Earthworms are a welcome sight for gardeners and farmers because the wiggling invertebrates recycle soil nutrients, making them more available to plants. When worms burrow, they consume almost everything in their path, including microscopic plastic pollution. Now researchers who report in ACS’ Environmental science and technology observed that earthworms actually prefer soil containing certain types of microplastics but digest polymers differently, which the team says could impact animal health and the ecosystem.
The ground is increasingly contaminated with plastic fragments – especially microplastics less than 5mm wide – that have broken off from larger plastic waste or have been directly released from products as small particles. Previously, researchers have shown that earthworms ingest these synthetic particles, breaking them into even smaller pieces. But during the digestion process, animals could potentially be harmed by the microplastics themselves or by the toxic substances they carry. Currently, companies are producing alternatives to petroleum-based plastics that are plant-derived, biodegradable, or both. Just like traditional plastics, these “bioplastics” can also fragment into microscopic particles, but there is little information on whether earthworms will ingest and break down these materials as well. So, Lei Wang and his colleagues wanted to compare earthworms’ willingness to consume soil mixed with microscopic pieces of bioplastic and petroleum-derived plastic, studying the in vitro digestion and excretion of particles.
By placing earthworms in chambers containing different types of plastics at certain locations in the soil, the researchers found that the worms preferred soils containing bio-based polylactic acid (PLA) particles or polyethylene terephthalate (PET) particles. derived from petroleum, but actively avoided certain semi-synthetic plastics. When lactic acid and terephthalic acid, sour-smelling monomers that make up PLA and PET, respectively, were introduced to the soil, worms were also attracted, suggesting that animals were attracted by odors as potential food cues. In another experiment, researchers put earthworms in soil mixed with microscopic particles of PLA or PET. Analysis of the creatures’ excretions showed that their digestive systems broke PLA down into much smaller fragments than seen with PET plastics. The animals also excreted PLA much more slowly. The researchers say the results show that earthworms can promote the breakdown of bioplastics, such as PLA, in soil. They add that further studies are needed to determine how the slow excretion of PLA fragments affects the health of these animals and whether worms are an option for removing degradable plastics from the environment.
The authors acknowledge funding from a key national research and development project of China, the National Natural Science Foundation of China, the Tianjin Municipal Bureau of Science and Technology, the Ministry of Education (China), and the Tianjin research innovation project for postgraduate students.
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