Different chronic effects of conventional PET and biodegradable PLA microplastics: Insights from apical and...

Although biodegradable microplastics are promoted as environmentally safer alternatives to conventional plastics, their long-term biological impacts remain poorly understood. Standard ecotoxicological assessments often rely on physical traits that may mask hidden cellular adjustments and distinct polymer-specific toxicity mechanisms. To address this gap, this study investigated the chronic responses of Daphnia magna to biodegradable polylactic acid (PLA) and conventional polyethylene terephthalate (PET) microplastics by integrating a 21-day life-history assay with global transcriptomic profiling. At the organism level, both PLA and PET exposure reduced somatic growth while leaving reproductive output unaffected, yielding no significant differences between the polymer types. However, transcriptomic analysis revealed that these matching physical outcomes are driven by unique, polymer-dependent molecular patterns. Both treatments showed a coordinated suppression of structural and growth-related pathways alongside the enrichment of regulatory, transport, and protein turnover functions. Yet, PLA induced a broader engagement of stress-responsive functional categories particularly oxidative stress, tetrapyrrole/heme-binding functions, and metabolic control pathways, suggesting a higher regulatory and energetic burden. In contrast, PET elicited a more constrained transcriptomic profile dominated by structural, transport, and RNA-regulatory processes, with minimal activation of redox pathways. Furthermore, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed a shared downregulation of ribosome biogenesis under both polymers, but a much stronger enrichment of spliceosome and RNA-regulatory machinery under PET exposure. These findings demonstrate that similar organism-level phenotypes can overlook distinct internal metabolic costs. Consequently, evaluating alternative materials purely by physical symptoms can misrepresent their actual biological impact, highlighting the importance of using transcriptomic profiling to accurately assess the safety of emerging alternative polymers.