Considerations of Environmentally Relevant Test Conditions for Improved Evaluation of Ecological Hazards of Engineered Nanomaterials

Engineered nanomaterials (ENMs) are increasingly entering the environment with uncertain consequences including potential ecological effects. Various research communities view differently whether ecotoxicological testing of ENMs should be conducted using environmentally relevant concentrations-where observing outcomes is difficult-versus higher ENM doses, where responses are observable. What exposure conditions are typically used in assessing ENM hazards to populations? What conditions are used to test ecosystem-scale hazards? What is known regarding actual ENMs in the environment, via measurements or modeling simulations? How should exposure conditions, ENM transformation, dose, and body burden be used in interpreting biological and computational findings for assessing risks? These questions were addressed in the context of this critical review. As a result, three main recommendations emerged. First, researchers should improve ecotoxicology of ENMs by choosing test end points, duration, and study conditions-including ENM test concentrations-that align with realistic exposure scenarios. Second, testing should proceed via tiers with iterative feedback that informs experiments at other levels of biological organization. Finally, environmental realism in ENM hazard assessments should involve greater coordination among ENM quantitative analysts, exposure modelers, and ecotoxicologists, across government, industry, and academia.

Projected Dietary Intake of Zinc, Copper, and Cerium from Consumption of Carrot (Daucus carota) Exposed to Metal Oxide Nanoparticles or Metal Ions

The expanding production and use of engineered nanomaterials (ENMs) have raised concerns about the potential risk of those materials to food safety and human health. In a prior study, the accumulation of Zn, Cu, and Ce from ZnO, CuO, or CeO2, respectively, was examined in carrot (Daucus carota L.) grown in sand culture in comparison to accumulation from exposure to equivalent concentrations of ionic Zn(2+), Cu(2+), or Ce(4+). The fresh weight concentration data for peeled and unpeeled carrots were used to project dietary intake of each metal by seven age-mass classes from child to adult based on consumption of a single serving of carrot. Dietary intake was compared to the oral reference dose (oral RfD) for chronic toxicity for Zn or Cu and estimated mean and median oral RfD values for Ce based on nine other rare earth elements. Reverse dietary intake calculations were also conducted to estimate the number of servings of carrot, the mass of carrot consumed, or the tissue concentration of Zn, Cu, or Ce that would cause the oral RfD to be exceeded upon consumption. The projections indicated for Zn and Cu, the oral RfD would be exceeded in only a few highly unrealistic scenarios of exceedingly high Zn or Cu concentrations in the substrate from ZnO or CuO or consumption of excessive amounts of unpeeled carrot. The implications associated with the presence of Ce in the carrot tissues depended upon whether the mean or median oral RfD value from the rare earth elements was used as a basis for comparison. The calculations further indicated that peeling carrots reduced the projected dietary intake by one to two orders of magnitude for both ENM- and ionic-treated carrots. Overall in terms of total metal concentration, the results suggested no specific impact of the ENM form on dietary intake. The effort here provided a conservative view of the potential dietary intake of these three metals that might result from consumption of carrots exposed to nanomaterials (NMs) and how peeling mitigated that dietary intake. The results also demonstrate the potential utility of dietary intake projections for examining potential risks of NM exposure from agricultural foods.

Environmental Effects of Nanoceria on Seed Production of Common Bean (Phaseolus vulgaris): A Proteomic Analysis

