Biodegradable polylactic acid microplastics rewire the soil food web by impeding amoebae predation.

Soil amoebae function as both keystone microbial predators and sensitive indicators of soil ecosystem health. Although polylactic acid (PLA) biodegradable plastics are increasingly adopted to reduce persistent plastic pollution, their ecological impacts, particularly during the active degradation phase, remain poorly understood. In this study, we established multiple co-culture interfaces to investigate the interactions and effects between soil amoebae and polylactic acid microplastics (PLA MPs). Our results show that PLA MPs fundamentally reshape amoeboid behavioral strategies and restructure soil microbial networks. Specifically, PLA MPs induce particle-size-dependent shifts in microbial community composition and functional potential, trigger strong avoidance behavior in amoebae that impairs its chemotactic capacity to locate bacterial prey, and disrupt the development of the PLA-associated plastisphere by suppressing microbial interactions. PLA MPs also impose significant fitness costs on amoebae, reducing spore production and altering developmental morphology. This study establishes soil amoebae as powerful bioindicators for evaluating the ecological impact of biodegradable plastics and demonstrates that PLA-driven disruption arises not only from its persistence but also from its degradation intermediates. These findings uncover hidden ecological trade-offs in the adoption of biodegradable plastics and offer key mechanistic insights to support environmental risk assessment.