Long-term exposure to silver nanoparticles affects periphyton community structure and function

Abstract

Silver nanoparticles (AgNPs) are widely used as antimicrobial agents. During their life cycle, some of the AgNPs are released into natural environments, where chronic exposure to them can continue to cause harmful effects on microorganisms. However, very little is known about long-term impacts on important ecosystem compartments such as periphyton, a microbial community of algae and bacteria that covers submerged surfaces and contributes importantly to primary production and other ecosystem processes. Thus, the present study focused on assessing the accumulation of citrate-coated AgNPs and dissolved Ag⁺ in periphyton and on testing chronic effects on periphyton community structure and a range of functional endpoints. Stream periphyton grown in microcosms was exposed to 0.1, 1 and 10 μM AgNPs and 0.1 μM AgNO₃ (a source of Ag⁺ ions) for up to 21 days. By that time, 84 to 98% of the total available silver in the microcosms was strongly associated with the periphyton. The strongest and broadest impacts on functional endpoints were observed at the highest AgNP concentration (10 μM), which caused a decline in algal primary production and microbial respiration but a simultaneous increase in bacterial secondary production and total biomass accrual. The community structure of both phototrophs and heterotrophs was also changed. Overall, our results reveal that periphyton strongly accumulates AgNPs, leading to a shift of community metabolism towards heterotrophy, with possible consequences for trophic transfer in aquatic food webs exposed to Ag contamination.