Soil environment have become a major sink of plastics due to agricultural activities, including through amendments of sewage/sludge and organic fertilizers, as well as through the use of plastic films and mulches. Degradation of plastic released not only nanoplastic and microplastic but also organic chemicals as well.
Earthworms are known as soil engineers due to consuming almost everything in their path, including microscopic plastic pollution.
Previously, study reported that earthworms may ingest the plastics, even breaking it apart into smaller pieces. But during the digestion or metabolism process, animals may harmed themselves either by microplastics or by other chemicals they carry.
Using biodegradable or plants derived materials is emerging strategy to tackle this mounting challenge, but there is limited information on whether earthworms will also ingest and decompose these materials. So, Lei Wang and his team wanted to compare the willingness of earthworms to uptake soil contaminated with microscopic pieces of bioplastic and petroleum-derived plastic, studying the in vitro digestion and excretion of the particles.
By placing earthworms in cubic boxes with different types of plastics contaminated soil, Lei Wang team found that worms preferred soils with bio-based polylactic acid (PLA) particles or petroleum-derived polyethylene terephthalate (PET) particles. However, earthworms showed significant avoidance behavior in soil contaminated with semi-synthetic plastics. This might be due to the taste difference caused by the particles’ different mechanical strength and flexibility, or that the harder PET particles can help earthworms grind food in the gizzard.
In another experiment, earthworms were incubated in soil mixed with microscopic PLA or PET particles to investigate the digestion ability. Earthworm casts analysis revealed that digestive systems can broke down PLA into much smaller fragments than observed with PET plastics. Earthworms also excreted PLA at lower rate 45 h than PET 9.3 h. The team finally concluded that earthworms could significantly promote the breakdown of bioplastics, such as PLA, in soil system. However, further studies are needed to understand how the slow excretion of PLA fragments affects the health of engineered invertebrates and whether the earthworms can be potential candidate to promote sustainable biodegradable plastic remediation in soil system?
Lei Wang, Yawen Peng, Yali Xu, Junjie Zhang, Chunguang Liu, Xuejiao Tang, Yuan Lu, and Hongwen Sun Environmental Science & Technology, DOI: 10.1021/acs.est.1c08066