The impact of microplastic on nematodes: Soil type, plastic amount and aging as determinants for the fitness of Caenorhabditis elegans

The presence of microplastics (MPs) in the environment is increasing, however, there is still limited knowledge on the potential threat to biodiversity and function of terrestrial ecosystems. Although soil is a primary sink for MPs, the effects on the soil fauna have not been extensively investigated. This applies in particular to ecotoxicological tests using soil as a medium to mirror a natural environment. In this study, we investigated the impact of conventional plastic, low-density polyethylene (LDPE), and biodegradable plastic, a blend of poly(lactic acid) and poly(butylene adipate- co -terephthalate) (PLA/PBAT), with particle sizes of <180 μm and 180–500 μm. As a representative of the species-rich and abundant group of soil nematodes, the model nematode Caenorhabditis elegans was cultivated in three different soil substrates: a standard soil (LUFA - loamy sand), an artificial substrate (OECD - sand with peat) and an arable soil fertilized with farmyard manure (RFYM – sandy loam). The selected MPs were used at 0.01, 0.1, and 1 % w/w as pristine or artificially aged (degradation by sunlight or soil microorganisms) material. With pristine MP, reproduction and growth of C. elegans were negatively affected by LDPE in both RFYM and LUFA soils at 1 %, while PLA/PBAT did not cause any impairment of nematodes across soils. The environmentally relevant amount of 0.01 % did not show any significant effect. Linear regression analysis showed a negative relationship between the amount of LDPE and the relative offspring and growth of nematodes across all three soil types, while for PLA/PBAT, this was only evident at a size of 180–500 μm in OECD soil. The toxicity to C. elegans persisted with aged LDPE, while the aged PLA/PBAT generally had a greater toxicity than in its pristine state. The two simulated aging methods did not differ significantly in their negative impact on nematode offspring, while light aging showed a stronger inhibition of growth than microbial aging. Interestingly, aging time did not alter the effects on nematode offspring. In conclusion, the environmentally relevant concentration of MPs was not toxic, and the toxicity of the tested MP to nematodes was affected by soil type and related to MP amount and aging. The latter, however, rendered the biodegradable PLA/PBAT toxic to C. elegans . These findings provide crucial insights into the ecotoxicological risks of MPs to soil nematodes under semi-natural conditions in soil substrates. • Microplastic toxicity to C. elegans was tested in semi-natural soil substrates. • Environmentally relevant concentration (0.1 g kg −1 ) did not cause any impairment. • Aging can render the biodegradable PLA/PBAT toxic to C. elegans . • Negative effects of conventional LDPE persisted with aging. • Soil type but not aging type (light, microbial) affected the impact on nematodes.