A polystyrene-munching beetle larvae called a "superworm", can gain weight on an exclusive diet of polystyrene, researchers have found.
They hope their findings can pave the way for technology to degrade and recycle the plastic on a large scale, but others are sceptical of the approach.
Fed only polystyrene (aka styrofoam) for three weeks, the superworms — larvae of the darkling beetle (Zophobas morio) — survived at comparable rates to larvae fed on a diet of bran, according to the study published today in Microbial Genomics.
The fact that the superworms were able to gain a small amount of weight from eating polystyrene exclusively is an indication that they're able to convert the material into energy, according to study co-author Christian Rinke from the University of Queensland's School of Chemistry and Molecular Biosciences.
Groups of superworms were fed either bran, polystyrene, or given no food for the three-week trial period.
The bran-fed worms more than doubled their weight over the three-week period, the styrofoam-fed worms added a small amount of weight, whereas those on the starvation diet roughly maintained their starting weight.
The worms fed bran or polystrene were also far more active compared to the starvation diet worms, according to Dr Rinke.
"In terms of activity, the worms that had the bran were super active, the polystyrene ones were a bit slower, and the ones that were starving were basically still all the time."
The rate of pupation after the three-week trial was also looked at. The "worm" or larval phase of the superworm, is followed by the development of pupae — where the animal cocoons itself and metamorphoses into a darkling beetle.
As expected, the rate was highest in the bran-fed beetles with more than 90 per cent becoming pupae.
But there was also a significantly higher pupation rate in the polystyrene-fed beetles compared to the starved beetles — more than 60 per cent compared to 10 per cent, again indicating that the polystyrene diet was at least superior to the starvation diet.
For every larva that entered the pupa phase from each diet group, all successfully hatched as beetles.
Key enzymes in the microbiome
At the beginning and end of the study, a portion of the worms from each group were frozen and their gut contents and microbiome analysed.
Bacteria in the microbiome produce enzymes that, among other functions, can aid in digestion.
Though the plastic diet was found to have a negative impact overall on the worms' gut microbiome diversity and health, the researchers identified several enzymes in the superworm gut that they think can degrade the polystyrene.
While it's been known for a while that some types of beetle larvae can consume polystyrene, this study takes it a step further, according to Colin Jackson from the ANU's Research School of Chemistry, who wasn't involved with this study.
"This study goes a long way towards understanding how the bacteria in [the worms'] gut do this at the molecular level," Professor Jackson said.
"[That's] important for translation and use of this type of approach in recycling."
To get that information, the researchers used genetic analysis, according to Dr Rinke.
"The novelty here is we've used a technique called metagenomics — that allows us to identify all the genes in the microbiome," he said.
Although the researchers in this case inferred which enzymes were breaking down the polystyrene, no experiment to date has managed to isolate the critical enzymes and demonstrate the process in a test tube.
What that means is, although the researchers think they know which enzymes are responsible for breaking down the plastic, there is still some room for doubt.
Some researchers remain sceptical that the polystyrene is able to be biologically degraded.
It's possible, some argue, that any weight gain associated with consumption of the product is more likely to be related to other chemical elements in the styrofoam, such as flame retardants or expanding agents.
Polystyrene is primarily made from styrene, a hydrocarbon.
Although it can be synthesised, styrene is also a naturally occurring, highly volatile chemical that is used to make plastics, rubber and foam.
The idea, according to Dr Rinke, would be to degrade polystyrene to styrene, then use that to create new materials.
"It's still early days," he said.
"The next step would be to engineer [the enzymes], to increase that efficiency.
"In an ideal scenario … if it all works out you can add the enzymes or the microbes that degrade the polystyrene to a bunch of chemical compounds and then use those compounds to form other products like bioplastics."
Where to from here?
While the findings are promising, it's not the first biological agent found to be able to consume plastics.
Certain species of fungi, marine microbes and even bacteria found in cows' stomachs have previously been found to have some plastic-degrading potential.
But if the worms are able to consume and degrade polystyrene, there are still many difficulties in developing a feasible recycling technology.
The first is the capacity to scale this type of technology to where it may have some significant impact on plastic pollution, Professor Jackson said.
"The scale-up and translation of research like this is always a challenge, which is magnified in the area of plastics by the incredible scale of the problem and the economics in terms of how cheap new plastic is to produce," he said.
Muxina Konarova from the University of Queensland's School of Chemical Engineering, who wasn't involved in this study, said even if the technology could be scaled up, you'd potentially be creating a raft of new environmental and health problems.
"There are carcinogenic co-products that could potentially come out of the styrofoam," Dr Konarova said.
Benzene is one of the chemicals used to make styrofoam, and is listed as a carcinogen on Australia's National Pollutant Inventory.
"That's the main reason they don't recycle polystyrene, because you end up with these compounds that you don't want to touch," Dr Konarova said.
There are also greenhouse gas emissions associated with the potential by-products of the process as well, she said.
"Even if we convert this to biodiesel, then we're going to burn the biodiesel, which is going to make CO2," Dr Konarova said.
"I don't want to give the impression that there's a solution [to styrofoam pollution] when there's not.
In Australia that may soon be happening, at least in part.
In 2021, the federal government launched a plan to phase out certain uses of polystyrene in packaging and containers.
By July this year, the aim was to end the use of loose-fill styrofoam packaging, and moulded consumer packaging — like the type that televisions, computers and homewares come encased in.
By the end of this year, styrofoam food and drink containers are also supposed to be phased out.
Although broadly supported at the time, the plan was also criticised for being voluntary and lacking legislated targets.
The phase-out doesn't include polystyrene in building and construction, in "business-to-business" packaging, or in medical applications.
In response to questions from the ABC regarding the phase-out, a spokesperson for Environment Minister Tanya Plibersek said she was currently receiving briefings from her department on her new portfolio.
"However, Australia should be ambitious in reducing waste and phasing out problematic plastics and polystyrenes," they said.
