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Exposure to mm-scale microplastic particles does not cause weight loss in two earthworm species belonging to different ecological groups
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Researchers tested whether relatively large microplastic fragments (1-5 mm) from polyethylene film cause weight loss in two common earthworm species by diluting their food supply. Contrary to their expectations, the microplastics did not reduce earthworm body weight even in food-limited conditions, despite both species ingesting the particles. The findings challenge the widely cited food dilution hypothesis and suggest that the effects of larger microplastics on soil organisms may be less straightforward than assumed.
The aim of this study was to estimate the effect of relatively large (1-5 mm) fragments of high-density polyethylene films on two widespread earthworm species belonging to different ecological groups-endogeic Aporrectodea caliginosa and epigeic Lumbricus rubellus. In a microcosm experiment lasting 8 weeks, we tested the food dilution hypothesis, which suggests that the adverse effect of microplastic on earthworms is caused by the dilution of food by plastic, which has zero energetic value. Both earthworm species ingested plastic particles, and both species were seemingly limited by the availability of food. In particular, the addition of food substrate (aspen litter) to the soil had a significant positive effect on the weight of A. caliginosa. In contrast to our expectations, microplastic at relatively high concentrations (0.3% and 2.3% w/w in the soil for A. caliginosa, and 33% and 48% w/w in the litter for L. rubellus) had no significant effect on earthworm biomass. This suggests that the food dilution effect is not likely to be the main mechanism of the adverse effect of microplastic on earthworms. Our work adds to the growing evidence that in many cases microplastic does not harm soil animals.
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Earthworms exposed to polyethylene microplastics in soil for 30 days lost more body weight than unexposed worms, especially at the smallest particle size and highest concentration. Mortality, cocoon production, and gut bacteria diversity did not show clear differences, and effects on gut bacteria types and their predicted functions were limited.
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Researchers exposed earthworms to polyethylene microplastics of two sizes and found that smaller particles (13 micrometers) were more toxic than larger ones (130 micrometers), reducing survival and growth more severely. The microplastics caused oxidative stress in the worms and altered key soil properties including pH and organic carbon content. Since earthworms play a vital role in maintaining healthy soil for agriculture, this damage could affect soil quality and ultimately the food grown in microplastic-contaminated farmland.
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When a large batch of papers lands in the Atlas, we read through it and send a short write-up of what stood out.