Ecological impacts of polylactic acid and polylactic acid-polyethylene microplastics on freshwater ecosystems: Insights from a water–Vallisneria natans–sediment system

Microplastics (MPs) are widely distributed in freshwater and seawater ecosystems around the world, posing a serious threat to ecological security and environmental health, and have become a global environmental problem. In this study, the effects of polylactic acid (PLA) MPs and polylactic acid-polyethylene (PLA-PE) MPs on freshwater ecosystems were simulated in a natural environment by constructing a water-Vallisneria natans-sediment system. The changes in the physicochemical parameters of the system's water quality, morphological characteristics of the plants, antioxidant enzyme system, and the structure of the microbial communities were comprehensively discussed. The results demonstrated that MPs had no discernible impact on the water physicochemical parameters within the system. PLA MPs did not influence the growth of Vallisneria natans, whereas the root length of Vallisneria natans was not affected in the PLA-PE MPs treatment groups. Low concentrations of MPs enhanced the activity of antioxidant enzymes (CAT, SOD, and POD) and increased oxidative stress markers (MDA). The PLA-PE MPs treatment groups exhibited a positive correlation between MDA content and CAT activity in response to the addition of PLA MPs. The microbial communities in the sediment and rhizosphere of Vallisneria natans exhibited less impact from PLA MPs and PLA-PE MPs, and the variability in community structure was not statistically significant. These findings provide valuable insights for refining the ecological risk assessment framework of PLA and PLA-PE MPs within integrated water-plant-sediment systems.