A chemical found in sunscreen could be making plastic pollution in the ocean even harder to break down, new research from the University of Stirling has found.

The chemical Ethylhexyl Methocxycinnamate – commonly known as EHMC – could be slowing the degradation of discarded plastic in our seas and helping biofilm bacteria, which have greater protection from harsh conditions, to thrive.

Led by Dr Sabine Matallana-Surget, scientists at the University of Stirling carried out the analysis, which is the first to study co-pollution – the process by which plastics in the sea act as carriers for other chemical contaminants, including ultraviolet filters from sunscreen.

Dr Matallana-Surget – an associate professor in the Faculty of Natural Sciences, is appealing to policymakers to take urgent action to tackle what she calls ‘the invisible threat of sunscreen.’

“These changes matter,” she said. “By suppressing the aerobic bacteria that help degrade plastic, and selecting those that stabilise or reinforce the biofilm, UV filters would prolong the life of plastics in the ocean – making them more resistant to breakdown by sunlight or microbes. 

“Targeted research and policy interventions are therefore urgently needed to mitigate these compounded ecological threats.”

Plastic waste in the ocean provides a new surface where microbes can grow, forming slimy layers called the plastisphere. As well as forming the plastisphere, plastics also absorb other pollutants, including sunscreens washed off human skin that are insoluble in water, and these can then attach to marine plastic surfaces.

Sunscreens, like oil, are hydrophobic – meaning they do not dissolve in water. This makes them a combined threat, as they can accumulate on plastics and remain in the environment. The role of the plastisphere has been investigated by scientists before, but little remains known about how additional chemicals such as EHMC affect the microbes living on the plastic.

This new study – ‘The Invisible Threats of Sunscreen as a Plastic Co-Pollutant: Impact of a Common Organic UV Filter on Biofilm Formation and Metabolic Function in the Nascent Marine Plastisphere’ – was published in the Journal of Hazardous Materials. 

It shows that when plastics are co-contaminated with EHMC, not only do pollutant-degrading bacteria like Marinomonas decline, but bacteria like Pseudomonas develop more proteins that stabilise biofilms and improve their ability to survive.

Pseudomonas includes species known for being resilient in polluted environments and for their ability to break down a wide range of contaminants including pesticides, heavy metals, and hydrocarbons. However, some Pseudomonas strains are also classified as ‘opportunistic pathogens’ capable of causing serious infections that can require antibiotic treatment – raising potential public health concerns that researchers now hope will be further investigated.

“The UV-protective properties of EHMC, combined with its suppression of hydrocarbonclastic bacteria, may indirectly protect plastics from photodegradation and biodegradation, further contributing to their persistence in marine environments,” said Dr Matallana-Surget. 

“This impact, combined with the enrichment of potentially pathogenic bacteria, raises significant concerns for ecosystem stability and human health, particularly in coastal regions with high rates of tourism, and high levels of plastic pollution.”

Dr Matallana-Surget led this research in collaboration with Dr Charlotte Lee who undertook the core experimental work, alongside Dr Lauren Messer at the University of Stirling with Professor Ruddy Wattiez at the University of Mons in Belgium. Stemming from Matallana-Surget’s initial concept of investigating the emerging double pollution issue, the project builds on 15 years of joint work between the teams at the two universities.

Last year, Matallana-Surget published a study uncovering the crucial roles of bacteria living on plastic debris. She has also published a study assessing the impact of the Deepwater Horizon oil spill on microscopic seaweed bacteria that perform a significant role in the ecosystem.

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