What happens when sunlight breaks down plastic in the ocean?

According to researchers from Boston’s Northeastern University, 4 to 10 million tons of plastic enter the oceans each year, most of which floats in subtropical currents. A research team led by Professor Aron Stubbins decided to check what happens when plastics start to break down into prime factors? It has been discovered that hundreds of chemicals are produced as they degrade under sunlight, which could have a negative impact on the ocean’s carbon cycle and chemistry, as well as potential consequences for human health.

Plastics are polymers made of long carbon chains, which make them solid on the one hand, and hydrophobic, i.e. insoluble in water, on the other. Polyethylene, polypropylene and expanded polystyrene constitute approximately 70% of plastics produced in the world, which translates into their dominant presence in the oceans. Polyethylene and polypropylene are also particularly common in subtropical gyres, where ocean currents change in such a way that floating materials accumulate in a sort of blob. Expanded polystyrene eventually absorbs water and sinks, so it does not usually enter coastal waters.

Although plastics can last for hundreds of years, that doesn’t mean they don’t degrade – especially when exposed to the intense ultraviolet rays from the sun hitting the subtropical oceans. Professor Stubbins and PhD student Lixin Zhu set out to find out what chemicals are produced by the photodegradation of polyethylene, polypropylene and expanded polystyrene. To do this, they collected microplastics from the ocean and cut other store-bought plastic containers, such as shampoo bottles and food containers, into 3-millimeter pieces. They sealed them in quartz jars half-filled with seawater collected from the South Atlantic.

The researchers then measured the amount of plastic and the chemical composition of dissolved organic carbon in the jars. Some of the samples were placed in the dark and some in the light. After two months, the researchers again measured the amount of plastic and the chemical composition of dissolved organic carbon in the jars, finding that those under the light had less plastic and more dissolved organic carbon. This indicates that sunlight has begun to dissolve plastics, creating new chemicals. Stubbins found that between 319 and 705 chemicals were formed from plastics exposed to light. Expanded polystyrene produced a greater variety of chemicals than polyethylene and polypropylene.

The resulting chemicals could change the composition of the marine environment, in particular the composition of the “microlayer” on the surface of the world’s oceans, which plays an important role in the exchange of materials – including aerosols, chemicals and trace gases – between the atmosphere and the ocean. Although many of the dissolved chemicals produced by plastic degradation are consumed by microorganisms, some are not consumed and may actually inhibit bacterial growth, which can alter the microbial ecology.

Investigating both potential effects is one of the next steps in the research. Additionally, scientists are examining some of the chemicals produced and the rates of chemical reactions that occur as plastics degrade. Meanwhile, polypropylene and polyethylene produced dissolved fatty acids resembling the parent material. However, these chemicals can be used as food by microorganisms in the ocean.

“Today we know that plastics are everywhere on Earth, and understanding how they move around the planet and how they break down is important if we want to understand how we and other organisms are exposed to them,” says Stubbins .


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