The rapidly growing literature on the response of edible plants to nanoceria has provided evidence of its uptake and bioaccumulation, which delineates a possible route of entry into the food chain. However, little is known about how the residing organic matter in soil may affect the bioavailability and resulting impacts of nanoceria on plants. Here, we examined the effect of nanoceria exposure (62.5-500 mg/kg) on kidney bean (Phaseolus vulgaris) productivity and seed quality as a function of soil organic matter content. Cerium accumulation in the seeds produced from plants in organic matter enriched soil showed a dose-dependent increase, unlike in low organic matter soil treatments. Seeds obtained upon nanoceria exposure in soils with higher organic matter were more susceptible to changes in nutrient quality. A quantitative proteomic analysis of the seeds produced upon nanoceria exposure provided evidence for upregulation of stress-related proteins at 62.5 and 125 mg/kg nanoceria treatments. Although the plants did not exhibit overt toxicity, the major seed proteins primarily associated with nutrient storage (phaseolin) and carbohydrate metabolism (lectins) were significantly down-regulated in a dose dependent manner upon nanoceria exposure. This study thus suggests that nanoceria exposures may negatively affect the nutritional quality of kidney beans at the cellular and molecular level. More confirmatory studies with nanoceria along different species using alternative and orthogonal "omic" tools are currently under active investigation, which will enable the identification of biomarkers of exposure and susceptibility.

Penetration and Toxicity of Nanomaterials in Higher Plants

Nanomaterials (NMs) comprise either inorganic particles consisting of metals, oxides, and salts that exist in nature and may be also produced in the laboratory, or organic particles originating only from the laboratory, having at least one dimension between 1 and 100 nm in size. According to shape, size, surface area, and charge, NMs have different mechanical, chemical, electrical, and optical properties that make them suitable for technological and biomedical applications and thus they are being increasingly produced and modified. Despite their beneficial potential, their use may be hazardous to health owing to the capacity to enter the animal and plant body and interact with cells. Studies on NMs involve technologists, biologists, physicists, chemists, and ecologists, so there are numerous reports that are significantly raising the level of knowledge, especially in the field of nanotechnology; however, many aspects concerning nanobiology remain undiscovered, including the interactions with plant biomolecules. In this review we examine current knowledge on the ways in which NMs penetrate plant organs and interact with cells, with the aim of shedding light on the reactivity of NMs and toxicity to plants. These points are discussed critically to adjust the balance with regard to the risk to the health of the plants as well as providing some suggestions for new studies on this topic.

Uptake and accumulation of bulk and nanosized cerium oxide particles and ionic cerium by radish (Raphanus sativus L.)

The potential toxicity and accumulation of engineered nanomaterials (ENMs) in agricultural crops has become an area of great concern and intense investigation. Interestingly, although below-ground vegetables are most likely to accumulate the highest concentrations of ENMs, little work has been done investigating the potential uptake and accumulation of ENMs for this plant group. The overall objective of this study was to evaluate how different forms of cerium (bulk cerium oxide, cerium oxide nanoparticles, and the cerium ion) affected the growth of radish (Raphanus sativus L.) and accumulation of cerium in radish tissues. Ionic cerium (Ce(3+)) had a negative effect on radish growth at 10 mg CeCl3/L, whereas bulk cerium oxide (CeO2) enhanced plant biomass at the same concentration. Treatment with 10 mg/L cerium oxide nanoparticles (CeO2 NPs) had no significant effect on radish growth. Exposure to all forms of cerium resulted in the accumulation of this element in radish tissues, including the edible storage root. However, the accumulation patterns and their effect on plant growth and physiological processes varied with the characteristics of cerium. This study provides a critical frame of reference on the effects of CeO2 NPs versus their bulk and ionic counterparts on radish growth.

Dissolved cerium contributes to uptake of Ce in the presence of differently sized CeO2-nanoparticles by three crop plants

Cerium uptake into plants in the presence of CeO₂ nanoparticles occurs not only in nanoparticulate form, but also as dissolved ions.

