Phytotoxicity of Silver Species to Egeria Densa and Applications to Phytoremediation
Bernas, Leonard James
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Silver species pose a significant threat to aquatic ecosystems because the use of silver nanoparticles (AgNps) for industrial applications is significantly increasing, as are discharges into waterways. The effects have only been studied on select fish and aquatic plants, so more research is needed to understand the risks associated with silver species in the environment. Egeria densa (E. densa, also known as Elodea densa), a common freshwater submerged macrophyte, was exposed to 0-1.25 ppm silver cations and 2 and 15 nm AgNps in an aqueous environment for 7 days. E. densa was chosen for this study because it is a fastgrowing aquatic organism. It is also commonly used to monitor water quality and heavy metal accumulation and toxicity in plants. Silver is toxic to aquatic organisms and the effects on E. densa have not been studied in detail. Effects were characterized based on photosynthetic function, lipid peroxidation, soluble carbohydrates, total phenolics, total flavonoids, percent scavenging, percent chelating, and total silver absorption. Silver cations appear more toxic to the plants because they enter cells through ion channels and generate reactive oxygen species (ROS), which induces oxidative stress throughout the plant. Declines were observed in all of the parameters studied, demonstrating the deleterious effects of Ag+ on plants. The nanoparticles were mainly adsorbed to the exposed leaf surfaces on stalks of E. densa and the 2 nm AgNps induced significant lipid peroxidation at low concentrations of AgNps. This caused declines in photosynthesis, as seen by the degradation of chlorophyll a and carotenoids. The 15 nm AgNp exposures showed similar effects to the 2 nm AgNp exposures, but also caused damage to the antioxidant response system. Because of similarities to the Ag+ exposures, the mechanism of the 15 nm AgNp toxicity was attributed to the release of silver cations from the nanoparticles adsorbed to the surface of the cell walls. The silver cations caused dose-dependent increases in silver content in the stem tissue, whereas the AgNp exposures caused increases in silver content in the leaves, demonstrating different mechanisms of translocation into the plants. Silver removal from solutions was also monitored for 48 hours in the presence of E. densa. There was 87% removal of the Ag+ after 48 hours in 0.25 and 0.5 ppm Ag+ exposures, and over 70% removal of 2 and 15 nm AgNps at like concentrations. These results demonstrate that E. densa can remove silver from its aqueous environment and serve as an effective phytoremediant.