Coral meals come with side order of microplastics

Under a black light, fluorescent green microplastics are seen in the water during a small demonstration experiment. In the 2018 experiment described in the paper, cauliflower coral (above) ingested microplastics when prey was also present in the water, but avoided eating microplastics when no prey was there. (Credit: Dennis Wise/U. Washington)

Researchers have found that some corals are more likely to eat microplastics when they consume other food, but they don’t have a taste microplastics alone.

Two coral species researchers tested responded differently to the synthetic material, suggesting variations in how corals adapt to life with microplastics, according to the new study in Scientific Reports.

Tiny microplastic particles are about as common in the ocean today as plastic is in our daily lives. But scientists aren’t exactly sure how the ubiquitous material affects ocean life. Synthetic clothing, containers, bottles, plastic bags, and cosmetics all degrade and release microplastics into the environment where corals and other marine organisms eat them.

“The more plastic we use, the more microplastics there are, and the more corals are going to be exposed,” says lead author Jeremy Axworthy, a doctoral student in the School of Aquatic and Fishery Sciences at the University of Washington.

“Our study found that some corals probably won’t eat microplastics and will keep going about their daily business. But some might—and if they happen to be sensitive to warmer ocean temperatures or other stressors, this is just another compounding factor to be worried about.”

A researcher uses a tool to take a small coral out of a tank filled with them
Jeremy Axworthy removes a young coral from a tank at the team’s lab. The two coral species shown here are Pocillopora damicornis (left), known as cauliflower coral, and Montipora capitatae (right), or rice coral. Cauliflower coral was more likely to ingest microplastics during the team’s experiments. (Credit: Dennis Wise/U. Washington)

Coral diet shifts

Corals are tiny animals rooted to the reef or rocks on the ocean floor. They use tentacle-like arms to sweep food into their mouths. Many rely on algae for energy, but most also consume drifting animals to survive.

The new study is the first to examine whether corals eat microplastics when exposed to warmer water, expected to accelerate with climate change. Rising ocean temperatures can prove deadly for coral: warm water stresses them, causing corals to lose their symbiotic algae partner that undergoes photosynthesis and provides energy for them to survive. When this happens, coral bleaching and eventual death can occur.

But some corals have shifted their diets to feed on tiny marine organisms called zooplankton to adapt to bleaching. This provides an alternate energy source. Since they munch on these small animals—often the same size as microplastics—the research team wondered whether they also ate plastic fragments.

A coral sits in a small tub of water with a black light turning the water purple
In this demonstration experiment, a black light illuminates microplastics, shown in fluorescent green, in a small tank with one coral species, Pocillopora damicornis, commonly called cauliflower coral. (Credit: Dennis Wise/U. Washington)

The new study shows they do eat microplastics when they switch to a zooplankton diet, adding one more stressor in a changing ocean environment.

“Microplastics are not as simple as a life-or-death threat for corals—it’s not that black or white,” says senior author Jacqueline Padilla-Gamiño, assistant professor at the School of Aquatic and Fishery Sciences.

“It’s about total energy lost. If corals constantly are dealing with microplastics, it might not kill them, but there will be less energy for them to grow and to reproduce.”

Microplastic ‘tastiness’

The researchers collected two species of common corals off the east coast of Oahu, Hawaii, and exposed half of each species to warmer water for several weeks to induce stress and bleaching. Then they ran four different feeding experiments on both bleached and non-bleached corals: only microplastics; only a type of zooplankton; microplastics and zooplankton; or nothing.

After dissecting the coral polyps, researchers found that corals stressed by warmer temperatures actually ate much less than their counterparts in normal seawater. The researchers didn’t expect this and say stress from high water temperatures may be the cause.

However, one of the two species, known for its voracious eating habits in the wild, consumed microplastics only while also eating zooplankton. Neither coral species ate microplastics alone.

The researchers don’t know why one species of coral readily ate microplastics in the presence of other food, but avoided microplastics when they were the only thing on the menu.

They suspect that this species of coral can read certain chemical or physical cues from the plastics and the prey, but might not be able to distinguish between the two when both are present.

It’s also possible the plastic researchers used in this experiment is less desirable to corals, and that plastics with a different chemical makeup could, in fact, be tasty to corals. The researchers plan to test the “tastiness” of other types of microplastics, such as synthetic fibers from clothing.

Ultimately, some coral species likely face greater risks from exposure to microplastics than others, the study shows. The researchers will look next at the effects on the physiology of corals exposed over a longer period to microplastics.

“Knowing that will provide a lot more context to this work,” Axworthy says. “We need to know the full physiological impacts of chronic exposure to microplastics on corals, especially at increased temperatures, to understand how serious the problem is.”

In the meantime, the problem of microplastics isn’t going away. A 2014 estimate found between 15 and 51 trillion microplastic particles in the oceans. Scientists expect plastic waste entering the oceans will increase tenfold between 2010 and 2025.

“It’s important when talking about waste management to think big picture—what are we putting in the oceans?” Padilla-Gamiño says. “We don’t know where plastic goes, where it stays, who grabs it, and what are the mechanisms by which we get it back. We are just at the tip of understanding these implications.”

The National Science Foundation funded the work.

Source: University of Washington