Coral reef ecosystems are intricate and diverse collections of species that interact with each other and the physical environment. Coral is a class of colonial animal that is related to hydroids, jellyfish, and sea anemones.
Stony corals, a type of coral characterized by their hard skeleton, are the bedrock of the reef. Stony coral colonies are composed of hundreds of thousands of individual living polyps. Polyps are capable of drawing dissolved calcium from seawater, and solidifying it into a hard mineral (calcium carbonate) structure that serves as their skeletal support. When you look at a coral colony, only the thin layer on its surface is live coral; the mass beneath is the calcium carbonate skeleton that may be decades old.
The slow growth of polyps and expansion of the hard skeletal structures build up the permanent coral reef structure over time.
Polyps of reef-building corals contain microscopic algae called zooxanthellae, which exist with the animal in a symbiotic relationship. The coral polyps (animals) provide the algae (plants) a home, and in exchange the algae provide the polyps with food they generate through photosynthesis. Because photosynthesis requires sunlight, most reef-building corals live in clear, shallow waters that are penetrated by sunlight. The algae also give a coral its color; coral polyps are actually transparent, so the color of the algae inside the polyps show through.
A surprising amount of plastic pollution in the ocean may wind up in a previously overlooked spot: the skeletons of living corals.
Up to about 20,000 metric tons of tiny fragments called microplastics may be stored in coral skeletons worldwide every year, says ecologist Jessica Reichert of Justus Liebig University Giessen in Germany. That corresponds to nearly 3 percent of the microplastics estimated to be in the shallow, tropical waters where corals thrive.
Corals have been observed eating or otherwise incorporating microplastics into their bodies (SNS: 3/18/15). But scientists don’t know how much of the debris reefs take up globally. So Reichert and colleagues exposed corals in the lab to microplastics to find out where the particles are stored inside corals and estimate how much is tucked away.
Corals consumed some of the trash, or grew their skeletons over particles. After 18 months, most of the debris inside corals was in their skeletons rather than tissues, the researchers report October 28 in Global Change Biology. After counting the number of trapped particles, the researchers estimate that between nearly 6 billion and 7 quadrillion microplastic particles may be permanently stored in corals worldwide annually.
It’s the first time that a living microplastic “sink,” or long-term storage site, has been quantified, Reichert says.
Scientists are learning how much microplastic is being introduced to the oceans. But researchers don’t know where it all ends up (SN: 6/6/19). Other known microplastic sinks, such as sea ice and seafloor sediments, need better quantification, and other sinks may not yet be known.
Reefs are typically found near coasts where polluted waterways can drain to the sea, placing corals in potential microplastic hot spots.
“We don’t know what consequences this [storage] might have for the coral organisms, [or for] reef stability and integrity,” Reichert says. It “might pose an additional threat to coral reefs worldwide.”
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