Microplastic contamination and earthworms: Current trends and research needs

Abstract

Microplastic pollution in agricultural soils is a growing global concern that demands an urgent understanding of its effects on soil function and ecosystem services. Soil-dwelling earthworms exhibit strong subsoil bioturbation, which contributes to processes associated with many ecosystem services such as water regulation, nutrient cycling, biomass production, pollution control and remediation, among others. This functional association between earthworms and soil has made these organisms suitable candidates for assessing microplastic pollution in agricultural soils. However, based on the literature systematically reviewed here, current knowledge of microplastic toxicity in earthworms mainly derives from a recurrent methodological pattern, which implies lab-scale standardised toxicity testing. Eisenia fetida and E. andrei have been the model earthworm species (86% of screened publications), leading to the risk of biased data. Additionally, the ecological characteristics of Eisenia spp. raise uncertainties about the environmental and agronomic meaning of microplastic toxicity testing outcomes. These species are absent in agricultural lands and inhabit the soil surface, avoiding the ingestion of mineral soil. Accordingly, a more environmentally realistic assessment of microplastic pollution is needed, which involves field soil, native earthworm species as possible and aged microplastics from the environment. This review identifies specific knowledge gaps regarding the use of Eisenia species in the environmental risk assessment of microplastic pollution, and suggests moving ecotoxicity assessment towards a retrospective approach, which involves complex setups (field-simulated and ex-situ microcosms) and microplastic interactions with biomolecules and other environmental contaminants (e.g. protein corona and eco-corona) that may modulate microplastic toxicity.