Analyzing This: Corals stash microplastics in their skeletons

Analyzing This: Corals stash microplastics in their skeletons

Plastic pollution is piling up all over Earth and entering the oceans too. That includes tiny bits called microplastics. These are pieces that are about the size of a grain of rice or smaller. Now, researchers have found that corals stash a surprising amount of those tiny plastic shreds in their tissues and skeletons.

Millions of metric tons of plastic may find its way into the ocean every year. But the amount of plastic floating in seawater seems too low compared with what gets into the oceans. Researchers don’t yet know where all that missing plastic goes. But one destination may be corals. Researchers have actually seen some corals munching on plastics. But it isn’t clear if that’s the only way plastics get into corals and whether corals hang onto plastic for a long time.

Jessica Reichert is an ecologist at Justus Liebig University Giessen in Germany. In the lab, Reichert’s team grew four species of coral that build reefs. For 18 months, the researchers exposed these creatures to pieces of black polyethylene. This is one of the most common plastics in the ocean. It’s used in bags and bottles. The black color made the plastic easy to see when corals gobbled it up or grew their skeletons over it.

At the end of the experiment, the researchers tallied up how much plastic the corals took in. Most plastic debris inside corals was in their skeletons rather than tissues. The researchers shared these results October 28 in Global Change Biology.

The researchers then estimated how much plastic corals might be accumulating worldwide. Their calculations suggest that corals may cache between almost 6 billion and 7 quadrillion microplastic pieces each year. That could be up to 20 million kilograms (44 million pounds) of plastic, Reichert told Science News. That’s the weight of 10,000 cars. It’s not clear yet how those plastic fragments impact reefs. Corals already face dangers due to warming waters and ocean acidification. Microplastics “might pose an additional threat to coral reefs worldwide,” Reichert said.

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A plastic tally

These graphs show how much plastic corals took into their tissue (right graph) or their skeletons (left graph). These “box and whisker plots” show how data is distributed over a range. Each orange or aqua circle represents a data point collected from an individual coral. The thick line in the middle of the box is the middle value (the median) if all the observations were lined up smallest to largest. The box contains the half of the data that falls in the middle when the values are lined up. So the highest quarter of values and lowest quarter of values fall outside the box. The “whiskers” represent the values that most of the data is likely to lie between. Data points outside of the whiskers are called “outliers.”

two graphs showing how much microplastics were stored in the skeletons of four different corals

Data Dive:

  1. What are the highest and lowest values for how much plastic each of the four corals took into their tissue? What are the median values (marked by the dark black lines)?
  2. What are the highest and lowest values for how much plastic each of the four corals held in their skeletons? What are the median values?
  3. How would you go about finding the average of values for either the tissue or skeleton for any given species? Can you do that from these graphs?
  4. For each species, roughly how much more plastic was in the skeletons than the tissue?
  5. In the two graphs, which values are outliers? How far are these values away from the median?
  6. What’s another way you could show this data?
  7. In this experiment, all the corals were exposed to the same type and amount of microplastic. How might the experiment setup be changed to more closely reflect what happens in the ocean?

Power Words

More About Power Words

acidification: A process that lowers the pH of a solution. When carbon dioxide dissolves in water, it triggers chemical reactions that create carbonic acid.

average: (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

biology: The study of living things. The scientists who study them are known as biologists.

coral: Marine animals that often produce a hard and stony exoskeleton and tend to live on reefs (the exoskeletons of dead ancestor corals).

data: Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.

debris: Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.

ecologist: A scientist who works in a branch of biology that deals with the relations of organisms to one another and to their physical surroundings.

median: (in mathematics) The value or quantity that lies at the midpoint of a group of numbers that had been listed in order from lowest to highest.

microplastic: A small piece of plastic, 5 millimeters (0.2 inch) or smaller in size. Microplastics may have been produced at that small size, or their size may be the result of the breakdown of water bottles, plastic bags or other things that started out larger.

outliers: Events or cases that fall outside some normal range. That makes them unusual and may make them seem unlikely or suspicious.

plastic: Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation. (adj.) A material that is able to adapt by changing shape or possibly even changing its function.

polyethylene: A plastic made from chemicals that have been refined (produced from) crude oil and/or natural gas. The most common plastic in the world, it is flexible and tough. It also can resist radiation.

quadrillion: A very big unit of measure equal to 1,000 trillion. It would be written with a 1 followed by 15 zeros.

reef: A ridge of rock, coral or sand. It rises up from the seafloor and may come to just above or just under the water’s surface.

sea: An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

species: A group of similar organisms capable of producing offspring that can survive and reproduce.


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