Silver-Form-Driven Nanotoxicity Reveals <i>Spirodela intermedia</i> W. Koch as a Potential Bioindicator for Aquatic Systems

Silver nanoparticles (AgNPs) and silver sulfide nanoparticles (Ag2SNPs) are increasingly being used in diverse industries due to their unique physicochemical properties. However, their release into aquatic environments raises concerns regarding their potential ecological impact, making them an excellent model system for evaluating potential biomonitors. In this study, we evaluated and compared the phytotoxic effects of AgNPs and Ag2SNPs suspensions on the free-floating macrophyte Spirodela intermedia W. Koch, a species with broad distribution from Mexico to South America and promising potential as a bioindicator. The designed protocol involved both types of nanoparticles, which were synthesized and characterized via UV–vis spectrophotometry and other techniques. In addition, morphometric parameters (frond number, frond abscission, and chlorosis), as well as enzymatic activity of ascorbate oxidase (AO) and chlorophyll-a variable fluorescence, were assessed after exposure to environmentally relevant concentrations. Results indicated that AgNPs induced significantly higher toxic effects than Ag2SNPs, with marked frond abscission, chlorosis, AO activity reduction, and damage to the photosynthetic apparatus. These findings indicate that, as expected, the chemical form of silver nanoparticles plays a key role in phytotoxicity and that S. intermedia exhibits adequate sensitivity to be proposed as a bioindicator candidate in aquatic ecosystems, thereby expanding the global portfolio of bioindicator species and highlighting the ecological value of native flora for sustainable water quality monitoring.