TAKE-AWAY meals over the years have become synonymous with expanded polystyrene foam (EPS), often incorrectly referred to by the brand name Styrofoam.
Whether it is the traditional fish and chips, a cup of coffee, chicken wings or a hamburger, it often comes packaged in polystyrene.
However, this lightweight, buoyant and water resistant type of plastic is also used in bean bags, house insulation, buoys, cool boxes, marina platforms, and as a protective packaging material.
Unfortunately, polystyrene’s popularity and potential to rapidly disintegrate into smaller pieces, makes it one of the most common forms of plastic pollution on planet Earth. Eventually, much of the polystyrene ends up in the ocean where the waves break it into millions of even smaller pieces.
Due to its chemical components such as styrene, polystyrene has been linked to some serious diseases in humans, including cancer. Research has also indicated that polystyrene can cause serious harm to animals that often mistake it for food.
When eaten, polystyrene reduces fertility, decrease energy and alters juvenile development of invertebrates in the sea. It also creates a false feeling of fullness in animals or physical blockages that often lead to internal damage.
These are the very reasons why several countries have banned or limited the use of polystyrene food-contact products in recent years, such as take-away containers and cups.
Plastics are relatively inexpensive and widely used organic polymers with a high durability that impedes biodegradation. In fact, due to its recalcitrant macromolecular structure polystyrene foam takes so many decades to biodegrade that it has been classed as non-biodegradable.
Over the years the demand for polystyrene has been growing and since recycling of polystyrene is almost non-existent, most of the polystyrene foam is ending up on landfills and in the oceans. Together with the high durability of plastics, this has a wide range of negative environmental impacts.
However, earlier studies by the School of Life Sciences at the Beijing Institute of Technology in China have found that larvae of the mealworm beetle (tenebrio molitor) were able to degrade and mineralise expanded polystyrene used in packaging due to the work of Serratia fonticola bacteria in the gut of the mealworm larvae.
In June, 2022 researchers from the University of Queensland in Australia published some interesting findings in the journal Microbial Genomics. Their research found that superworms, a common name for the larval stages of the darkling beetle (Zophobas morio), were able to consume and excrete polystyrene foam. The superworms passed it through their gastrointestinal system, where it came into contact with the microbial gut communities of which the Bacilli genera Lactococcus and Enterococcus, were the most abundant.
According to Christian Rinke, the researchers were the first to identify potential polystyrene-degrading enzymes in the microbes of the superworm gut, as well as the bacterial lineages Pseudomonas, Rhodococcus and Corynebacterium to possess genes associated with polystyrene-degrading capabilities. These microbes produced a class of enzymes called hydrolases that use water to degrade the plastic polymer into styrene monomers, which are then further broken down inside the bacterial cells to carbon dioxide.
The researchers divided 171 superworms into three groups that were fed either wheat bran, polystyrene or no food at all for three weeks. Within one day, the polystyrene groups began to chew their way into blocks of polystyrene. The research team was thus able to prove that superworms could survive on a solely polystyrene diet and even gained some weight when compared to the starvation-control group. The polystyrene-reared superworms were also able to complete their entire life cycle, formed pupae and emerged as adult beetles.
A major contribution of the study is that superworms can help to reduce polystyrene waste by digesting it.
The researchers also isolated the polystyrene-degrading microbes of the superworm microbiome and characterised the enzymes involved in the polystyrene degradation pathways. If followed by enzyme engineering and large-scale production, this important finding could possibly be used in future to degrade and recycle plastic on a large scale and tackle the enormous plastic-pollution crisis of the world.
However, much future work will have to be done to tweak the enzymes and optimise the composition of microbial communities for efficient degradation of polystyrene to styrene, which could then be used to create new materials.
Although the University of Queensland researchers have deduced which enzymes were degrading the polystyrene, no experiment to date successfully isolated the critical enzymes in a test tube. Although the findings are important and meaningful, there are still many difficulties in developing a feasible recycling technology. Two of the biggest problems are the scaling of the technology and the possibility of carcinogenic co-products.
Until the final breakthrough, perhaps the better approach is to ban polystyrene foam used in packaging and containers – if we want to save our planet.
*Professor Louis C H Fourie is an extraordinary professor at the University of the Western Cape.
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