title: 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.
authors.forename: John P
authors.initials: JP
authors.lastname: Stegemeier
authors.forename: Fabienne
authors.initials: F
authors.lastname: Schwab
authors.affiliation: ∥Department of Biology, Duke University, Durham, North Carolina 27708, United States.
authors.forename: Benjamin P
authors.initials: BP
authors.lastname: Colman
authors.affiliation: ⊥Stanford Synchrotron Radiation Lightsource, Stanford University, Menlo Park, California 94025, United States.
authors.forename: Samuel M
authors.initials: SM
authors.lastname: Webb
authors.affiliation: #Center for Advanced Radiation Sources, The University of Chicago, Chicago, Illinois 60637, United States.
authors.forename: Matthew
authors.initials: M
authors.lastname: Newville
authors.affiliation: #Center for Advanced Radiation Sources, The University of Chicago, Chicago, Illinois 60637, United States.
authors.forename: Antonio
authors.initials: A
authors.lastname: Lanzirotti
authors.affiliation: ⊗ICTAS Nanoscale Characterization and Fabrication Laboratory, Virginia Tech, Blacksburg, Virginia 24061, United States.
authors.forename: Christopher
authors.initials: C
authors.lastname: Winkler
authors.forename: Mark R
authors.initials: MR
authors.lastname: Wiesner
authors.forename: Gregory V
authors.initials: GV
authors.lastname: Lowry
mesh_terms: D001682:Biological Availability; D001692:Biological Transport; D003593:Cytoplasm / Q000187:drug effects / Q000378:metabolism; D004785:Environmental Pollutants / Q000493:pharmacokinetics*; D000455:Medicago sativa / Q000187:drug effects* / Q000378:metabolism; D053768:Metal Nanoparticles / Q000737:chemistry; D046529:Microscopy, Electron, Transmission; D018517:Plant Roots / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism; D012834:Silver / Q000032:analysis / Q000493:pharmacokinetics*; D018030:Silver Compounds / Q000493:pharmacokinetics*; D012987:Soil; D013052:Spectrometry, X-Ray Emission; D062065:Wastewater / Q000737:chemistry
chemical_list: D004785:Environmental Pollutants; D012834:Silver; D018030:Silver Compounds; D012987:Soil; D062065:Wastewater; C013251:silver sulfide