Distinct exposure impact of non-degradable and biodegradable microplastics on freshwater microalgae (Chlorella pyrenoidosa): Implications for polylactic acid as a sustainable plastic alternative

Microplastics (MPs) are increasingly recognized as significant sources of harm to biota in various environments. However, the detrimental impacts of aged MPs with different structures and degradability remain poorly understood. In this study, aged MPs from polylactic acid (PLA), polyethylene (PE), and polystyrene (PS), representing biodegradable, aliphatic, and aromatic plastics, respectively, were prepared to examine their effects on microalgae (Chlorella pyrenoidosa). Structural and property analyses indicated the presence of aging and oxygen-containing functional groups on the surfaces of the MPs, which correlated with an increase in negative electrical charge (i.e., aged PLA > aged PE ≈ aged PS). Aged PLA MPs affected microalgae biomass, promoted protein synthesis, and elevated mild oxidative stress. In contrast, aged PE and PS MPs not only affected biomass, protein, and carbohydrate synthesis but also inhibited photosynthetic pigment production and activity, resulting in intracellular oxidative stress. Excitation-emission-matrix spectra analysis showed that PLA induced microalgae to secrete large amounts of humic acid-like extracellular polymers, whereas aged PE and aged PS groups contained only small amounts of them and proteins. This study addresses critical knowledge gaps in the toxicology of various aged MPs on microalgae and provides valuable insights into the potential of PLA as a sustainable alternative to conventional plastics in microalgae culture industry.