Low Concentrations of Silver Nanoparticles in Biosolids Cause Adverse Ecosystem Responses under Realistic Field Scenario

<div><p>A large fraction of engineered nanomaterials in consumer and commercial products will reach natural ecosystems. To date, research on the biological impacts of environmental nanomaterial exposures has largely focused on high-concentration exposures in mechanistic lab studies with single strains of model organisms. These results are difficult to extrapolate to ecosystems, where exposures will likely be at low-concentrations and which are inhabited by a diversity of organisms. Here we show adverse responses of plants and microorganisms in a replicated long-term terrestrial mesocosm field experiment following a single low dose of silver nanoparticles (0.14 mg Ag kg⁻¹ soil) applied via a likely route of exposure, sewage biosolid application. While total aboveground plant biomass did not differ between treatments receiving biosolids, one plant species, <i>Microstegium vimeneum,</i> had 32 % less biomass in the Slurry+AgNP treatment relative to the Slurry only treatment. Microorganisms were also affected by AgNP treatment, which gave a significantly different community composition of bacteria in the Slurry+AgNPs as opposed to the Slurry treatment one day after addition as analyzed by T-RFLP analysis of 16S-rRNA genes. After eight days, N₂O flux was 4.5 fold higher in the Slurry+AgNPs treatment than the Slurry treatment. After fifty days, community composition and N₂O flux of the Slurry+AgNPs treatment converged with the Slurry. However, the soil microbial extracellular enzymes leucine amino peptidase and phosphatase had 52 and 27% lower activities, respectively, while microbial biomass was 35% lower than the Slurry. We also show that the magnitude of these responses was in all cases as large as or larger than the positive control, AgNO₃, added at 4-fold the Ag concentration of the silver nanoparticles.</p> </div

Silver fate in terrestrial mesocosms.

<p><b>A</b> Recovery of silver by ecosystem compartment after 50 days exposure to biosolid Slurry (white bars), Slurry+AgNPs (gray bars), or Slurry + AgNO₃ (black bars), and <b>B</b> EXAFS linear combination fit (k-space) of AgNPs after 15 minute exposure to biosolid slurry. In B, Lines indicate the data (black line), the linear combination fit (light gray dashed line), and the individual fit components Ag⁰ (gray line) and Ag₂S (dark gray line) are shown, and represent 75±2% and 25±6% percent of the silver, respectively. The model R-factor  =  0.0672, chi²  =  86.64, and the reduced chi²  =  0.4867 (parameters describing goodness of fit of the model to the data). Error bars in panel <b>A</b> are standard errors of the mean (n = 6). Since all treatments showed the same pattern in ANOVA post-hoc testing, differences for each treatment within each ecosystem compartment were denoted with brackets with letters, where shared letters denote no significant difference at p<0.05 between ecosystem compartments within a treatment</p

Mesocosm plant aboveground and belowground biomass affected by Ag.

<p><b>A</b> Aboveground plant biomass of <i>Microstegium vimineum</i>, <b>B</b> root biomass in 0–1 cm soils. Error bars are standard error of the mean, and shared letters denote no significant difference at p<0.05 between treatments differences (n = 6)</p

Microbial abundance, activity, and composition affected by Ag.

<p><b>A</b> Microbial biomass in 0–1 cm soils on Day 50 of the experiment; <b>B</b> N₂O flux from soil on day 8; <b>C</b> activity of the proteolytic extracellular enzyme leucine aminopeptidase (LAP), on day 50; <b>D</b> activity of the organophosphorous degrading enzyme phosphatase on day 50; <b>E</b> NMS ordination of bacterial community composition with day of experiment designated by shapes: Day 0 (triangles), Day 1 (squares), and 50 (circles); and treatment designated by colors: Control (white), Slurry (black), Slurry+AgNPs (gray), and Slurry+AgNO₃ (red). All error bars are standard error of the mean, and shared letters denote no significant difference at p<0.05 between treatments in panels A–D (n = 6)</p

TEM characterization of particles and aggregates.

<p>TEM image <b>A</b> of a mixture of primary particles and particle aggregates <b>B</b> particle size distribution, and <b>C</b> detailed view of AgNP aggregate. (<b>A</b> and <b>B</b> are adapted from reference 23, and reproduced by permission of the Copyright Clearance Center on behalf of Elsevier)</p