Direct and indirect ecological impacts of microplastic fibers on host-parasite and host-microbiota interactions

Microplastic (MP) pollution poses a growing threat to freshwater ecosystems, yet its effects on ecological interactions, such as host-parasite or host-microbiota dynamics, are poorly understood. Using leopard frog (Lithobates pipiens) tadpoles and their interactions with a common trematode parasite as a model, this study experimentally manipulated MP fiber size (short: ∼0.24 mm; long: ∼1.5 mm) and concentration (10 or 40 μg L⁻¹) to assess effects on tadpole survival, growth, development, behavior, gut microbiota, and parasite susceptibility. High MP concentrations significantly reduced tadpole survival, with long fibers causing greater mortality. Sublethal exposures inhibited development at both concentrations, while 40 μg L⁻¹ also reduced mass and length. Shorter fibers were more frequently ingested, coinciding with altered host behavior and elevated infection intensities, suggesting impaired parasite avoidance. MP exposure caused modest shifts in gut microbiota. In contrast, tadpole gut microbiota was minimally altered by the parasite and MP-parasite treatments. Our findings revealed that MP fiber morphology and concentration interact to shape amphibian health and susceptibility to parasitic infection. These results underscore the importance of incorporating ecological interactions into risk assessments of MP and highlight how environmentally-relevant fiber sizes can disrupt key ecological interactions in freshwater systems.