Dominant effects of elevated CO2 over microplastics on physiological and microbial responses of submerged aquatic plants in eutrophic waters

Eutrophication has become a global ecological issue, exacerbated by the increasing presence of carbon dioxide (CO₂) and microplastics in aquatic environments. Currently, there is limited understanding of the synergistic ecological effects of eutrophication, microplastics, and CO₂ on submerged aquatic plants. Therefore, it is essential to investigate their interactions with submerged aquatic plants. In this study, two submerged aquatic plants, Vallisneria natans (Lour.) H. Hara and Ceratophyllum demersum L., were selected to compare their stress response strategies under microplastic and elevated CO₂ (eCO₂) exposure. The results indicated that under eutrophic conditions, polystyrene microplastics (PSMPs) did not directly affect either plant species. In contrast, eCO₂ significantly activated antioxidant defense mechanisms, enhanced the accumulation of photosynthetic pigments, and modified metabolite contents. Moreover, both PSMPs and eCO₂ induced shifts in epiphytic microbial communities, implying that microplastics may affect plants indirectly via microbial pathways. Additionally, eCO₂ improved nitrogen and phosphorus uptake in the plant systems, indicating that submerged plants may contribute more actively to aquatic nutrient cycling under future CO₂ conditions. In summary, this study reveals that in eutrophic waters, eCO₂ influences submerged aquatic plants more directly than microplastics, primarily through physiological process and microbial community regulation, offering important implications for aquatic ecosystem management.