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The bay of Illa Grossa, located on a remote island in the Columbretes Islands marine reserve off the coast of Spain, is home to the only reef-forming stony coral species in the Mediterranean: Cladocora caespitosa. Despite its isolated location, approximately 55 kilometers from the mainland and being free from local sources of pollution, a new international study led by Kiel University reveals that this strictly protected habitat is heavily polluted with microplastics and micro-rubber.

These tiny particles accumulate in the coral's sediments, potentially impairing feeding processes and weakening the corals' ability to withstand heat stress. In some locations, researchers found more than 6,000 microplastic particles per kilogram of sediment—four times the average level in the study area and among the highest levels recorded in the Mediterranean regions to date. The findings were recently in the Marine Pollution Bulletin.

"Our results clearly demonstrate that even remote, strictly protected habitats are not safe from plastic pollution," said Dr. Lars Reuning, lead author of the study and geoscientist at Kiel University. "Cladocora caespitosa is the only reef-building coral in the Mediterranean. Its ability to form reefs in a temperate climate creates an important habitat for numerous other species—a characteristic typically associated with tropical corals. It also serves as a key indicator of environmental change in the region."

A key species in the Mediterranean under pressure

Unlike most coral reefs, which are typically found in tropical waters, C. caespitosa forms stable reef structures in temperate conditions—a rare phenomenon. As a keystone species, it creates three-dimensional habitats that host diverse marine life.

The coral feeds both through photosynthesis, via symbiosis with microscopic algae, and by filtering plankton from the water. This heterotrophic feeding method that is especially vital during periods of stress, such as high temperatures or low light in winter. However, high concentrations of microplastics may disrupt this flexible feeding mechanism, which is important during critical periods. This could have serious consequences for the coral's energy supply and stress resistance.

A remote hotspot of plastic pollution

The research team—mainly scientists from the Spanish Institute of Marine Sciences (CSIC)—collected sediment samples at five sites within the volcanic caldera of Illa Grossa. The samples were analyzed in laboratories at Kiel University and the Helmholtz Center Hereon using advanced spectroscopic methods.

The corals acted as physical sieves; the denser the colony, the smaller the retained particles, indicating hydrodynamically driven accumulation process. On average, the researchers identified 1,514 microplastic and micro-rubber particles per kilogram of sediment. One sample contained as many as 6,345 particles from within the coral colony.

"We found microplastics in every sample. The highest concentrations were observed in sediments embedded within the coral structures," explains Dr. Diego Kersting, co-author and researcher at the Torre de la Sal Aquaculture Institute (IATS-CSIC). "These values are far higher than those observed to date in other parts of the western Mediterranean."

Laboratory analysis using laser direct infrared spectroscopy (LDIR) identified the size, shape, and polymer types of the particles. The most common polymers included polyethylene (PE, 28%), polyethylene terephthalate (PET, 25%), polystyrene (PS, 19%), polyurethane (PU), and micro-rubber (around 16% total).

"More than 90% of the particles were smaller than 250 micrometers—small enough to be ingested by corals," explains Dr. Daniel Pröfrock from the Helmholtz Center Hereon. "Polyurethanes, in particular, are suspected of being especially harmful to marine organisms due to their chemical properties suggest that they may be toxic," adds his colleague, Dr. Lars Hildebrandt.

Plastic pathways: The role of ocean currents

The crescent-shaped bay of Illa Grossa opens to the northeast, efficiently trapping floating plastic debris. The Northern Current, a major ocean current in the western Mediterranean, transports plastic waste from the densely populated coastal regions of Spain, southern France, and northern Italy to the Columbretes Islands. Once the debris enters the bay, it becomes trapped. Additionally, microrubber particles from tire abrasion are carried to the sea via rivers, contributing further to the pollution of this marine protected reserve.

"Our findings are deeply concerning. Even though they pertain to a limited area of the Mediterranean, they highlight that even protected areas are severely affected by global plastic pollution, which particularly endangers sensitive coral species. These results underscore the urgent need to expand research on such impacts and to intensify efforts to reduce global plastic emissions," says Dr. Reuning.