The tiny particles of microplastics not only collect in the water and sediment of the oceans – they are also built into their tissues by corals, as an experiment has now revealed. According to this, reef-forming corals can absorb up to 84 particles per cubic centimeter and store them in their calcareous skeleton. Projected, the coral reefs worldwide could bind up to 20,000 tons of microplastic. However, the long-term consequences of this for the reefs have not yet been researched.
The problem has long been known, but for the time being there is hardly any improvement in sight: the world’s oceans are becoming more and more polluted by plastic waste and microplastics. The mostly tiny plastic particles have long been found even in the most remote places – from the deep sea to the beaches of uninhabited tropical islands to the water of the Arctic Sea. Much of this microplastic is deposited in the sediments of the ocean floor over time if it does not first enter the food chain.
Built into the tissue and skeleton
But especially in the tropics, microplastics apparently also accumulate in the coral reefs, as Jessica Reichert from the University of Giessen and her colleagues have now found out. For their study, they examined how reef-building corals thrive in the presence of microplastics. To do this, they raised four types of coral from the Indo-Pacific in seawater aquariums: antler corals, paw corals, small polyp hard corals and blue corals. Over a period of 18 months, these corals grew up in tanks with a high level of pollution of around 200 plastic particles per liter. The researchers chose black polyethylene particles around 170 micrometers in size as the microplastic.
At the end of the 18 months it became clear that the corals had stored the microplastics both in their tissue and in their calcareous skeleton. On average, they found between four and 37 particles per cubic centimeter of tissue – the densities in the skeleton were ten times higher than in living coral tissue. One coral in the experiment even absorbed up to 600 microplastic particles while doubling its body size from five to ten centimeters, as the team reports. But why does this installation take place? “Corals normally excrete such inedible particles,” says Reichert. “But sometimes something goes wrong with self-cleaning. The coral swallows itself, so to speak, and the particle stays in the body. “
Cleansing helper of the seas – but at what price?
These observations suggest that corals take up and store at least some of the microplastics in seawater. The reef-building animals thus contribute to the purification of the seawater. “Corals are the first organisms to be discovered as a living sink for microplastics in the sea,” says Reichert. If you extrapolate the amounts absorbed, she and her team estimate that corals in the reefs around the world could bind up to 20,000 tons of microplastic per year. That corresponds to about one percent of the microplastics in the reef water – for this one group of animals alone. “Our study shows coral reefs in a new light,” says Reichert. “They can not only help to maintain the ecological balance of the oceans, but also serve as long-term storage for microplastics.”
However, it is still largely unclear what effects the incorporation of the plastic has on the corals. In laboratory tests, the microplastics did not seem to affect the corals very much, but earlier studies have shown that some coral species grow worse when exposed to microplastics or develop coral bleaching or necrosis. “We do not know what long-term consequences the storage of microplastics will have for the corals,” emphasizes Reichert. “But it could affect the stability and resilience of the reefs. Microplastics would then be an additional threat to coral reefs around the world, which are already endangered by climate change. “
Source: Justus Liebig University Giessen; Technical article: Global Change Biology, doi: 10.1111 / gcb.15920