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Meta-analysis reveals contrasting effects of biodegradable and conventional microplastics on earthworm fitness, physiology, and gut microbiota
Summary
A large analysis of over 100 studies found that both regular plastic pollution and "biodegradable" plastics harm earthworms, shrinking their survival, growth, and gut bacteria, while causing cell damage from oxidative stress. The key takeaway: switching to biodegradable plastics doesn't actually protect these important soil creatures, so it's not a guilt-free fix. Since earthworms keep soil healthy for growing our food, this matters for the whole food chain, including us, and it's a reminder that "biodegradable" labels don't always mean "harmless."
Microplastics (MPs) have become widespread contaminants in terrestrial ecosystems and may pose risks to soil fauna, particularly earthworms, a key group of soil ecosystem engineers. Because laboratory ecotoxicological studies have demonstrated that MPs disrupt earthworm physiological and cellular functions, then we must establish whether earthworms respond consistently to the conventional MPs and newer types of biodegradable MPs. Here, we conducted a quantitative meta-analysis of peer-reviewed studies published from database inception to 8 December 2025 to assess the effects of conventional and biodegradable MPs on earthworm fitness, physiology, and gut microbiota. A total of 2124 observations extracted from 102 articles were evaluated for the effect of MPs on 15 indicators of earthworm. Overall, MPs exposure reduced survival and growth rates, increased oxidative stress and oxidative DNA damage, and inhibited gut bacterial diversity and richness. Meta-regression analysis indicated that conventional MPs exhibited stronger dose-dependent toxicity patterns, characterized by increased catalase (CAT), and peroxidase (POD) activities, elevated malondialdehyde (MDA), and decreased survival and growth at higher doses. In contrast, biodegradable MPs primarily affected reproduction, CAT activity and glutathione (GSH) level in a size-dependent manner. Among the MPs and experiment features, the MP shape, dose, and exposure duration were most likely to cause earthworm responses, with MDA, 8-hydroxy-2'-deoxyguanosine (8-OHdG), and reactive oxygen species (ROS) consistently acting as sensitive physiological indicators of MPs exposure in earthworms. Overall, MP pollution poses substantial risks to earthworms and gut microbial communities, and biodegradable MPs do not eliminate the earthworm biological responses. This suggests that substituting biodegradable MPs for conventional MPs might not improve earthworm survival, growth and reproduction, which has implications for earthworm populations and community ecology.