Experimental Exposure of Lumbricus terrestris to Microplastics

The effects of microplastic exposure in aquatic organisms have been widely reported. Nonetheless, there is limited evidence of the effects of exposure in soil systems. Thus, the objective of this study was to evaluate the effects of microplastic exposure using as a bioindicator the species Lumbricus terrestris, exposed to different concentrations of microplastic (2.5%; 5%, and 7% w/w). Avoidance bioassays were carried out for 48 h in soil with and without microplastic; the gastrointestinal tract—crop/gizzard, foregut, and midgut—was dissected and acetylcholinesterase (AChE) was used as a biomarker of neurotoxicity stress. In parallel, bioassays of microplastic ingestion were carried out, and after 48 h of initiating the ingestion assay, using a stereo and fluorescence microscope, the microplastic distribution was observed in the different earthworm segments. The results obtained in the avoidance assay indicated a lack of preference for either soil type; however, upon moving, the earthworms lost surface mucus, resulting in burns and lesions on their bodies, which were reflected in the increase in AChE enzyme levels, which was not directly related to microplastic ingestion, but rather likely acts as an external physical stress agent. The results of the ingestion bioassay showed that microplastic was present in all the earthworm segments, with a higher number of particles in the hindgut. The Lumbricus terrestris did not distinguish microplastics from soil particles, and given the high exposure concentrations, microplastics produced physical lesions on the mucus membranes of earthworms. Lumbricus terrestris showed to be a suitable bioindicator for testing the exposure to microplastic contamination in soil.