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Microplastic pollution a possible threat to Great Barrier Reef coral

  • The Great Barrier Reef off the coast of northeastern Australia is currently under threat from a floating layer of microplastic particles.
  • Plastic debris floats easily and spreads throughout the seas, gradually fragmenting into smaller and smaller particles, making them tiny enough to be ingested by ocean organisms, including seagulls, mussels and coral.
  • Microplastics have penetrated down the food chain into plankton – a major food source for coral and many other species throughout the world’s oceans.
Dipsastrea coral, the genus used in the plastics experiments. Coral tentacles expanded for feeding. Photo credit: Markus Mende.

The Great Barrier Reef off the coast of northeastern Australia – considered the largest living organism on earth – is currently under threat from a floating layer of microplastic particles, according to a new study from James Cook University in Queensland, Australia.

“Microplastic contamination is one threat to marine ecosystems that is difficult to manage because such tiny particles are invisible and, hence, can escape the awareness of the general public,” says Dr. Mia Hoogenboom, lead scientist on the study and Senior Lecturer at the School of Marine and Tropical Biology at James Cook University.

Microplastics are exactly what they sound like: “plastic fragments less than five millimeters in diameter.” They enter the world’s oceans from a variety of sometimes surprising sources including paint chips from ship hulls, ropes, buoys, debris from water treatment facilities and other on-land sources. Plastics of this size and smaller, are particularly dangerous because of their ability to absorb troubling contaminants, such as heavy metals and persistent organic pollutants. “Neither plastics nor these contaminants easily degrade in the environment, or during digestion by organisms, enabling them to bioaccumulate in the food chain,” says the study.

Plastic debris floats easily and spreads throughout the seas, gradually fragmenting into smaller and smaller particles, making them tiny enough to be ingested by ocean organisms, including seagulls, mussels, coral and plankton. “Experimental feeding trials revealed that corals mistake microplastics for prey,” writes Dr. Hoogenboom.

Dipsastrea coral, the circled areas indicate digestive filaments that can be extended outside of the coral tissue. Photo credit: Markus Mende.

In their study to determine if microplastics were present on the Great Barrier Reef (GBR), and whether corals were consuming them, Dr. Hoogenboom and three other scientists sampled waters at various reef sites. “We found low quantities of microplastics in the waters around Orpheus and Pelorus Islands on the central GBR,” Dr. Hoogenboom notes. “A previous study done by [Julia] Reisser… found microplastics in several sites on the northern GBR. This would suggest that microplastic pollution is not uniform throughout the reef, but has varying degrees of intensity in different areas depending on pollution sources.”

Dr. Hoogenboom admits that it will take “an intensive sampling effort to work out how much plastic is present on the Great Barrier Reef.” The small size of microplastics made it difficult for the team to fully assess just how much are there.

“Although microplastics were only present in GBR waters in relatively low concentrations, our estimates are likely to underestimate actual plastic concentrations because we could not have detected particles smaller than [.3 millimeters] in diameter, and we only tested the subset of microscopic particles from the plankton tows that we suspected to be plastic based on visual estimation,” the study notes.

After conducting ‘plankton tows’ to sweep for the presence of microplastics, the team conducted laboratory feeding trials. The stony coral Dipsastrea, a species commonly found on the Great Barrier Reef, was exposed to concentrations of microplastics over a period of 48 hours to determine if any was ingested. The team dissected the exposed coral and discovered that 20 percent were eating the plastic, with “ingested microplastics found wrapped in mesenterial tissue within the coral gut cavity.” Mesenterial tissue is the glandular tissue found within the polyps of the corals.

Coral reef. Photo taken by Dr. Mia Hoogenboom.

Additional evidence that corals were ingesting microplastics was observed through increased levels of mucus secretion. Corals produce mucus to protect themselves in response to stress and as a way to trap food. In Hoogenboom’s study, the team found that when microplastics were present, more mucus was produced. “We observed strings of mucus on the corals that were exposed to plastics and we’ve made similar observations in another follow up study,” she says.

The effects plastic microparticles are having on the corals was not determined, but evidence suggests it could hamper food digestion. “There are a few mechanisms by which plastic ingestion could affect coral health: blockage of normal digestion, prevention of normal feeding activities, mucus excretion, and toxicological effects, and we are doing follow up experiments to test these,” states Dr. Hoogenboom.

Of additional concern to the researchers is the finding that the plastics were wrapped deep in the mesenterial tissue of the coral polyps. “This observation raises the potential for plastic ingestion to impede coral digestion of natural food sources because the mesenterial tissues are the primary tissues responsible for digestion,” the researchers say.

Microplastics obstruction of coral digestion is a huge concern to marine experts, but it isn’t the only one. “As microplastic debris occupies the same size range as sand grains and planktonic organisms, it is available to a wide range of invertebrates near the base of the food chain,” write the researchers. “Ingestion of microplastics by amphipods, copepods and zooplankton is a potential concern for coral reef health since these planktonic organisms are the prey of corals.”

Coral reef. Photo taken by Dr. Mia Hoogenboom.

The study notes that microplastics have penetrated down the food chain into plankton – a major food source for coral and many other species throughout the world’s oceans. “Plankton can ingest microplastics and corals can possibly be exposed to plastics by feeding on contaminated plankton,” says Dr. Hoogenboom.

“In addition to blocking the digestive tract and preventing normal feeding, microplastic ingestion can damage the cells and tissues of organisms such as blue mussels, fish and crustaceans,” the study points out, referencing other microplastic studies.

“In my view microplastic pollution is a threat to the health of the GBR because plastic contamination can only increase through time, and because all organisms in the food web are potentially at risk,” says Dr. Hoogenboom.

The study urges more research to fully assess the spread of microplastics on the Great Barrier Reef and in other waters, and to determine the effects of microplastic ingestion on marine species. The researchers conclude: “Corals are the foundation species, and create much of the structural complexity of reefs, that in turn, provide habitat for thousands of species… these findings highlight the importance of the need for further investigation of whether and how microplastic contamination influences the physiology, growth and survival of marine organisms.”

Coral reef. Photo taken by Dr. Mia Hoogenboom.

Citations:

Hall, N. M., Berry, K. L. E., Rintoul, L., & Hoogenboom, M. O. (2015). Microplastic ingestion by scleractinian corals. Marine Biology, 162(3), 725-732.

Reisser, J., Shaw, J., Wilcox, C., Hardesty, B. D., Proietti, M., Thums, M., & Pattiaratchi, C. (2013). Marine plastic pollution in waters around Australia: characteristics, concentrations, and pathways. PLOS One DOI: 10.1371/journal.pone.0080466