Soil chloride content influences the response of bacterial but not fungal diversity to silver nanoparticles entering soil via wastewater treatment processing

Abstract

Silver nanoparticles (NPs) are among the most widely used nanomaterials and are entering soil ecosystems, mainly via biosolids in agriculture. When added directly to soils, metallic Ag-NPs have been shown to affect microbial communities, which underpin important ecosystem functions. During wastewater treatment processing, metallic Ag-NPs are rapidly converted to Ag₂S, which is relatively insoluble and less toxic. Furthermore, recent evidence indicates that silver bioavailability is influenced by soil chloride content. Hence there is a need to understand how Ag₂S, which forms from Ag-NPs during wastewater treatment, influences soil microbial diversity at varying salinity. In this study, after adding Ag-NPs to sludge (with most converted to Ag₂S), we then applied the sludge to soil and examined how salinity influences the effects of 0 mg, 1 mg and 10 mg kg^-1 Ag on bacterial and fungal diversity over time. Using high-throughput phylogenetic marker gene sequencing of 16S rRNA gene and ITS2 amplicons, we demonstrate that, despite being theoretically less toxic, wastewater treatment processed Ag-NPs can affect the composition of soil bacterial and fungal communities, and influence bacterial alpha diversity. In addition, we found that silver-associated changes in bacterial community composition were affected by soil chloride content, with more acute responses to silver being observed in more saline soils. This work highlights that the release of Ag-NPs and their conversion into Ag₂S prior to addition to soils via realistic exposure pathways can alter microbial diversity and that these effects may be influenced by soil chloride content.

Keywords: Microbial diversity; Phylogenetic marker gene sequencing; Silver nanoparticles; Soil salinity; Wastewater treatment processing.