By Dr Catriona Nguyen-Robertson MRSV

When life gives you plastic, you live with it, you break it down and you… eat it. At least that is what some organisms have evolved to do.

Plastic is one of the most enduring materials we humans have created. Millions of tonnes of plastic pollution (including microplastics) pervade the planet from the deepest oceans to the top of Mount Everest.^1,2 It can take hundreds of years for plastic to degrade alone, but nature may already have answers to our problem in the form of organisms that consume plastic. While for these forms of life, it might not be as “fantastic” as Aqua’s 1997 song may have led us to believe, it is certainly incredible how some organisms have adapted to utilise it – and these plastic-eaters may provide us with potential ways to clean up our mess.

A Home Made of Plastic

For some organisms, plastic debris literally provides a life raft. Around 8 to 10 million metric tons of plastic end up in the ocean each year, and some of it provides a home to entire biological communities.³ Scientists first discovered that microplastics are home to life in 1972, when two studies reported microorganisms living on plastic retrieved from the North Atlantic Ocean.^4,5 A further study dubbed the diverse and complex community of microbes found on ocean plastic, the “Plastisphere”. Since then, numerous studies have observed microbes and animals that are using plastic rafts to colonise the open ocean. The broad range of surface textures available on the garbage form ideal habitats for many species, from single-celled microbes and algae to barnacles and insects. In addition, plastic provides a place for coastal species to breed (e.g. lay eggs) and expand their populations into the open ocean when they would otherwise be confined to shore.

The bad news is that plastic dwellers could make ocean plastics more attractive as food for animals further up the food chain. The more creatures that reside on plastics, the harder it is for other animals to distinguish between plastic waste and food. Animals that accidentally eat plastic often suffer, as the ingested plastic fills their stomachs, reducing how much they eat, or larger pieces can also clog their gastrointestinal tract. Furthermore, marine species can hitch a ride on these plastic rafts for long periods of time. Following the 2011 earthquake and tsunami in Japan, scientists expected floating trash to wash up on other shores. What they did not expect was for Japanese mussels, barnacles, and sea squirts – 289 species in total – to survive a six-year trek across the Pacific Ocean.⁶ This raises concern that invasive species are able to travel from one shore to another, potentially invading new habitats and impacting local ecosystems.

The good news is that some plastic inhabitants may weigh down the plastic, decreasing plastic pollution level at the sea surface, where major environmental impacts occur. For example, diatoms – a silica-forming algae – that grows on ocean plastics in large numbers can weigh down its plastic home, causing tiny pieces of plastic debris to sink to the bottom of the ocean. Bacteria such as Pseudomonas can also grow as a biofilm on microplastics – a cluster of microbes that adhere to the surface – thereby weighing them down. Not only is this potentially beneficial in open water, but also in wastewater treatment, as our current processes do not sufficiently eliminate plastic waste and biofilms can be grown on plastic pieces to weigh them down to be sifted out. In addition, researchers have also seen colonies of microbes within the Plastisphere that seem to be “eating plastics” and thereby providing a form of biodegradation.

The Plastic Eaters

Plastics take anywhere from 20 to 500 years to decompose into microscopic particles, depending on the material and structure. This is simply not fast enough, nor is it good enough – we produce plastics faster than they break down, and most molecules of plastic that we have ever produced are still present in the environment.

Many plastics are hard to degrade and recycle, but with plastic having become an abundant “resource” in the environment, many organisms have evolved to eat it by adapting enzymes over time to break the sturdy bonds within plastic compounds. In 2021, a study of ocean and soil samples from around the world revealed that the plastic-degrading potential of microbes correlates with pollution trends: more plastic-degrading enzymes were present in habitats with more pollution.⁷

Many of the organisms that we now know to degrade plastic were discovered accidentally. The first bacteria discovered to break down and metabolise plastic was found in a Japanese bottle-recycling facility.⁸ The new species, Ideonella sakaiensis, breaks down PET to create basic building blocks for its growth. PET is one of the most common plastics and does not readily break down in the environment. All the wet wipes, water bottles, or product packs made of PET that head to landfill will stick around for centuries – unless hungry I.sakaiensis are around. Moreover, a Biohm biotech engineer also discovered a plastic-eating fungus after she realised that one of the fungi had eaten its way through the plastic seal of the jar she was growing it in.⁹ Some animals can even eat through plastic – granted, for many like the common Zophobas morio ‘superworm’ that can digest polystyrene (e.g. foam), it is thanks to bacteria in their gut.¹⁰ Wax worms were the first insect discovered to digest plastic themselves. Again, this was an accidental discovery: a scientist who was also an amateur beekeeper cleaned out her hives infested with wax worms and dumped the larvae in a plastic bag, only to find that the larvae saliva could break down polyethylene within hours allowing them to escape.¹¹

Organisms that degrade plastics typically only have the capability to digest one plastic type, over time, and under certain conditions. Furthermore, plastic is, unsurprisingly, not the most exciting food option (albeit, to some like the superworm, it does provide enough nutrition to make them gain weight).9 There is, however, great potential for scientists to work together with these organisms and their enzymes.

The 2021 study of plastic-eaters from around the world identified a total of 30,000 enzymes that degrade 10 types of plastics.7 While we may not see industrial “composts” in the near future with a community of organisms breaking down our plastic waste, we could engineer some of these enzymes using biotechnology. Scientists tweaked the enzyme from I.sakaiensis that digests PET, and inadvertently created a version of the enzyme that was even better at breaking down plastic bottles than the bacteria. After many iterations, they have produced a “super enzyme” that can break the tough chemical bonds in PET 10,000 times more efficiently.¹² Plus, when the building blocks generated by the enzyme are used to make new PET for plastic bottles, the bottles are just as strong as those made from conventional plastics. A research team at the University of Edinburgh have even engineered E.coli bacteria to turn a molecule derived from PET into the flavour molecule, vanillin (with the same structure as that obtained from a vanilla bean).¹³ Perhaps we could be recycling – and even eating derivatives – of plastic too.

Biodegradation is a plausible route toward sustainable management of the millions of tons of plastic waste that have accumulated in terrestrial and marine environments. With 30,000 enzymes in the environment to degrade plastic, there are plenty to use to our advantage. By breaking down microplastics into smaller components, we can recycle them into new products, such as 3D-printed plastic materials or recycled plastic bottles. When scientists work together with the many organisms that have evolved to live with our waste, we make a great team.

This piece appears in the August 2023 edition of Science Victoria magazine. All issues can be read online for free at rsv.org.au/Science-Victoria.

References:

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