Plant growth and yield were modestly reduced but importantly, nitrogen fixation was almost entirely eliminated. Nodule content of ceria approached 11 mg kg−1 in some instances and electron microscopy confirmed the complete absence of symbiotic bacteria. Similarly, Hernandez-Viezcas et al.used synchrotron μXRF and μXANES to observe nanoceria within soybean root nodules and pods, although up to 20% had been transformed from CeIJIV) to Ce. However, the inhibition of bacterial nitrogen fixation did not necessarily result in nitrogen shortage for the plants; soybeans exposed to high doses of nanoceria actually grew better those exposed to low doses of nanoceria in the Priester study,suggesting that the plants successfully used an alternative source of nitrogen for growth. In a related study, Bandyopadhyaya et al.observed that nanoceria at 31–125 mg l−1 significantly inhibited the growth of Sinorhizobium meliloti, the primary symbiotic nitrogen fixing bacteria of alfalfa. The authors reported that the negative impact of nanoceria on nitrogen fixing bacteria resulted from nanoparticle adsorption on the extracellular surface and the subsequent alteration of certain surface protein structures. These changes could potentially affect colonization of symbiotic bacteria on root surfaces and therefore negatively impact plant nitrogen cycling. Notably, this study was conducted in cell culture and more investigation in soil-based systems will be needed.
In a final soil study, Morales et al.noted that nanoceria at concentrations up to 500 mg kg−1 had no impact on cilantro shoot biomass and in some instances,vertical rack increased root growth. However, the authors did report FTIR-detected changes in carbohydrate chemistry, which raises the potentialfor altered nutritional content in edible tissues. A recent study with rice confirmed that exposure of 500 mg nanoceria/kg soil throughout the life cycle of rice substantially altered the nutritional values of rice grains.For examples, the authors reported that nanoceria generally reduced the sulfur and iron content of rice grains and the extent of reduction depended upon the variety of rice types. The authors also reported the alteration of macromolecule contents in rice grains by nanoceria exposure, providing the first direct evidence on the mitigation of nutritional values of agricultural grains by nanoceria.Due to their small sizes, nanoparticles can move through the macro and microporosity of the soil and be detrimental for soil microbial communities.They may have an impact on soil microorganisms via a direct effect , changes in the bioavailability of toxicants or nutrients, indirect effects resulting from their interaction with natural organic compounds and interaction with toxic organic compounds which would amplify or alleviate their toxicity.
In two soils contaminated with nanoceria at 100 mg Ce/kg of dry soil, no significant effect on both microbial biomass C and N were observed after 60 days.However nanoceria decreased microbial C/N ratio and increased the metabolic quotient , probably due to microbial stress and changes in the composition of microbial communities inhabiting soil. They found that nanoceria were associated to small aggregates rich in both labile organic C, microbial biomass and clays, suggesting that nanoparticles can interact with most of microbial communities inhabiting soil.So far, the only two terrestrial organism to have been used to assess nanoceria soil toxicity are the earthworm Eisenia fetidia and the nematode Caenorhabditis elegans. Lahive et al. compared the toxicity of cerium salt and three different nanoceria to E. fetida in exposed in standard Lufa 2.2 soil. While median lethal concentration and effective concentration values of 317.8 and 294.6 mg Ce kg−1 were found for survival and reproduction , respectively, neither of these endpoints were affected by even the highest Cerium particle concentrations of 10 000 mg Ce kg−1 . The three nanoceria used varied in size ranges , with one larger particle and the cerium salt used as controls. However, there was a dose-dependent increase in cerium in the organisms at all exposure concentrations, and for all material types. With earthworms exposed to CeO2 particles interestingly having higher concentrations of total cerium compared to those exposed to ionic cerium, without exhibiting the same toxic effect.
Additionally, histological observations in earthworms exposed to the particulate forms of CeO2 showed cuticle loss from the body wall and some loss of gut epithelium integrity. The data overall suggesting that while nanoceria do not affect survival or reproduction in E. fetida over the relatively short standard test period, then there were histological changes that could indicate possible deleterious effects over longer term exposures. In contrast to E. fetida, then C. elegans is most often exposed in aquatic media rather than soil and so it is also often considered an aquatic toxicity testing organism.Roh et al. assessed the interaction between nanoceria and C. elegans and encountered a marked size-dependent effect on the fertility and survival of C. elegans. Zhang et al. evaluated the in vivo effects of a positively charged coated nanoceria on C. elegans at low concentrations . The results indicated that nanoceria induced ROS accumulation and oxidative damage in C. elegans, and finally lead to a significant decreased lifespan even at the exposure level of 0.172 μg l−1 . Collin et al.2 showed that the toxicity and bio-accumulation of coated nanoceria in C. elegans were dependent on the surface charge of the nanoceria. The positively charged nanoceria were significantly more toxic to C. elegans and bioaccumulated to a greater extent than the neutral and negatively charged nanoceria. They measured a LC50 of 15.5 mg l−1 for L1 stage C. elegans exposed during 24 h to the positively charged coated nanoceria.The presence of NOM has been shown to influence the bioavailability and toxicity of other nanoparticles.The presence of humic acid in the exposure media had been shown to influence Ce bio-accumulation in C. elegans exposed to positively charged coated nanoceria.2 Ce bio-accumulation was influenced by the ratio between HA and nanoceria. For a relevant scenario, i.e. when the concentration of HA was higher than the nanoceria concentration, Ce bio-accumulation decreased. Interestingly, for all tested concentration, the presence of HA in the exposure media significantly decreased the toxicity of nanoceria to C. elegans.
