The effect of biogenic and chemically manufactured silver nanoparticles on the benthic bacterial communities in river sediments.

This study was conducted to determine and compare the effect of chemically-synthesised and biogenic silver nanoparticles on the benthic bacterial community structure in mesocosms containing sediment from three rivers in geographical sites with different population densities (low, medium, high), and therefore likely to be associated with respective low, moderate and high degrees of anthropogenic input. The nanoparticles were applied at the upper limit expected to accumulate in impacted environments (4 μg kgsed -1 ). The biomass, concentrations of elements, including selection metals (P, K, Na, K, Ca, Mg, Zn, Cu, Al, Ag) were all significantly higher at the high density than at the low density sites. Bacterial community profiling (terminal restriction fragment length polymorphism and amplicon sequencing) showed that the bacterial community structure in the sediments from the high population density site were resilient to environmental perturbations [adjustment from in-situ to ex-situ (laboratory) conditions], as well as to exposure to silver nanoparticles, with the converse being true for the low population density site. Results obtained from amplicon sequencing were interrogated to the lowest taxonomic level with a relative abundance >5%. Proteobacteria was the most abundant phylum in all the sediments. Notable resistance (increased relative abundance) to one or both forms of silver nanoparticles was seen in the class Thermoleophilia, and the orders Myxococcales, Bacteriodales, Pirellules CCU21 and iii 1-15 (class Acidobacteria 6). Conversely, sensitivity was demonstrated in the family Koribacteraceae and the orders Rhizobiales, Ellin 329 and Gemmatales. It is recommended that pro-active environmental monitoring is performed in aquatic systems receiving point source pollution from wastewater treatment plants in order to assess the accumulation of silver nanoparticles. If necessary, measures should be implemented to mitigate the entry of silver nanoparticles, especially into more vulnerable environments.