Earthworms and microplastics: Transport from sewage sludge to soil, antibiotic-resistant genes, and soil remediation

The massive production and use of plastic-based utensils has resulted in large amounts of microplastics (MPs) that can reach all ecosystem compartments. This is of great concern because MPs can act as vectors for organic and inorganic contaminants and thus magnify the hazardous potential of these pollutants. One of the key sources of MPs in soil systems is sewage sludge, which is widely used as an agricultural fertilizer. Besides MPs, antibiotic resistance genes (ARGs), which are associated with the overuse of antibiotics in humans and livestock, are also commonly found in sewage sludge. The presence of ARGs can drive changes in the soil microbiome and impact soil fauna and plants. Evidence of synergistic interactions between MPs and ARGs in sewage sludge and soil has also been reported. Earthworms can assist the degradation of MPs via the activity of associated enzymes and microbiota. The presence of earthworms has also been related to changes in the relative abundance of some strains of bacteria indirectly modulating their ARGs content and, subsequently, the presence of ARGs in soils. In this regard, vermicomposting has been proposed as an environmentally friendly approach to reduce the toxicity of sewage sludge before its application to soil. This chapter provides a review of the most recent literature on the interactions between MPs and ARGs and their effects on soil systems at different levels of biologic organization. Under this scenario, earthworms can act as targets of exposure to these pollutants or as a remediation tool. An overview of the relevant research conducted to date as well as a critical appraisal of the priority research topics and current knowledge gaps are addressed within this chapter.