Corals Found to Trap Microplastics, Offering Clues to the 'Missing Plastic' Problem

The issue of marine plastic pollution has become an urgent global environmental concern, with an estimated 4.8 to 12.7 million tons of plastic entering the ocean every year. Microplastics (MPs), defined as plastic particles smaller than 5 mm, pose a severe threat to marine ecosystems, and scientists have been puzzled by the 'missing plastic problem'—the discrepancy between the amount of plastic entering the ocean and the plastic found in surface waters. Recent studies suggest that much of this missing plastic may be accumulating in marine organisms and seabeds. A groundbreaking study, led by Suppakarn Jandang and his team, sheds light on this mystery, revealing a potential hidden reservoir for microplastics: the skeletons of reef-building corals.

This research, carried out in the Gulf of Thailand, shows that microplastics not only adhere to the mucus and tissue of corals but also accumulate within the skeletons of these organisms. These findings provide compelling evidence that corals act as a long-term sink for oceanic microplastics, potentially accounting for a significant portion of the missing plastic. The study offers crucial insights into the fate of microplastics in marine environments and highlights the importance of understanding coral ecosystems' role in managing global plastic pollution.

The 'Missing Plastic' Dilemma

Plastic pollution in the ocean has been widely studied, but one of the most perplexing issues has been the 'missing plastic' conundrum. While large quantities of plastic enter the oceans each year, only a fraction is observed floating on the surface. Scientists have hypothesized that these plastics either sink to the seabed, degrade into smaller particles, or are ingested by marine organisms. Yet, the precise fate of these plastics remains uncertain.

Recent studies have suggested that biological processes, such as ingestion and adhesion, play a significant role in the distribution and eventual accumulation of microplastics. However, where these plastics end up, particularly in regions rich in marine life, like coral reefs, has been an area of active research. Corals, being critical ecosystems that host diverse marine life, have increasingly come under scrutiny for their potential role in sequestering microplastics.

The Study: Corals as Potential Sinks for Microplastics

The study led by Jandang and colleagues focused on four species of reef-building corals—Lobophyllia sp., Platygyra sinensis, Pocillopora cf. damicornis, and Porites lutea—collected from Si Chang Island in the Gulf of Thailand. These corals were chosen for their ecological significance and widespread distribution in coral reefs globally. The researchers aimed to investigate how microplastics accumulate in different components of corals, including the mucus layer, tissue, and skeleton.

One of the study's major findings was that microplastics were found in all parts of the coral—mucus, tissue, and skeleton—demonstrating that these organisms interact with plastic pollution at multiple levels. The coral species Pocillopora cf. damicornis exhibited the highest level of microplastic accumulation, particularly in its skeleton, with over 52% of the detected particles found embedded within. These microplastics were predominantly in the form of fragments, accounting for over 75% of the particles, with black, white, and blue being the most common colors.

Detection Techniques: A New Approach to Uncovering Microplastics

A crucial aspect of this study was the development of a refined method to detect microplastics in different coral components, particularly the skeleton. Previous research had often focused on the surface mucus and tissue layers of marine organisms, neglecting the potential for plastics to be incorporated into the coral's calcified skeleton. The team employed a combination of sodium iodide (NaI) solutions, hydrogen peroxide (H2O2), and potassium hydroxide (KOH) to isolate microplastics from the mucus and tissue layers, while hydrochloric acid (HCl) was used to dissolve the coral skeleton for further analysis.

This new technique allowed the researchers to quantify the amount of microplastics present in each coral component accurately. By dissolving the coral skeleton, they were able to identify microplastics that had been integrated during the calcification process, highlighting the potential for corals to serve as a long-term repository for these pollutants.

The Role of Coral Skeletons in Storing Microplastics

The discovery of microplastics in coral skeletons is significant because it suggests that corals may act as long-term sinks for oceanic plastic pollution. Once microplastics are incorporated into the coral's skeleton, they are likely to remain there for extended periods, potentially on a geological timescale. This finding has profound implications for understanding the fate of microplastics in the ocean.

The fact that over half of the detected microplastics in Pocillopora cf. damicornis were found in the skeleton suggests that these particles are not merely transient pollutants but are becoming an integral part of the coral's structure. This could explain why so much of the ocean's plastic appears to be missing—much of it may be trapped in the skeletons of corals and other calcifying organisms, hidden from detection in the water column.

