Ecotoxicological effects of micronized car tire wear particles and their heavy metals on the earthworm (Eisenia fetida) in soil

As one of the most common and persistent emerging human-made pollutants, microplastics (MPs) have become a global environmental problem. Although many studies have demonstrated the impact of MPs on aquatic organisms, the potential effects on terrestrial fauna are relatively unknown, even though soil is a major sink for plastic pollution. In this study, earthworms (Eisenia fetida) were exposed to MPs from tires (TMPs) in artificial soil at different TMP concentrations (0, 1, 5, 10, and 20%, w/w), at different TMP size fractions (2 mm-350 μm; 350 μm-50 μm; 50 μm-25 μm; and <25 μm), and time periods (14 and 28 days). The purpose was to evaluate the impact of TMPs on the bioaccumulation of heavy metals (Zn, Cd, and Pb) and levels of oxidative stress in the earthworms. Fluorescence imaging of TMPs stained with Nile red dye clearly showed that the worms had ingested the TMPs, with smaller particles preferentially consumed. The ingested TMPs had altered the surface morphology, degradation that increased surface area and uptake of heavy metals into the organisms. Earthworms exposed to TMPs at the higher concentrations showed clear signs of oxidative stress, with significantly (p < 0.05) increased catalase and peroxidase activity and lipid peroxidation levels, and reduced activity of superoxide dismutase and glutathione S-transferase. SOD, POD, and CAT are important cell protection enzymes that prevent excessive levels of reactive oxygen species in cells. MDA represents the total scavenging ability of free radicals in the tissue. SOD decreases and MDA accumulates rapidly. GST has a detoxification effect. Overall, this study adds to the growing body of evidence that TMPs are harmful to certain terrestrial organisms.