Contrasting effects of conventional PET and biodegradable PHB microplastics on population dynamics and gut microbiome responses in Daphnia magna.

Research on the ecotoxicity of biodegradable microplastics (MPs) in Daphnia magna is rapidly growing, while their population-level effects remain unclear. This study compared the population-level effects of conventional MPs (polyethylene terephthalate, PET) and biodegradable MPs (polyhydroxybutyrate, PHB) on D. magna over 45 days. Gut microbiome and transcriptomic analyses were also conducted to elucidate the underlying mechanisms of the observed responses. PET MPs significantly (p < 0.05) decreased D. magna population biomass relative to control, whereas PHB MPs showed no significant effects. Both types of MPs altered the gut microbial community structure of D. magna, with PHB inducing a selective increase in potential degraders. Transcriptomic data showed that PET MPs significantly (p < 0.05) increased the expression of genes related to stress and defense responses. In contrast, PHB MPs significantly (p < 0.05) upregulated genes associated with metabolic processes. Further research incorporating direct assessment of PHB degradation, host energy assimilation, and wider range of exposure concentrations is required to clarify the mechanisms underlying the distinct responses to biodegradable and conventional MPs. This study highlights the importance of considering polymer types in MP environmental risk assessments and underscores the need of integrating multiple endpoints for a comprehensive evaluation.