Particle-size dependent accumulation and trophic transfer of cerium oxide through a terrestrial food chain

The accumulation and trophic transfer of nanoparticle (NP) or bulk CeO2 through a terrestrial food chain was evaluated. Zucchini (Cucurbita pepo L.) was planted in soil with 0 or 1228 μg/g bulk or NP CeO2. After 28 d, zucchini tissue Ce content was determined by ICP-MS. Leaf tissue from each treatment was used to feed crickets (Acheta domesticus). After 14 d, crickets were analyzed for Ce content or were fed to wolf spiders (family Lycosidae). NP CeO2 significantly suppressed flower mass relative to control and bulk treatments. The Ce content of zucchini was significantly greater when exposure was in the NP form. The flowers, leaves, stems, and roots of zucchini exposed to bulk CeO2 contained 93.3, 707, 331, and 119,000 ng/g, respectively; NP-exposed plants contained 153, 1510, 479, and 567 000 ng/g, respectively. Crickets fed NP CeO2-exposed zucchini leaves contained significantly more Ce (33.6 ng/g) than did control or bulk-exposed insects (15.0-15.2 ng/g). Feces from control, bulk, and NP-exposed crickets contained Ce at 248, 393, and 1010 ng/g, respectively. Spiders that consumed crickets from control or bulk treatments contained nonquantifiable Ce; NP-exposed spiders contained Ce at 5.49 ng/g. These findings show that NP CeO2 accumulates in zucchini at greater levels than equivalent bulk materials and that this greater NP intake results in trophic transfer and possible food chain contamination.

CeO₂ and ZnO nanoparticles change the nutritional qualities of cucumber (Cucumis sativus)

There is lack of information about the effects of nanoparticles (NPs) on cucumber fruit quality. This study aimed to determine possible impacts on carbohydrates, proteins, mineral nutrients, and antioxidants in the fruit of cucumber plants grown in soil treated with CeO2 and ZnO NPs at 400 and 800 mg/kg. Fourier transform infrared spectroscopy (FTIR) was used to detect changes in functional groups, while ICP-OES and μ-XRF were used to quantify and map the distribution of nutrient elements, respectively. Results showed that none of the ZnO NP concentrations affected sugars; however at 400 mg/kg, CeO2 and ZnO NPs increased starch content. Conversely, CeO2 NPs did not affect starch content but impacted nonreducing sugar content (sucrose). FTIR data showed changes in the fingerprint regions of 1106, 1083, 1153, and 1181, indicating that both NPs altered the carbohydrate pattern. ZnO NPs did not impact protein fractionation; however, CeO2 NPs at 400 mg/kg increased globulin and decreased glutelin. Both CeO2 and ZnO NPs had no impact on flavonoid content, although CeO2 NPs at 800 mg/kg significantly reduced phenolic content. ICP-OES results showed that none of the treatments reduced macronutrients in fruit. In case of micronutrients, all treatments reduced Mo concentration, and at 400 mg/kg, ZnO NPs reduced Cu accumulation. μ-XRF revealed that Cu, Mn, and Zn were mainly accumulated in cucumber seeds. To the best of the authors' knowledge this is the first report on the nutritional quality of cucumber fruit attributed to the impact of CeO2 and ZnO NPs.

Trophic transfer, transformation, and impact of engineered nanomaterials in terrestrial environments

Engineered nanomaterials (ENMs) are released into the environment with unknown implications in the food chain. Recent findings demonstrate that ENMs may accumulate and/or increase concentrations of the component metal or carbon nanomaterials in the fruits/grains of agricultural crops, have detrimental or beneficial effects on the agronomic traits, yield, and productivity of plants, induce modifications in the nutritional value of food crops, and transfer within trophic levels. Given this information, important questions needed to be resolved include a determination of actual or predicted concentrations of ENMs through the development of new and perhaps hybridized analytical tools, assessment of the nutritional content modifications and/or accumulation of ENMs, component metal, and cocontaminants in edible plants and their implications on human diet, nutrition, and health, assessment of the consequences of ENM-induced changes in soil health, physiological process, and yield on agricultural production and food security, and transfer of ENMs in trophic levels. Given the significant implications of ENMs exposure and the rather large knowledge gaps that exist, it will be prudent to observe judicious and targeted use of ENMs so as to minimize environmental release until a comprehensive environmental fate and effects assessment can be undertaken.