Corals dine on microplastics

Plastic trash is washing off of land and into the seas. And that pollution may be harming some of the ocean’s most important habitats: coral reefs. That’s the conclusion of a new Australian study.

Coral reefs are the most biologically diverse habitats in the ocean. Their nooks and crannies provide shelter for thousands of species of animals, both big and small. That huge variety of reef organisms also provides food for a wide range of other critters. If the corals die, though, lots of those other species will have trouble surviving. The new study raises concerns about the survival of some coral species — and the complex ecosystems that depend on them.

The animals that build reefs are called polyps. Coral polyps are small. Their soft bodies also lack a hard outer covering to protect them from potential predators. So the polyps make their own protective home out of calcium carbonate. Coral polyps continually add to these homes. And over time, communities of millions of polyps craft the large, rocky apartment complexes that we know as reefs.

Polyps hide in their homes by day. At night, they extend their arm-like appendages out to snatch small snacks, usually plankton, from the water. Those snacks are truly tiny — a mere 400 micrometers (0.016 inch) in diameter or less, notes Mia Hoogenboom. She is a marine biologist at James Cook University in Townsville, Australia.

Unfortunately, she points out, scientists are finding more and more bits of plastics in the ocean ecosystem. Those microplastic pieces are less than 5 millimeters (0.2 inch) in size. That makes many just the right size for corals to gobble up.

Hoogenboom’s team lives and works near the Great Barrier Reef, the world’s largest coral system. It stretches across more than 2,000 kilometers (1,240 miles) of Australia’s northeast coast. It’s also home to the greatest diversity of species in the world. But that biodiversity could be at risk from plastics. Hoogenboom and her co-workers wanted to find out how plastics might be affecting those reef corals.

Food or plastic?

Corals get some energy from single-celled algae that live amidst the corals’ tissues. These algae produce their energy through photosynthesis. But corals also must eat plankton and other foods to obtain certain vital nutrients important for growth and reproduction. So Hoogenboom’s team started its investigation by probing whether corals might be mistaking plastics for food. This is a concern because many other marine animals make that error.

The team brought pieces of one type of coral into their lab. The species is known as brain coral because its round shape and fold-like pattern make it resemble the human brain. Then the researchers shredded a blue ice cream tub made of polypropylene (PAAH-lee-PRO-pih-leen). This is one of the plastics most commonly found in the ocean. The scientists added the plastic microbits to the water in which the corals were being kept.

Two days later, the researchers examined the polyps’ stomachs. One out of every five of the coral animals had eaten plastic. What’s more, pieces of the blue plastic had gotten stuck deep in the animals’ stomachs. That suggests the polyps cannot get rid of the plastic once it is swallowed, says Hoogenboom.

Next, the researchers added a precise amount of microplastics to the corals’ water. Twelve hours later, the scientists measured how much had disappeared. This showed the polyps had eaten microplastic bits at the same rate they normally eat plankton.

None of this matters if microplastics are not polluting the waters of the Great Barrier Reef. So the final step by Hoogenboom’s group was to sample water at various reef sites. And at each one, they found bits of plastic that had broken off of larger pieces of packaging or items used in fishing. So corals definitely are at risk of eating plastic, the researchers conclude.

The Australian team published its findings online February 4 in Marine Biology.

It’s an interesting study, says Stephanie Wright. A marine biologist at the University of Exeter in England, she was not involved with the study. The new study did not give corals a choice of foods, she points out. They could only eat microplastics. Future steps should look at how easily corals ignore plastic when true food is around. But, she notes, the study does add to a growing body of knowledge about the risks that microplastics in the sea may pose.

Power Words

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algae     Single-celled organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.

biodiversity  (short for biological diversity) The number and variety of species found within a localized geographic region.

calcium carbonate  The main chemical compound in limestone, a rock made from the tiny shells of ancient marine organisms. Its formula is CaCO₃ (meaning it contains one calcium atom, one carbon atom and three oxygen atoms).

coral  Marine animals that often produce a hard and stony exoskeleton and tend to live on the exoskeletons of dead corals, called reefs.

ecosystem  A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra.

habitat  The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.

marine   Having to do with the ocean world or environment.

marine biologist   A scientist who studies creatures that live in ocean water, from bacteria and shellfish to kelp and whales.

microplastic   A small piece of plastic, 5 millimeters (0.2 inch) or smaller in size. Microplastics may have been produced at that small size, or their size may be the result of the breakdown of water bottles, plastic bags or other things that started out larger.

photosynthesis  (verb: photosynthesize) The process by which green plants and some other organisms use sunlight to produce foods from carbon dioxide and water.

plankton  A small organism that drifts or floats in the sea. Depending on the species, plankton range from microscopic sizes to organisms about the size of a flea. Some are tiny animals. Others are plantlike organisms. Although individual plankton are very small, they form massive colonies, numbering in the billions. The largest animal in the world, the blue whale, lives on plankton.

plastic  Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.

pollutant   A substance that taints something — such as the air, water, our bodies or products. Some pollutants are chemicals, such as pesticides. Others may be radiation, including excess heat or light. Even weeds and other invasive species can be considered a type of biological pollution.

polypropylene  The second most common plastic in the world. It is tough and durable. Polypropylene is used in packaging, clothing and furniture (such as plastic chairs).

predator  A creature that preys on other animals for most or all of its food.        

reef  A ridge of rock, coral or sand. It rises up from the seafloor and may come to just above or just under the water’s surface.

species  A group of similar organisms capable of producing offspring that can survive and reproduce.

tissue  Any of the distinct types of material, comprised of cells, which make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues. And brain tissue will be very different from bone or heart tissue.

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