Cadmium Accumulation in the Epiphytic Biofilm of Vallisneria natans L.: Implications for Bioindicator Monitoring of Heavy Metal Pollution

Submerged plants are common bioindicators of heavy metal contamination in aquatic ecosystems. Traditional research often focused on the overall metal content within plants or their tissues, and overlooked the role of epiphytic biofilms, which may affect the accuracy of bioindicators’ function. This study conducted cadmium (Cd) exposure experiments on Vallisneria natans L. (V. natans) to investigate the Cd accumulation in the plant tissues and their associated epiphytic biofilms. It was observed that phyllosphere and rhizosphere biofilms accounted for 27.3–60.6% and 31.3–41.7% of the total Cd in the leaf and root areas, respectively. The phyllosphere biofilm rapidly accumulated Cd directly from the water column, showing a strong correlation with aqueous Cd levels, whereas Cd in leaf tissue was primarily root-derived. Microbial analysis revealed that Proteobacteria dominated the biofilm communities, and the relative abundance of specific genera (e.g., Nitrospira) correlated with Cd exposure. Our findings underscore a critical refinement for bioindication: distinguishing metal accumulation in epiphytic biofilms from that in plant tissues significantly enhances source attribution: phyllosphere biofilms reflect water pollution, while rhizosphere biofilms indicate sediment–root interface contamination. This refined approach provides a pathway toward more precise environmental monitoring, contributing to sustainable water quality management by enabling earlier detection of pollution and informing targeted remediation efforts to protect aquatic ecosystem health.