Emerging investigator series: prompt response of estuarine denitrifying bacterial communities to copper nanoparticles at relevant environmental concentrations

Abstract

The effects of metallic nanoparticles (NPs) on the estuarine biota have mostly been shown for concentrations higher than those actually measured or predicted in these environments. To address this gap, a range of concentrations expected to occur in estuarine environments (from 0.01 to 1 μg g⁻¹) was employed in microcosms studies to assess the impact of Cu NPs in the denitrification pathway. This was achieved by quantifying the gene expression in estuarine sediments exposed to Cu NPs for up to six days and potential denitrification rates. The expression of nitrite (nirS) and nitrous oxide (nosZ) reductase genes was enhanced in a timewise manner upon exposure to Cu NPs <50 nm. For 1 μg g⁻¹ Cu NPs, an increase in the gene expression could be seen immediately after 1 h of exposure and continued to be enhanced up to 7 h. For 0.01 μg g⁻¹ Cu NPs, an increase in the gene expression could only be seen after 4 h or 7 h of exposure; however, it continued to rise until 24 h. Concomitantly to the increased gene expression, the potential denitrification rate was increased by 30%. In any case, after 48 h, expression levels were no longer different from that of the non-exposed control. For the exposure to Cu NPs <150 nm, no change in the gene expression could be seen. Our results suggest that the deposition and adsorption of Cu NPs <50 nm in estuarine sediments promote the immediate and transient expression of key genes of the denitrification pathway. The long-term impact of continuous inputs of Cu NPs into estuaries deserves renewed analysis to account for their effects not just on the biota but especially on ecosystem functions.