary rocks on the otherwise pristine Trindade island. Scientists report that human pollution is acting as a ‘geological agent’ in otherwise completely natural geological processes.The economic costs of plastic pollution are estimated to be around $13 billion per year due to impacts on tourism, fisheries and other industries. Additionally, the production of plastic relies heavily on fossil fuels, contributing to greenhouse gas emissions and climate change. These statistics are only a glimpse of the scale and the severity of the plastic environmental crisis. There is an urgent need for action to address plastic waste pollution that must combine scientific and social initiatives. But where to start?
One solution comes from an unlikely source. Polyethylene, typically used in packaging, is responsible for around 34% of plastic pollution. Moth larvae that infest beehives (known as waxworms) have the unique ability to digest this plastic. Researchers have discovered that the waxworms are able to break it down using a combination of physical and chemical mechanisms, though the details are not yet clear.
Waxworms have strong jaws that they use to munch through the plastic, breaking it down into smaller pieces. This increases the surface area making it more accessible to enzymes found in the digestive tract; these proteins drive chemical breakdown of plastics from chain-like polymers into monomers, the single molecular building blocks.
One of the reasons that these moth larvae are capable of digesting plastic is that they have evolved to feed on a variety of different substances, including plant material and animal products like beeswax. As a result, they have developed a specialised suite of powerful enzymes not found in most animals. Plastic, while a relatively ‘new’ synthetic material, is similar enough to the beeswax and plant fibers that the insects usually feed on.
Studies have shown that waxworms also rely on a community of gut microbes to assist with the breakdown of the plastic. Namely, Enterobacter asburiae and Pseudomonas species produce accessory enzymes that initially break down the plastic, yielding compounds that can be used as a food source by the waxworms. Ingested plastic is processed imperfectly by these microbes, such that the breakdown may result in release of harmful chemicals into the environment.
Researchers in Spain and Cambridge are investigating the potential for using waxworms to develop more effective methods for waste management. While polyethylene can persist for years in the environment, waxworm salivary enzymes were able to oxidize plastic in just a few hours without any pre-treatment. However, it is important to note that further research is needed to fully understand the potential of this approach and to assess its feasibility on a larger scale. There are several unknowns that need to be investigated. How aggressive are these enzymes? What if the gut microbes adapt to living outside the larvae? Uncontrolled implementation puts at risk countless structures held by plastic parts. There is also concern about the carbon dioxide released in digestion – is this benefit countered by accelerating climate change? Wax moths take their name from their love for the wax holding together the honeycombs of bees. In the US they already cause an annual loss of $4 million, and that’s without their industrial implementation. Extensive breeding of moth populations could have devastating effects on bees, who are already suffering from pesticides, habitat loss and predators.
Another downside is that larvae are limited to only certain types of plastic; there is an increasing belief that the original enthusiasm following their discovery was overestimating their ability to solve the larger problem of plastic pollution. It is possible that despite their amazing potential at first glance, the only effective solution may be cutting down plastic use and focusing on biodegradables.
The ‘ideal’ solution would be to reverse the polymerisation of hydrocarbons in oil to revert to a useful substance that can re-enter the production line. A team from the Kyoto Institute in Japan has identified a bacterium, Ideonella sakaiensis, that can do just that. This microbe grows on PET (polyethylene terephthalate) plastics and seems to be using PET as its main nutrient source. Ideonella associates with the plastics through tendril-like projections and uses the enzyme PETase to break them down into an intermediate called MHET. Further processing to terephthalic acid and ethylene glycol by MHETase rather conveniently churns out the same non-toxic starting material used in the manufacturing industry.
Unfortunately, this process is quite slow - Ideonella needs about six weeks at room temperature to go through a few grams of PET. One idea is to genetically engineer E. coli to take over the job - they have already proven themselves as powerful insulin factories to treat diabetes. E. coli in rich media has a doubling time of 20 minutes - you can grow many litres overnight relatively cheaply. Since it has been used as a model organism for many years, its genome, behavior and ‘idiosyncrasies’ have been extensively studied and are hence widely understood. Transferring the plastic-processing Ideonella genes into E. coli would allow us to draw on faster-growing system that has one more added benefit: it does not digest terephthalic acid. Instead, this breakdown product is secreted by default, and could be harvested and recycled.
Natural hydrolases such as PETase are few and far between. One equivalent enzyme, a cutinase, has been isolated in Thermobifida fusca. When sequenced, the PETase gene showed only 51% sequence homology to its T. fusca counterpart. The MHETase, on the other hand, belongs to the tannase family but does not work on the typical aromatic substrates of these enzymes. Have the Ideonella enzymes, that are upregulated in the presence of PET, evolved independently of their closest relatives? Has the brief (on an evolutionary scale) time of 70 years since the appearance of plastics been enough for evolutionary pressures to drive the development of a PET[1]specific system? If this is the case, what can we learn from them, and how can we harness their power to clear landfills and improve recycling processes? With our voracious appetite globally for all things plastic, we should be chasing these tantalising leads.
Philippa is a second year Biological Natural Sciences undergraduate at Trinity college. She has written on biopholtovotaics and sustainable solutions to plastic pollution.
Artwork by Biliana Todorova.
