Poly(L-lactide) mineralisation under environmental conditions is enhanced in earthworm guts

Microplastic accumulates in various habitats, posing a potential environmental threat. Biodegradable polymers like poly(L-lactide) (PLLA) is a possible eco-friendly alternative to conventional, non-biodegradable plastics. However, biodegradation of PLLA in soil is strongly limited, but is potentially enhanced by soil-dwelling organisms. We recently showed that PLLA exposure positively affected reproduction in the earthworm Eisenia fetida, and increased gut lactate concentrations, indicating the hypothesis of earthworm-enhanced PLLA biodegradation. Thus, 13C-labelled PLLA was used for a 13CO2-tracing approach to monitor PLLA mineralisation in presence and absence of the earthworm E. fetida. Mineralisation of 0.2% of initial PLLA was attributed to the activity of earthworms after two weeks of exposure. Extrapolation assuming zero-order kinetics and limitation of microbial growth suggested a substantially shorter half-life of PLLA in earthworm-amended soils. This finding provides strong evidence that conditions inside the earthworm gut are beneficial for PLLA degradation and provide a basis for the development of mitigation strategies for PLLA microplastic pollution.