EFFECTS FROM FILTRATION, CAPPING AGENTS, AND PRESENCE/ABSENCE OF FOOD ON THE TOXICITY OF SILVER NANOPARTICLES TO \u3ci\u3eDAPHNIA MAGNA\u3c/i\u3e

Relatively little is known about the behavior and toxicity of nanoparticles in the environment. Objectives of work presented here include establishing the toxicity of a variety of silver nanoparticles (AgNPs) to Daphnia magna neonates, assessing the applicability of a commonly used bioassay for testing AgNPs, and determining the advantages and disadvantages of multiple characterization techniques for AgNPs in simple aquatic systems. Daphnia magna were exposed to a silver nitrate solution and AgNPs suspensions including commercially available AgNPs (uncoated and coated), and laboratory-synthesized AgNPs (coated with coffee or citrate). The nanoparticle suspensions were analyzed for silver concentration (microwave acid digestions), size (dynamic light scattering and electron microscopy), shape (electron microscopy), surface charge (zeta potentiometer), and chemical speciation (X-ray absorption spectroscopy, X-ray diffraction). Toxicities of filtered (100 nm) versus unfiltered suspensions were compared. Additionally, effects from addition of food were examined. Stock suspensions were prepared by adding AgNPs to moderately hard reconstituted water, which were then diluted and used straight or after filtration with 100-nm filters. All nanoparticle exposure suspensions, at every time interval, were digested via microwave digester and analyzed by inductively coupled argon plasma–optical emission spectroscopy or graphite furnace– atomic absorption spectroscopy. Dose–response curves were generated and median lethal concentration (LC50) values calculated. The LC50 values for the unfiltered particles were (in μ/L): 1.1±0.1-AgNO3; 1.0±0.1-coffee coated; 1.1±0.2-citrate coated; 16.7±2.4 Sigma Aldrich Ag-nanoparticles (SA) uncoated; 31.5±8.1 SA coated. LC50 values for the filtered particles were (in μ/L): 0.7±0.1- AgNO3; 1.4±0.1-SA uncoated; 4.4±1.4-SA coated. The LC50 resulting from the addition of food was 176.4±25.5-SA coated. Recommendations presented in this study include AgNP handling methods, effects from sample preparation, and advantages/ disadvantages of different nanoparticle characterization techniques

Impact of Heavy Metals on Transcriptional and Physiological Activity of Nitrifying Bacteria

Heavy metals can inhibit nitrification, a key process for nitrogen removal in wastewater treatment. The transcriptional responses of <i>amoA</i>, <i>hao</i>, <i>nirK</i>, and <i>norB</i> were measured in conjunction with specific oxygen uptake rate (sOUR) for nitrifying enrichment cultures exposed to different metals (Ni­(II), Zn­(II), Cd­(II), and Pb­(II)). There was significant decrease in sOUR with increasing concentrations for Ni­(II) (0.03–3 mg/L), Zn­(II) (0.1–10 mg/L), and Cd­(II) (0.03–1 mg/L) (<i>p</i> < 0.05). However, no considerable changes in sOUR were observed with Pb­(II) (1–100 mg/L), except at a dosage of 1000 mg/L causing 84% inhibition. Based on RT-qPCR data, the transcript levels of <i>amoA</i> and <i>hao</i> decreased when exposed to Ni­(II) dosages. Slight up-regulation of <i>amoA</i>, <i>hao</i>, and <i>nirK</i> (0.5–1.5-fold) occurred after exposure to 0.3–3 mg/L Zn­(II), although their expression decreased for 10 mg/L Zn­(II). With the exception of 1000 mg/L Pb­(II), stimulation of all genes occurred on Cd­(II) and Pb­(II) exposure. While overall the results show that RNA-based function-specific assays can be used as potential surrogates for measuring nitrification activity, the degree of inhibition inferred from sOUR and gene transcription is different. We suggest that variations in transcription of functional genes may supplement sOUR based assays as early warning indicators of upsets in nitrification

Toxicity and Transcriptomic Analysis in <i>Hyalella azteca</i> Suggests Increased Exposure and Susceptibility of Epibenthic Organisms to Zinc Oxide Nanoparticles

Nanoparticles (NPs) are expected to make their way into the aquatic environment where sedimentation of particles will likely occur, putting benthic organisms at particular risk. Therefore, organisms such as <i>Hyalella azteca</i>, an epibenthic crustacean which forages at the sediment surface, is likely to have a high potential exposure. Here we show that zinc oxide (ZnO) NPs are more toxic to <i>H. azteca</i> compared with the corresponding metal ion, Zn²⁺. Dissolution of ZnO NPs contributes about 50% of the Zn measured in the ZnO NP suspensions, and cannot account for the toxicity of these particles to <i>H. azteca</i>. However, gene expression analysis is unable to distinguish between the ZnO NP exposures and zinc sulfate (ZnSO₄) exposures at equitoxic concentrations. These results lead us to hypothesize that ZnO NPs provide an enhanced exposure route for Zn²⁺ uptake into <i>H. azteca</i>, and possibly other sediment dwelling organisms. Our study supports the prediction that sediment dwelling organisms are highly susceptible to the effects of ZnO NPs and should be considered in the risk assessment of these nanomaterials

Toxicogenomic Responses of Nanotoxicity in <i>Daphnia magna</i> Exposed to Silver Nitrate and Coated Silver Nanoparticles

Applications for silver nanomaterials in consumer products are rapidly expanding, creating an urgent need for toxicological examination of the exposure potential and ecological effects of silver nanoparticles (AgNPs). The integration of genomic techniques into environmental toxicology has presented new avenues to develop exposure biomarkers and investigate the mode of toxicity of novel chemicals. In the present study we used a 15k oligonucleotide microarray for <i>Daphnia magna</i>, a freshwater crustacean and common indicator species for toxicity, to differentiate between particle specific and ionic silver toxicity and to develop exposure biomarkers for citrate-coated and PVP-coated AgNPs. Gene expression profiles revealed that AgNO₃ and AgNPs have distinct expression profiles suggesting different modes of toxicity. Major biological processes disrupted by the AgNPs include protein metabolism and signal transduction. In contrast, AgNO₃ caused a downregulation of developmental processes, particularly in sensory development. Metal responsive and DNA damage repair genes were induced by the PVP AgNPs, but not the other treatments. In addition, two specific biomarkers were developed for the environmental detection of PVP AgNPs; although further verification under different environmental conditions is needed