• Ultraviolet filters typically found in sunscreen lotions can accumulate in high concentrations in seagrass rhizomes, a new study shows.
• This discovery is raising concerns about the potential effect on important seagrass ecosystems, though the full ramifications remain unclear.
• The findings indicate that such components not only end up in organisms in the coastal environment but also tend to remain there for a long time, one expert says.
• UV filters are already known to accumulate in a variety of aquatic species, such as dolphins, sea turtles, fish and mussels, and can cause harm, including birth defects and reduced fertility, as well as damage to coral reefs.

Sharing sunscreen with family and friends is part and parcel of any trip to the seaside. Now, researchers have discovered that the seagrass Posidonia oceanica also partakes, accumulating ultraviolet filters contained in such products and cosmetics. This discovery is raising concerns about the potential effect on important seagrass ecosystems, though the full ramifications remain unclear.

Researchers sampled the rhizomes of P. oceanica at three different sites on the Mediterranean island of Mallorca: at the port of the capital Palma, near the port of Alcudia, and at Ses Salines, a “pristine” site with fewer tourists than the others. In all samples, the researchers found varying mixtures and concentrations of the sunscreen components oxybenzone, avobenzone, 4-methylbenzylidene camphor, benzophenone-4, and methyl parabens.

P. oceanica accumulated benzophenone-4 and methyl parabens at the highest levels, at 129 and 512 parts per billion dry weight, respectively. The rhizomes, a part of the stem, were between 10 and 22 years old, indicating to Silvia Díaz-Cruz, associate professor at the Spanish Council for Scientific Research in Barcelona and a co-author of the study, that these components persist in the environment and that continuous exposure enabled the concentrations to reach such levels.

Díaz-Cruz said she was concerned by the findings, published in March in the journal Marine Pollution Bulletin. In recent years, research has shown that UV filters can accumulate in a variety of aquatic species, such as dolphins, sea turtles, fish and mussels, and can cause harm, including birth defects and reduced fertility. Coral reefs, already threatened by climate change and ocean acidification, are also harmed by sunscreen components.

“What oxybenzone does is it drops the temperature that causes corals to bleach,” said Craig Downs, an ecotoxicologist and executive director of the Haereticus Environmental Laboratory in Virginia, U.S., who was not involved in the study. Research by Downs indicates that oxybenzone can cause deformations in coral larvae, inhibiting reproduction. Although sunscreen is not the primary cause of coral declines, it can be part of wider local pollution problems that lead to “death by a thousand cuts,” Downs said.

Concern for coral reefs has led several jurisdictions to ban sunscreen products containing potentially harmful components. The U.S Virgin Islands were the first to institute a ban in 2019, and the Pacific island republic of Palau became the first country to do so the following year. Hawai‘i, Aruba, Key West, Florida and Bonaire, a Dutch-administered island in the Caribbean, have also enacted their own bans. Last year, Thailand prohibited certain products in its marine parks.

Seagrass at risk?

Back in the Mediterranean, it remains unclear what impact, if any, the accumulated UV filters may have on P. oceanica. Seagrass meadows occur in shallow waters around all continents except Antarctica. They provide habitat and nurseries for marine species, support fisheries, buffer coastal areas against erosion, and are key allies in the fight against climate change as prolific stores of carbon. Yet, globally, these important ecosystems are in decline due partly to coastal development and pollution. Understanding whether sunscreen is among the threats is therefore vital, said Díaz-Cruz.

For Downs, the high concentrations the study found of benzophenone-4 and methyl parabens are cause for alarm. The substances are known endocrine disruptors in humans and wildlife, although their wider environmental effects remain poorly understood. “We don’t know what those impacts are, especially on the seagrasses,” he said. Some studies show that oxybenzone can impair photosynthesis in plants, which Downs said is another red flag. Concentrations of this substance were far lower in P. oceanica, however, and appeared only in samples taken from the port of Palma.

Scientists have yet to study how the UV filters found accumulating in P. oceanica might affect seagrass. This point is key for Christine Pergent-Martini, a seagrass specialist at the University of Corsica, who was not involved in the study. “[T]hese findings could be of concern if a real impact on the health of the meadow is demonstrated,” she said, but accumulation does not necessarily indicate harm.

“We have used the capacity of Posidonia meadows to accumulate trace-metal as a bioindicator of the contamination of seawater,” she said, but the metals only show negative effects on the seagrass at high concentrations in labs, not in the environment.

Potential harm to the seagrass itself aside, Díaz-Cruz said she’s concerned that the findings indicate a wider pollution problem. The contamination is likely to be coming from wastewater treatment plants in the port areas, she said; rhizomes from the two port sites had the highest concentration of all sunscreen UV blockers. Treated wastewater can include a host of other pollutants known to impair marine life, including pharmaceuticals, microplastics and caffeine. Other sources of the UV filters are direct contamination from swimmers and, potentially, beach showers.

“The findings of the present study are of major concern for many reasons and most importantly because they indicate that such components not only end up in organisms in the coastal environment but also tend to remain there for a long time, and consequently, [could] be recycled through the food web,” said marine scientist Zoi Mylona, who was not involved in the recent study.

A 2020 study, for example, found UV filters accumulate in juvenile loggerhead turtles (Caretta caretta), which spend a lot of time feeding among P. oceanica in the Mediterranean. This study linked UV filters to inflammation and oxidative stress in blood tissue samples.

While at the Aristotle University of Thessaloniki, Greece, Mylona conducted part of her Ph.D. research on the effects on seagrass of a different class of sunscreen UV filters: inorganic metal oxides. Her research team found that exposure to titanium dioxide nanoparticles inhibited the growth of leaves and caused cell death in the seagrass Halophila stipulacea.

It will take further research to understand whether seagrass species besides P. oceanica also accumulate organic UV filters like the ones identified in the present study, Mylona said.

Digging deeper

Díaz-Cruz and her teammates are seeking to answer many of the questions their initial finding raises. One project is set to investigate how UV filters affect P. oceanica meadows around Mallorca.

She said testing individual filters is important, but so is testing mixtures of common filters because such data are so far lacking. “We know that in nature, in real life, there are a lot of compounds together and the effect can be synergistic or antagonistic,” Díaz-Cruz said.

She said she hopes the results can inform decisions about which UV filters should be used, if the study demonstrates any negative effects on this important seagrass species.

“The idea is not that we stop using UV filters,” Díaz-Cruz said. “We should use those that respect the environment, and we need the research behind in order to see which of them are or are not environmentally friendly.”

This article was originally published on Mongabay.

Banner image: Mallorca, in Spain’s Balearic Islands, is a go-to destination for holidaymakers from Europe and further afield. Image by Kyle Taylor via Flickr (CC BY 2.0).