Abstract
Silver nanoparticles (AgNPs) enter estuaries via wastewater treatment effluents, where they can inhibit microorganisms, because of their antimicrobial properties. Ammonia-oxidising bacteria (AOB) and archaea (AOA) are involved in the first step of nitrification and are important to ecosystem function, especially where effluent discharge results in high nitrogen inputs. Here, we investigated the effect of a pulse addition of AgNPs on AOB and AOA ammonia monooxygenase (amoA) gene abundances and benthic nitrification potential rates (NPR) in low-salinity and mesohaline estuarine sediments. Whilst exposure to 0.5 mg L-1 AgNPs had no significant effect on amoA gene abundances or NPR, 50 mg L-1 AgNPs significantly decreased AOB amoA gene abundance (up to 76% over 14 days), and significantly decreased NPR by 20-fold in low-salinity sediments and by twofold in mesohaline sediments, after one day. AgNP behaviour differed between sites, whereby greater aggregation occurred in mesohaline waters (possibly due to higher salinity), which may have reduced toxicity. In conclusion, AgNPs have the potential to reduce ammonia oxidation in estuarine sediments, particularly where AgNPs accumulate over time and reach high concentrations. This could lead to long-term risks to nitrification, especially in polyhaline estuaries where ammonia-oxidation is largely driven by AOB.
Highlights
A recent explosion in the use of nanotechnology in consumer products has increased concerns regarding the adverse effects of nanoparticles on the environment and human health (Nowack et al, 2011; Richardson and Ternes, 2011)
The AgNPs were more soluble in the mesohaline (Wivenhoe) water compared to the low-salinity (Hythe) water (554 versus 72 μg L-1 free silver by day 25, respectively), while ultra-high purity UHP water fell approximately midway between the two (Fig. 2A and B)
One possible explanation for the observed sensitivity of ammonia-oxidizing bacteria (AOB) towards AgNPs is that there are significantly different predicted structures of archaeal AmoB and bacterial AmoB. Since it is the AmoB subunit of ammonia monooxygenase (AMO) that is thought to harbor the active site in ammonia oxidizers (Lieberman and Rosenzweig 2005; Balasubramanian et al, 2010), it is possible that there are differences in ammonia oxidation functionality of this protein in AOA compared to AOB (Walker et al, 2010)
Summary
A recent explosion in the use of nanotechnology in consumer products has increased concerns regarding the adverse effects of nanoparticles (i.e. particles between 1-100 nm) on the environment and human health (Nowack et al, 2011; Richardson and Ternes, 2011). AgNPs or silver ions (Ag+) may be released directly (e.g. from washing AgNP-containing textiles) or indirectly (e.g. leaching from nanosilver-enhanced products) into rivers and estuaries (Cleveland et al, 2012; Sun et al, 2014). One major potential route for AgNP entry into aquatic environments is via wastewater treatment plants (WWTPs), where ammonia-oxidizing microbial communities are important for nitrogen removal. Little is currently known about the effect of AgNPs on AOB and AOA communities in receiving waters and sediments, where the fate, behaviour and toxicity of AgNPs may be vastly different to that in WWTP sludge (Fabrega et al, 2011)
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