Effects of Ag and Ag₂S nanoparticles on denitrification in sediments

The widespread use of commercial silver nanoparticles (Ag NPs) inevitably results in their increased release into natural waters and subsequent deposition in sediments, requiring the environmental impact of such deposition to be closely investigated. Hence, the effects of Ag NPs, polyvinylpyrrolidone (PVP)-Ag NPs, and sliver sulfide nanoparticles (Ag₂S NPs) on denitrification-induced gas production (N₂O and ¹⁵N-N₂), and denitrifying microbes in freshwater sediments were investigated. Slurry experiments (8 h) combined with a ¹⁵NO₃^- addition technique were performed to determine the gaseous production. The abundance of relative functional genes (nirK, nirS and nosZ) and the composition of functional community were determined through RT-PCR and high-throughput sequencing, respectively. The obtained results showed that the toxicity of NPs on denitrification depended on their type (Ag⁺ > Ag NPs > PVP-Ag NPs > Ag₂S NPs) and concentration, e.g., all 1 mg/L NPs exhibited no effects on denitrification, whereas evident hormesis effect-induced acceleration was observed in the case of Ag⁺. Conversely, 10 mg/L Ag⁺ and Ag NPs significantly inhibited the release rates of N₂O and N₂ by decreasing the abundance of functional genes (nirK and/or nirS) and the predominant bacteria Paracoccus. PVP-Ag and Ag₂S NPs had no effects on N₂ release rates and the composition of denitrifiers, however, inhibited the emission of N₂O (by reducing the abundance of nirK), suggesting that normal denitrification-induced N₂ formation in sediments could still be sustained when the N₂O production decrease lied within a certain range. Further, the inhibiting ability of Ag-containing NPs was caused by their intrinsic nanotoxicity to functional microbes rather than by the general toxicity of Ag⁺. Besides, Ag₂S NPs (as a main detoxification form of AgNPs) were revealed to be intrinsically nanotoxic to denitrifiers, albeit showing the lowest inhibitory effect among the three tested NPs. Thus, this study demonstrated that the inhibitory effect of Ag-containing NPs on denitrification in sediments depends on their morphology and type, implying that the stability and toxicity of Ag-containing NPs should be considered with caution.