Hydrilla verticillata defends against toxic ionic Ag through its biotransformation into Ag nanoparticles

Heavy metals are common pollutants in aquatic environments. They are accumulated by aquatic plants and thus incorporated into living cells. Ionic silver finds its way to water bodies through silver (Ag) plating industries, photographic processing effluents, e-waste, sewage sludge, etc. We conducted investigations to evaluate the impact of silver ions on Hydrilla verticillata.

Plants were incubated in different concentrations viz. 0.25, 0.5, 0.75, and 1 mM of silver nitrate (AgNO₃) for 12 and 36 h. The impact of Ag ions was evaluated by measuring (i) the stress markers, viz. MDA and proline, and (ii) the uptake of silver content in treated plants using an atomic absorption spectrophotometer (AAS). Exogenously generated Ag nanoparticles (AgNPs) were characterized through UV-Vis spectroscopy, Transmission Electron Microscopy (TEM), Selected Area Electron Diffraction (SAED), Energy Dispersive X-ray (EDX), and Fourier Transform-Infra Red (FTIR) spectroscopy investigations characterized exogenously generated AgNPs.

The study examined the effects of silver ions (Ag⁺). Results indicated that treatment with 1 mM AgNO₃ for 36 h led to a 5-fold reduction in MDA levels and a 3-fold decrease in Proline. Additionally, a color change in the AgNO₃ solutions, from a clear, colorless solution to a brown, colloidal suspension. The UV-Vis absorption spectra of these colloidal solutions exhibited a plasmon resonance band at 430-480 nm. A peak at this position confirms the presence of AgNPs. TEM coupled with SAED revealed the presence of crystalline spherical AgNPs in the size range of 5-50 nm in the brown colloidal solution. These results suggest that Hydrilla biotransforms ionic silver into AgNPs.