Ecological Implications: Impact on Coral Reefs and Marine Life

While the accumulation of microplastics in coral skeletons may offer an explanation for the missing plastic problem, it also raises concerns about the ecological impacts on coral reefs. Coral reefs are some of the most biodiverse ecosystems on the planet, providing habitat for thousands of marine species. The infiltration of microplastics into coral skeletons could have unforeseen consequences for the health and resilience of these ecosystems.

Microplastics have been shown to cause physical and chemical stress in marine organisms, and their presence in coral reefs could disrupt critical processes like calcification, reproduction, and nutrient cycling. The study found that corals interact with microplastics through ingestion and adhesion, which could lead to tissue damage, reduced growth rates, and impaired physiological functions. Additionally, the accumulation of microplastics in coral skeletons may affect the structural integrity of reefs, making them more vulnerable to erosion and bleaching events.

The research also suggests that corals with different physiological traits may exhibit varying levels of microplastic accumulation. For example, large-polyp corals like Lobophyllia sp. displayed significant variation in the types of polymers found in their tissues and skeletons, suggesting that species-specific factors play a role in microplastic accumulation. These differences highlight the complexity of microplastic pollution in coral ecosystems and the need for further research into how different species and habitats are affected.

Addressing the Problem: The Need for Action

The findings of this study underscore the urgent need to address plastic pollution in marine environments, particularly in coral reefs. While corals may serve as sinks for microplastics, their role in this process comes at a cost to their health and the ecosystems they support. Reducing plastic waste at its source is essential to mitigate the impacts of microplastics on coral reefs and other marine organisms.

Efforts to curb plastic pollution must focus on both land-based and ocean-based sources, including improved waste management practices, the reduction of single-use plastics, and increased awareness of the environmental impacts of plastic waste. Additionally, further research is needed to explore the long-term effects of microplastic accumulation in corals and to develop strategies for protecting these critical ecosystems from the threats posed by plastic pollution.

Assistant Professor Suppakarn Jandang (right) and team are preparing to collect coral samples for microplastic analysis. Credit: Kyushu University/Isobe Lab

The discovery of microplastics in coral skeletons offers a potential solution to the missing plastic problem, providing evidence that a significant portion of oceanic microplastics may be sequestered within reef-building corals. This research highlights the need for a deeper understanding of the role that marine organisms play in the global plastic cycle and the ecological impacts of plastic pollution on coral reef ecosystems.

As plastic pollution continues to threaten marine environments, it is crucial to develop strategies for reducing plastic waste and protecting coral reefs from further degradation. By uncovering the hidden reservoirs of microplastics in coral skeletons, this study opens new avenues for research and action, offering hope for addressing one of the most pressing environmental challenges of our time.

Fig. 1. Location map of the study site in the Tham Phung coastal region on Si Chang Island in the upper Gulf of Thailand. The red point indicates the sampling location.

Fig. 2. (A) Schematic diagram depicting the potential incorporation of microplastics (MPs) in corals and (B) MP sample process employed for the mucus layer, tissue, and skeleton of the coral samples.

Fig. 3. Proportion of microplastics (MPs) obtained from the mucus, tissue, and skeleton parts for A) all the four coral species and B) each individually species.

Fig. 4. Characteristics of the microplastics (MPs) identified in the coral samples.

Fig. 5. Composition profiles of microplastics (MPs) in the four coral species with respect to (A) shape, (B) color, (C) size (μm), and (D) polymer type. Abbreviations: PP (polypropylene), EVA (ethylene-vinyl acetate), PE (polyethylene), EVOH (ethylene vinyl alcohol), PAM (polyacrylamide), PVAL (polyvinyl alcohol), PU (polyurethane), PET (polyethylene terephthalate), PES (polyester), POM (polyoxymethylene).

Fig. 6. Proportion of low- and high-density microplastics (MPs) with respect to (A) all the coral species and (B) individual species.

Fig. 7. Composition profiles of microplastics (MPs) in the four coral species considered in this study with respect to each component: (A) shape, (B) color, (C) size (μm), and (D) polymer type. Abbreviations: PP (polypropylene), EVA (ethylene-vinyl acetate), PE (polyethylene), EVOH (ethylene vinyl alcohol), PAM (polyacrylamide), PVAL (polyvinyl alcohol), PU (polyurethane), PET (polyethylene terephthalate), PES (polyester), POM (polyoxymethylene). M = Surface mucus layer, T = Tissue, and SK=Skeleton.