Coral reefs are among the richest, but at the same time most endangered, habitats on Earth. Reefs are composed of thousands of small sac-like organisms called polyps that secrete an exterior skeleton with a specific shape and color and group into coral colonies, also with different shapes and colors (Figure 1). When many of these coral colonies come together, a coral reef is formed. Home for more than 25% of all marine species and with more than 850 million people depending on them for livelihoods and tourism, the health of coral reefs is closely monitored. In the last decades, coral reefs have been highly challenged due to climate change, primarily by the rising seawater temperature and ocean acidification. Oceans are becoming more acidic because they absorb the excessive amounts of carbon dioxide that humans release into the atmosphere. These challenges cause corals to expel the microscopic algae living inside their cells (Figure 1), losing their vibrant colors and turning white—a process called coral bleaching. Coral bleaching can ultimately kill the corals. Recent forecasts indicate that more than 75% of coral reefs will be severely bleached in the next 50 years due to temperature increase alone. Coral health may be worsened if corals face additional challenges [1], including plastic pollution. Because plastics continue to accumulate in the oceans, we must look with more attention into the harm they are causing to corals.

Big Little Enemies

Since the development of the first synthetic plastic in 1907, plastic production has increased by 200-fold and there is now ~275 million tons of plastic waste spread throughout the world. Up to 70% of the litter in many areas of the ocean is plastic, and up to 12.7 million tons of plastics are added to the oceans each year (Figure 2). By 2025, at least 155 million tons of plastics are expected to accumulate in the oceans [2]. Because it takes plastics 500–1,000 years to degrade and they never completely break down, all plastics created since 1907 still exist, even if they have degraded to smaller plastic particles, such as microplastics (<5 mm) or nanoplastics (<0.1 mm). The actions of waves and sunlight break down larger pieces of plastic in the oceans into microplastics and nanoplastics, but other tiny plastic particles, originating from clothing, car tires, and personal care products, may also end up in oceans via wastewater, road runoff, winds, and waterways.

In 2014, the oceans were estimated to contain 15–51 trillion microplastic particles [1], which can cause both physical and chemical harm to corals, and many other species, big and small. The increased abrasion (tissue scraping) by the plastic particles can wound the corals. These injuries may become infected by fungi or bacteria, killing some cells or even the entire coral. Each coral species reacts differently to microplastics. Scientists found that a mushroom coral removed microplastics from the water by capturing them from water currents and sticking them to its wrinkled skeleton [3]. Another study revealed that a species of finger coral tried to clean itself of microplastics [4]. In both cases, dealing with the microplastics requires increased energy expenditure that negatively affects corals. Finger corals and table corals also grow over the microplastics, incorporating them into their tissues [4], which occurs when the tiny particles are wrapped into the cell membrane. Nanoplastics have also been found in the brains, livers, and circulatory systems of other animals [5], clearly indicating that these tiny plastic particles can cross cell membranes and therefore may be a major risk for corals.

Tiny Particles, Big Impact

Microplastics are likely to be eaten by corals because they have a similar shape and size to microscopic organisms called zooplankton that live in the water. Under normal conditions, reef corals get most of their food from their partnership with the microscopic algae that they harbor in their cells (Figure 1). However, as coral bleaching becomes more severe and the corals lose their algae partners, some corals start ingesting more zooplankton, which they filter from the seawater. The risk of corals ingesting microplastics increases with increased feeding on zooplankton [1]. For instance, ~112 plastic particles were found in each polyp of wild northern star corals, 73.4% of which were fibers, coming mainly from clothes. In a lab experiment, northern star corals were fed with their favorite meal in the wild (brine shrimp eggs) mixed with similar-sized microplastic beads. The corals chose the microplastics, which inhibited further feeding on brine shrimp eggs [2]. Thus, one potential consequence of ingesting microplastics is a false feeling of “fullness” and subsequent reduction of real food intake, leading to low coral growth and decreased survival. Ingestion of microplastics may also cause blockages throughout the gut cavity of corals, as well as internal damage. Although several experiments have shown that corals clean off the microplastics within 6 h after intake, this requires spending a high amount of energy that can no longer be used for growth and survival.

