A possible explanation for the ‘missing plastic problem’: New detection technique finds microplastics in coral skeletons

Researchers from Japan and Thailand investigating microplastics in corals have discovered that all three parts of the coral anatomy (surface mucus, tissue and skeleton) contain microplastics. The discovery was made possible thanks to a new microplastic detection technique developed by the team and applied to corals for the first time.

These findings may also explain the “missing plastics problem” that has puzzled scientists, where about 70% of plastic waste that enters the ocean goes undetected. The team hypothesizes that corals may absorb microplastics from the ocean and act as “sinks.” The findings were published in the journal Science of the Total Environment.

Humanity’s addiction to plastics has brought unprecedented convenience to our lives, but it has also caused immeasurable damage to ecosystems in ways that researchers are only beginning to understand. In the oceans alone, it’s estimated that between 4.8 and 12.7 million tonnes of plastic enter the marine environment each year.

“Plastic pollution is a major problem in Southeast Asia. Overall, about 10 million tonnes of plastic waste is dumped there every year, which is one-third of the global total,” explains Suppakarn Jandan, assistant professor at Kyushu University’s Research Institute for Applied Mechanics (RIAM) and lead author of the study. “Some of this plastic ends up in the ocean, where it breaks down into microplastics.”

To study the problem of plastic pollution in Southeast Asia, RIAM partnered with Chulalongkorn University in Thailand to establish the Marine Plastics Research Centre in 2022. This international institute is headed by Professor Atsuhiko Isobe, who led the research team behind these latest findings.

The team wanted to investigate the impact of microplastics on local coral reefs, so they focused their fieldwork off the coast of Koh Sichang in the Gulf of Thailand, an area known for its small, shallow coral reefs and a popular site for anthropological research.

“Corals have three main anatomical parts: surface mucus on the outside of the coral body, tissue inside the coral, and the skeleton, a hard deposit of calcium carbonate that the coral produces. Our first step was to develop a method to extract and identify microplastics from coral samples,” Jandin continues.

“We put the samples through a series of simple chemical washes designed to break down the anatomical layers. After each layer dissolved, we filtered the contents and worked on the next layer.”

In total, the researchers collected and examined 27 samples from four species of coral. They found 174 microplastic particles in the samples, most of which were between 101 and 200 microns in size, roughly the width of a human hair. Of the microplastics detected, 38% were distributed in the surface mucus, 25% in the tissues, and 37% in the skeleton.

In terms of the types of microplastics, the team found that nylon, polyacetylene, and polyethylene terephthalate (PET) were the most common, making up 20.11%, 14.37%, and 9.77% of the samples identified, respectively.

These new findings also suggest that corals may be able to act as marine plastic “sinks,” keeping plastic waste out of the ocean, in the same way that trees sequester carbon dioxide from the air.

“The ‘missing plastic problem’ has vexed scientists tracking marine plastic waste, but this evidence suggests that corals may be the source of that missing plastic,” Jandan said. “Because coral skeletons remain intact after death, these deposited microplastics could be preserved for hundreds of years – similar to mosquitoes in amber.”

Further research is needed to fully understand how these findings impact coral reefs and global ecosystems.

“The corals surveyed in this study are distributed all over the world. To get a more accurate picture, we need to conduct more extensive surveys on a global scale targeting various coral species,” concludes Isobe. “We also do not know the health effects of microplastics on corals and coral reefs as a whole. There is still a lot to be done to accurately assess the impact of microplastics on ecosystems.”