A chemical commonly found in sunscreen could be making plastic in oceans even harder to break down, according to University of Stirling research.

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

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

Now, 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.

She said: “These changes matter. 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 that they do not dissolve in water. This makes them a combined threat, as they can accumulate on plastics and remain in the environment.

Scientists have previously studied the role of the plastisphere, but little is known about how additional chemicals such as EHMC affect the microbes living on the plastic.

The new study published today [Friday 4 July] 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 hope will be further investigated.

One key finding of the study is the much higher level of a protein called outer membrane porin F (OprF) in Pseudomonas that was exposed to EHMC. This protein plays a crucial role in maintaining the structure of biofilms, protective layers that help bacteria survive hostile environments.

Researchers also observed a shift toward anaerobic respiration – where cells can generate energy in the absence of oxygen – revealing a complete shift in the microbial metabolism within the plastisphere.

The research shows that EHMC could hinder the development of useful aerobic bacteria that help break down plastic pollutants at an early stage, by favouring more stress-tolerant anaerobic biofilm-forming bacteria.

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

“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.”

The paper, 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, is published in the Journal of Hazardous Materials.

Dr Matallana-Surget led the research in collaboration with Dr Charlotte Lee – who undertook the core experimental work – and Dr Lauren Messer at the University of Stirling, alongside Professor Ruddy Wattiez at the University of Mons in Belgium.

The project, which stemmed from Dr Matallana-Surget’s initial concept of investigating the emerging double pollution issue, has built on 15 years of joint work between the teams in Stirling and Mons.

Research was funded by the UKRI Natural Environment Research Council (NERC) and the National Research Foundation Singapore. It was further supported by the European Regional Development Fund and the Walloon Region, Belgium.

It builds on previous research published last year by Dr Matallana-Surget which uncovered the crucial roles of bacteria living on plastic debris.

Dr Matallana-Surget has also published a study assessing the impact of the Deepwater Horizon oil spill on microscopic seawater bacteria that perform a significant role in ecosystem functioning.