The Harmful Footprint of Aged Biomicroplastics on Algal Development: A Comparative Study of Polylactic Acid, Polyhydroxybutyrate, and Cellulose Acetate

Biopolymers are increasingly produced as sustainable alternatives to plastics, but their degradation in aquatic ecosystems raises ecological concerns. This study demonstrates that the photodegradation of polylactic acid (PLA), polyhydroxybutyrate (PHB), and cellulose acetate (CA) in artificial seawater substantially increased toxicity under the elevated laboratory conditions used, compared with virgin counterparts, adversely affecting the development and growth of seaweeds. Those aged biopolymers and their leached substances impaired the growth and development of the green macroalgae <i>Ulva</i> (Chlorophyta) under standardized conditions, but the ecological relevance at natural seawater concentrations is likely lower. Chlorophyll <i>a</i> fluorescence, median lethal concentration (LC₅₀-values), and HR-MS analysis corroborated these findings, emphasizing detrimental impacts on the fitness and development of <i>Ulva mutabilis</i>. Toxic effects were linked to substances released during photodegradation and hydrolysis, especially for PLA and PHB. PLA exhibited a 6.7-fold increase in toxicity following UV exposure; LC₅₀-values indicate increased toxicity from direct polymer exposure and from leached compounds at the applied concentration. PHB exhibited the strongest degradation-related toxicity, with over a 10-fold increase. CA was the most toxic overall, with an LC₅₀-value of <0.5 mg/mL in aged samples under laboratory conditions. Even nonaged CA showed effects at these elevated concentrations, but environmental relevance remains uncertain. Although this is an explorative model study in lab scale using elevated concentrations, the observed persistence and (post-)-degradation effects of biomicroplastics highlight potential polymer-specific toxicity and emphasize the need for further research on the degradation of biopolymers in the marine environment.