Microplastics can also harm corals in other ways. Micro- and nanoplastics can contain large amounts of toxic chemicals. Many of these chemicals are added during the production of plastics, but micro- and nanoplastics may also adsorb toxic pollutants from the ocean water, due to their porous and rough surfaces [5]. Thus, micro- and nanoplastics surfing the oceans can increase the uptake of pollutants by corals, and these toxic pollutants can then be transferred to other organisms along the food web. In addition, toxic chemicals may leak from the tiny plastic particles when they are inside corals, causing toxins to accumulate in the corals, posing an additional risk to their growth, reproduction, and survival. An experiment showed that hood coral polyps pulled in after being exposed to chemicals leaked from plastics [6]. This action reduces the uptake of pollutants by the coral, but it also reduces food intake, photosynthetic rate, and oxygen exchange with the surrounding seawater. Thus, coral exposure to micro- and nanoplastics can also mean exposure to dangerous pollutants.

Wolves in Sheep’s Clothing

Corals catch zooplankton by relying on the signals these tiny organisms emit into the seawater. So, corals are more likely to ingest micro- and nanoplastics when these particles are covered by biological materials, such as algae, fungi, and bacteria [3]. This coating of organisms, which is called biofouling, is highly nutritional for corals that can digest it, and biofouling increases the time that micro- and nanoplastics stay inside the coral. Plastic particles not covered by biofouling are more easily recognized as alien particles and more readily spat off. However, biofouling may also contain harmful microorganisms and therefore plastic particles may serve as disease carriers in the ocean. Although these studies are still in their early stages, when lab-raised corals were fed microplastic beads covered with disease-causing bacteria, the particles were spat off within 48 h, but the corals still became ill and died.

Keeping the Oceans Healthy Without Plastics

Overall, the ingestion of micro- and nanoplastics affects corals in many ways (Figure 3): it reduces feeding on nutritious prey and increases the energy spent on spitting out or cleaning off the plastic particles; it increases corals’ exposure to pollutants stuck to the plastic particles or released in nearby seawater; and it increases corals’ exposure to harmful microorganisms. All these negative effects are not restricted to the corals—they affect all living creatures in lakes, rivers, and oceans. Moreover, if a fish eats a coral contaminated with plastic particles it also becomes contaminated; and this will be repeated all the way up in the food web, eventually reaching humans.

Plastic, as well as the pollutants and harmful microorganisms they may carry, would not be found in the oceans if plastic waste was not there to begin with. Reducing plastic production and plastic waste are therefore critical for improving the health and survival of coral reefs and will also be important for improving our own health. So how can you help? The easiest and most direct way is by reducing your use of single-use plastics: refuse straws, bags, and takeout cutlery and containers, if these items are made of plastic. If and when you do use plastics, please recycle them. If you are looking for a more hands-on action, participate in or even organize a cleanup at your local beach or waterway: you will be amazed by the number of plastic particles you will be preventing from reaching the ocean.

Glossary

Polyp: ↑ Each of the tiny animals that make up coral reefs. Polyps can live individually (like mushroom corals) or in large colonies that create an entire reef structure.

Ocean Acidification: ↑ The ocean is becoming more acidic (like lemon juice) as its water absorbs carbon dioxide from the atmosphere. Carbon dioxide is produced by factories, cars, or used fuels like coal or oil. This change in the properties of ocean water can be harmful for plants and animals.

Coral Bleaching: ↑ Many corals live with algae, which give them their vibrant color. However, the coral tissue is white. When corals lose their algae after a stress, only the white tissue remains, and this is called coral bleaching.

Microplastics: ↑ Microplastics are tiny bits of plastic of <5 mm (smaller than an ant). Often, they are too small to be seen. They pollute water in many ways.

Nanoplastics: ↑ Nanoplastics are bits of plastic even smaller than microplastics. A billion nanoplastics can fit on the head of a pin.

Zooplankton: ↑ Tiny, free-floating animals that can be found in the oceans, seas, rivers, and ponds.

Adsorb: ↑ The sticking of molecules from a gas, liquid, or dissolved solid to a surface, such as the surface of a plastic particle.

Biofouling: ↑ Aquatic organisms can build up on surfaces and structures exposed to seawater. This is biofouling, most often composed of tiny algae and bacteria.

Conflict of Interest

JM was employed by company Scite – Science Crunchers.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We would like to sincerely thank the Prince Albert II foundation for promoting this work, and the young readers of the International School of Monaco for improving the first version of this manuscript. We also thank the young reviewers for improving the submitted manuscript.