Microplastics Found Lurking Inside Coral Skeletons

Microplastics have made their way into nearly every environment on Earth and have been detected inside thousands of animals around the world.
Now, according to a paper in the journal Science of the Total Environment, microplastics have even been discovered hidden inside the mineral skeletons of corals.
This could explain the so-called missing plastic problem, which describes how around 70 percent of plastic pollution known to have entered the ocean is apparently nowhere to be seen. The researchers suggest that coral could be acting as a microplastic “sink,” absorbing it from the water and storing it inside of themselves.
Microplastics are tiny chunks of plastic pollution, less than 5 millimeters in size, that mainly result from the degradation of larger plastic items such as synthetic textiles, tires, road markings, plastic bags, personal care products and engineered plastic pellets. Microplastics have been found inside thousands of animal species, in our food, in the air we breathe and inside our blood. Between 4.8 million and 12.7 million tons of plastic are thought to enter the oceans every year, and microplastic pollution is forecast to double by 2040.
“In Southeast Asia, plastic pollution has become a significant issue. Collectively, nearly 10 million tons of plastic waste are dumped annually, equivalent to 1/3 of the world’s total,” said study co-author Suppakarn Jandang in a statement.
“Some of this plastic is discharged into the ocean, where it degrades into microplastics,” said Jandang, an assistant professor at the Research Institute for Applied Mechanics at Japan’s Kyushu University.
In the paper, the researchers describe how they collected samples of coral from around the coast of Ko Si Chang Island, which is in the Gulf of Thailand. They investigated four species of coral, taking a total of 27 samples.
“Coral has three main anatomical parts: the surface mucus, the outside of the coral body; the tissue, which is the inner parts of the coral; and the skeleton, the hard deposits of calcium carbonate they produce. Our first step was to develop a way to extract and identify microplastics from our coral samples,” Jandang said.
“We put our samples through a series of simple chemical washes designed to break apart each anatomical layer. After each subsequent layer was dissolved, we would filter out the content and then work on the next layer,” Jandang said.
The researchers discovered 174 microplastic particles across the samples, most of which were about as thick as a human hair. Microplastics were also found in all of the other layers of the coral—surface mucus and tissue—and the majority of these microplastics were tiny fragments that were mostly blue, black or white. Thirty-seven percent of the particles were found on the coral’s skeleton, 25 percent in the tissue and 38 percent on the surface mucus.
These findings indicate that corals may be mopping up large volumes of plastic from the oceans, which could explain why the plastic appears to be missing.
“The ‘missing plastic problem’ has been troubling scientists who track marine plastic waste, but this evidence suggests that corals could account for that missing plastic,” Jandang said. “Since coral skeletons remain intact after they die, these deposited microplastics can potentially be preserved for hundreds of years. Similar to mosquitos in amber.”
The researchers hope to further investigate more species of coral to see if they too are infested with microplastic particles.
“The corals that we studied this time are distributed all around the world. To get a more accurate picture of the situation we must conduct extensive studies globally across an array of coral species,” said Atsuhiko Isobe, a professor at the Research Institute for Applied Mechanics, in the statement.
“We also do not know the health effects of microplastics on coral and the larger reef community. There is still much to be done to accurately evaluate the impact of microplastics on our ecosystem,” Isobe said.
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Jandang, S., Alfonso, M.B., Haruka, N., Phinchan, N., Darumas, U., Viyakarn, V., Chavanich, S., & Isobe, A. (2024). Possible sink of missing ocean plastic: Accumulation patterns in reef-building corals in the Gulf of Thailand. The Science of the Total Environment. https://doi.org/10.1016/j.scitotenv.2024.176210