Susceptibility of constructed wetland microbial communities to silver nanoparticles: A microcosm study

Abstract

Silver nanoparticles (AgNPs) are increasingly used as an antimicrobial agent in various consumer products. Silver release from these products occurs during use, washing and disposal at varying rates, into the wastewater system and eventually into aquatic ecosystems. Constructed wetlands (wetlands designed for water pollution control) represent a unique type of water treatment system which are beginning to receive growing amounts of influent AgNP loadings. In order to examine potential impacts to the health and ongoing utility of constructed wetlands we evaluated the susceptibility and the developed resistence of constructed wetland microbial communities to two types of AgNPs (citrate and polyvinylpyrrolidone (PVP)-coated), and ionic Ag (Ag+). Microcosms representing an unplanted batch-fed subsurface flow constructed wetland were exposed to an AgNP contaminated (100μg/l) simulated wastewater for 28days. Experiments were triplicated and included unexposed controls. Analysis of total Ag in the interstitial water and biofilm matrix via inductively coupled plasma mass-spectrometry (ICP-MS) revealed the majority (>98%) of AgNPs partitioned from the interstitial water and accumulated in the biofilm and presumably the sediment matrix. No significant alteration to the microbial community function (assessed via community-level physiological profiling, CLPP) or structure (using denaturing gradient gel electrophoresis, DGGE) was observed following the initial 28day exposure. Subsequent ex-situ dose-response testing over a wide concentration range (0–5mg/l) provided some evidence for increased resistance to Ag toxicity in the previously exposed microbial communities when compared to the controls. The ex-situ evaluated susceptibility of the microbial communities to AgNPs varied between treatments with significant reductions in catabolic activity observed at 0.5mg/l for ionic Ag, 1mg/l for citrate coated AgNPs and 5mg/l for PVP coated AgNPs. These findings suggest that wetland microbial communities can tolerate and develop resistance to lower levels of in-situ AgNP exposure in a relatively short time, however ex-situ toxicity is observed past certain threshold concentrations and this threshold varies depending on the original form of the AgNP.