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A common plant-based plastic marketed as compostable has been found not to degrade when it ends up in the ocean, remaining intact for more than a year.

Compostable “bioplastics” have been touted as a solution to plastic waste, which enters the ocean at the pace of . A leading alternative to traditional oil-based plastics is polylactic acid (PLA), a plant-derived material used in clothing, single-use cups and containers. PLA can be composted in industrial facilities, but researchers weren’t sure if the material would decompose naturally once in the ocean.

To find out, at the University of California, San Diego, and her colleagues compared how several materials aged both at the ocean’s surface and suspended 10 metres below in a fine mesh cage. They used palm-sized swatches of textiles made from oil-based plastics, bioplastics like PLA and natural materials like cotton. Each week, they checked and photographed the samples, which were next to a pier in La Jolla, California, and took a small portion of each swatch to assess visually and chemically.

After 14 months at sea, their PLA sample remained just as intact as oil-based plastics like polypropylene and polyethylene terephthalate (PET). In contrast, natural materials like cotton-based fibres completely disintegrated and decomposed in about a month.

The team also mirrored the experiment in the lab with seawater and bacteria to mimic the natural environment. Neither the PLA nor the oil-based fabrics put off any carbon dioxide gas, confirming the plant-based plastics weren’t chemically degrading either. In the sea and in the lab, “they didn’t degrade at all”, says Royer.

The take-home message, says at the University of Twente in the Netherlands, is that “biodegradation always needs to consider the end-of-life scenario”.

That is why, when it comes to compostable plastics, descriptors like “biodegradable” can be misleading. Just because a bioplastic can be composted in a high-temperature, high-pressure facility doesn’t mean it will break down in a cold, wet environment.

“Consumers in general are not aware of what they are buying,” says Royer. She recommends avoiding single-use plastics, opting for reusable containers and purchasing less clothing.