Coral reefs are home to some of our planet’s most biodiverse ocean regions, sustaining over 25 percent of marine life. And yet, some coral will choose to eat bits of plastic waste over brine shrimp eggs, even if it carries fatal bacteria.
Plastic degradation can take 500-1,000 years, which mean nearly all plastic created still exists on Earth, often weathered down to smaller particles less than 5 millimeters long. Although lead author Randi Rotjan of Boston University previously discovered microplastics on the surface of seagrasses, her team's current findings took her by surprise.
"I never expected to find so many microplastics per polyp – to me, that was a big surprise," said Rotjan to IFLScience. "To our knowledge, this is the first report of microplastics accumulating in wild corals, though several other studies have shown coral consumption of microplastics in the laboratory."
The team collected colonies of northern star coral off the coast of Rhode Island and exposed them to either microplastics, brine shrimp eggs, both, or microbeads coated in bacteria. After feeding the coral, the team took the marine creatures out, cut them open, and inspected their stomach contents. All of the coral had microplastics in their guts, with an average of 112 pieces per polyp. Plastics in the shape of fibers were the most common at over 73 percent of the total, followed by round particles at 15 percent.
Overall, the coral ate almost double the number of microbeads as brine shrimp eggs, providing them with little-to-no nutritive value. More research needs to be done on the effects, but the team suggests that there is a high, repetitive energetic cost to eating microplastics.
In the next phase of the study published in the journal Proceedings of the Royal Society B: Biological Sciences, the team coated microplastics in bacteria and released them into the corals' chambers. After two days, the polyps spewed the plastic back out but the bacteria remained inside them.
"Our results demonstrate that this microbial hitchhiking is possible, even if the bacteria are pathogenic, and that's scary," said co-author Koty Sharp of Roger Williams University.
"Nearly every surface in the ocean is coated with microbes, but bacterial communities on microplastics are different. In fact, they are so different that researchers have assigned a new word for them – the "plastisphere," added Sharp. "We are barely getting to know the plastisphere, and we know almost nothing about the fate of the plastisphere in the environment."
Many questions still remain. "Will corals starve to death with stomachs full of plastic? Our study found corals alive despite high numbers of fibers, but how much is too much? Where is the tipping point? How does plastic ingestion impact tropical corals? Does plastic ingestion make them more vulnerable to disease? More vulnerable to bleaching? These are the next questions, and we need answers," said Rotjan.
In 2015, the US banned microbeads, but that doesn’t mean they have vanished from Earth. Plastics linger for centuries and northern star corals are just one of many species that continue to feel the impact. Plastics have been found everywhere, from the air above the Pyrenees Mountains and in the dark depths of the ocean to remote islands and in the food on our plates. Models estimate that somewhere in the realm of 4.8 to 12.7 million tons of plastics end up in Earth's oceans every year, often swept away via road runoff, wastewater, winds, and waterways.
"I hope people are inspired to make the necessary changes to clean up our world. It won't be easy, but we have to do it. I think we all, collectively, suffer from the delusion that when we do something in a human setting – be it use plastics or release CO2 or any other action – it only affects humans," said Rotjan. "But in reality, our actions are impacting all of the citizens of Earth – all organisms and ecosystems."
"Plastics are co-occuring with global climate change, human population growth, and increased land and ocean exploitation. I became a biologist because I love the beauty and complexity of nature; I never intended to study humans, or human nature. But it turns out that in today’s world, it's impossible to disentangle the two. In order to study nature again, we can no longer ignore ourselves."