Shape-driven toxicity of polystyrene microplastics: Impacts on physiology and gut microbiota in Daphnia magna

Microplastic pollution has emerged as a global issue that poses serious risks to aquatic ecosystems. Although Daphnia spp. are widely used as model organisms to study the effects of microplastics on their fitness, their microbiome response remains largely unexplored. This study investigated the effects of ground polystyrene microplastics (G-PS; fragments below the EC₁₀ value) and commercial polystyrene microplastics (C-PS; beads below the EC₁₀ value) on the physio-biochemical responses and gut microbiota of Daphnia magna. The toxicity of polystyrene microplastics to D. magna was shape-dependent, with G-PS being more toxic than C-PS. Exposure to G-PS and C-PS triggered Reactive oxygen species (ROS) production in D. magna. Although G-PS increased the abundance of both harmful (Fusobacterium) and beneficial bacteria (Blautia and Subdoligranulum) in the gut microbiota of Daphnia, C-PS only increased the abundance of beneficial bacteria (Lactobacillus, Ligilactobacillus, and Aerococcus), which may mitigate the toxicity of microplastics. Functional predictions based on amplicon sequencing suggested that altered microbiota may support the growth of D. magna by modulating associated metabolic pathways. D. magna exposed to G-PS exhibited a significantly higher abundance of gut microbiota pathways and enzymes associated with the detoxification of harmful compounds than those exposed to C-PS. This suggests that the higher toxicity of G-PS requires a stronger adaptive response from the gut microbiota. Overall, these findings highlight microplastic shape as a key factor influencing toxicity in D. magna and its associated microbiota.