Speciation Matters: Bioavailability of Silver and Silver Sulfide Nanoparticles to Alfalfa (Medicago sativa)

Abstract

Terrestrial crops are directly exposed to silver nanoparticles (Ag-NPs) and their environmentally transformed analog silver sulfide nanoparticles (Ag2S-NPs) when wastewater treatment biosolids are applied as fertilizer to agricultural soils. This leads to a need to understand their bioavailability to plants. In the present study, the mechanisms of uptake and distribution of silver in alfalfa (Medicago sativa) were quantified and visualized upon hydroponic exposure to Ag-NPs, Ag2S-NPs, and AgNO3 at 3 mg total Ag/L. Total silver uptake was measured in dried roots and shoots, and the spatial distribution of elements was investigated using transmission electron microscopy (TEM) and synchrotron-based X-ray imaging techniques. Despite large differences in release of Ag(+) ions from the particles, Ag-NPs, Ag2S-NPs, and Ag(+) became associated with plant roots to a similar degree, and exhibited similarly limited (<1%) amounts of translocation of silver into the shoot system. X-ray fluorescence (XRF) mapping revealed differences in the distribution of Ag into roots for each treatment. Silver nanoparticles mainly accumulated in the (columella) border cells and elongation zone, whereas Ag(+) accumulated more uniformly throughout the root. In contrast, Ag2S-NPs remained largely adhered to the root exterior, and the presence of cytoplasmic nano-SixOy aggregates was observed. Exclusively in roots exposed to particulate silver, NPs smaller than the originally dosed NPs were identified by TEM in the cell walls. The apparent accumulation of Ag in the root apoplast determined by XRF, and the presence of small NPs in root cell walls suggests uptake of partially dissolved NPs and translocation along the apoplast.

Elucidating the mechanisms for plant uptake and in-planta speciation of cerium in radish (Raphanus sativus L.) treated with cerium oxide nanoparticles

“…Consistent with CeO 2 NPs studies, previous research has shown that lettuce root exudates improved the solubilization of TiO 2 NPs and Fe 3 O 4 NPs in the rhizosphere [33]. Stegemeier et al also found that the root exudates of Alfalfa (Medicago sativa L.) could partially dissolve Ag 2 S-NPs, making them more bioavailable to plants [34]. In this in vitro study, we showed for the first time that the enhanced dissolution of CeO 2 NPs is primarily due to the organic acids with low molecular weight in root exudates (e.g.…”

The toxicity of silver to soil organisms exposed to silver nanoparticles and silver nitrate in biosolids‐amended field soil

“…Although speciation techniques applied to the bulk soil suggested complete sulfidization, the researchers hypothesized that the amorphous, nano‐scale Ag 2 S and organically complexed Ag‐thiol transformation products likely presented more phyto‐available Ag than crystalline or micron‐scale Ag 2 S, particularly given further transformations likely to occur within the rhizosphere . Furthermore, there is increasing evidence that dispersed, Ag(0)‐enriched nanoparticles and nano‐sized Ag 2 S can accumulate within plant tissue and that Ag is more phyto‐available in soils amended with biosolids from WWTPs contaminated with engineered AgNPs than with more soluble (i.e., AgNO 3 ) inputs .…”

Ecotoxicity and fate of a silver nanomaterial in an outdoor lysimeter study

“…The AgNM uptake by plants was reviewed by Gardea-Torresdey et al (2014) who discussed the possibility of AgNM enrichment in the food chain. However, Stegemeier et al (2015) conducted hydroponic exposure of alfalfa and indicated AgNM accumulation along the apoplasts of the roots, but only a low translocation to the shoot system. The current results likewise show root uptake but no accumulation of AgNM in the grains of wheat and canola (Fig.…”

“…However, due to the high retention of AgNM in soil, as shown in the fate part of the experiment, this can be eliminated (Cornelis et al 2014; Hoppe et al 2015). Moreover, translocation of AgNM in the plant roots (Lowry et al 2012; Stegemeier et al 2015) even over a test period of over 2 years and reduction of the AgNM concentration due to the uptake into the plant biomass cannot explain the effects.…”