The decrease of toxicity was explained by the profound modifications induced by the adsorption of humic acid such as a change of the ZP or the formation of μ size aggregates, which were too large to be absorbed by C. elegans.First of all, the aggregation state appears to be an important parameter to consider when dealing with exposure of aquatic organisms to nanoceria due to their low solubility. On a large scale, aggregation/sedimentation of nanoceria in aquatic environments will leave a small portion of the total mass of nanoceria available for direct uptake by planktonic organisms , while the majority will be in contact with benthic organisms . In this case, sediments should be regarded as a sink for nanoceria discharged to the aquatic environment. Not only can the exposure pathway be different upon aggregation,vertical farming hydroponic but the mechanisms of internalization can also vary. Like the aggregation, the chemical stability of nanoceria can change in environmental biological pH/Eh conditions. Metals such as Ce exhibit various possible redox states IJCeIJIII), Ce) for which stability is a function of Eh and pH values. Intracellular Eh is controlled by metabolic processes as the oxidative phosphorylation in mitochondria. It is based on a series of redox reactions at near circumneutral pH for which potentials are in a – 0.32 to 0.29 V . Extracellular Eh is generally controlled by thiol/ disulfide redox systems for which Eh vary in a – 0.140/–0.08 V range. In such intra- and extra-cellular Eh conditions, Ce can be redox unstable which lead to electron exchange between nanoparticle surface and surrounding media. This could be the starting point of disequilibrium of the redox balance and then to oxidative stress toward micro- and macro-organisms. Regarding microorganisms, up to now, no undisputable evidence of nanoceria uptake by cells has been obtained. The nanoceria were either found in direct contact with the bacterial wall or trapped in the exopolysaccharides layer surrounding the microorganisms.For instance, studies have shown that Escherichia coli exposed to nanoceria in a simplified exposure media were covered by a thin and regular monolayer of nanoceria surrounding the cell wall. But for Synechocystis, nanoparticles could not form a shell at the cell surface because they were adsorbed onto the protecting layer of EPS bound to cell membranes. These nanoparticlestrapping EPS likely explains the higher level of nanoceria adsorption onto Synechocystis as compared to E. coli. Several studies have been conducted investigating toxicity in microorganisms. The toxicity of nanoceria was found to be strain- and size-dependent for E. coli and B. subtilis, whereas they did not affect S. oneidensis growth and survival.EC was near 5 mg l−1 for E. coli and ranged from 0.27 to 67.5 mg l−1 for Anabaena in pure water.Chronic toxicity to algae P. subcapitata with 10% effect concentrations between 2.6 and 5.4 mg l−1 was observed. Van Hoecke et al.observed that the presence of NOM decreased the toxicity of nanoceria to P. sub-capitata.
They assumed that the adsorption of NOM to the nanoceria surface prevented the particle from directly interacting with algal cells thereby decreasing the bio-availability of the particles. Under exposure to nanoceria, N. europaea cells show larger sedimentation coefficient than the control.The toxicity of nanoceria was either exerted by direct contact with cells,membrane damage,cell disruption,release of free CeIJIII).No oxidative stress response was detected with E. coli or B. subtilis, but nanoceria and CeCl3 alter the electron flow, and the respiration of bacteria.Pelettier et al.also observed the disturbance of genes involved in sulfur metabolism, and an increase of the levels of cytochrome terminal oxidase transcripts known to be induced by iron limitation. Rodger et al.also monitored the growth inhibition of P. sub-capitata and reported EC50 value of 10.3 mg l−1 of a 10- to 20 nm nanoceria. This inhibitory mode of action was mediated by a cell-particle interaction causing membrane damage and likely photochemically induced. Even if free Ce is toxic, release of Ce from the nanoceria did not explain by itself the toxicity observed in these studies . However, the reduction of the Ce into Ce at the surface of the nanoceria correlates with the toxicity. Using XANES at Ce L3-edge, Thill et al.and Auffan et al.98 showed that the cytotoxicity/genotoxicity of nanoceria could be related to the reduction of surface Ce atoms to Ce. But, further research is needed to find out whether the oxidative activity of ceria could be responsible. Regarding inverterbrates, one of the most favorable routes for nanoceria uptake by aquatic organisms is ingestion. For instance, ingestion via food chain was the main route of nanoceria uptake by the microcrustaceans Daphnia pulex. The adsorption of nanoceria on algae during the exposure to sub-lethal doses of nanoceria enhanced by a factor of 3 the dry weight concentration of Ce on the whole D. pulex. Nanoparticles were localized in the gut content, in direct contact with the peritrophic membrane,and on the cuticle.Interestingly, the depuration was not efficient to remove the nanoceria from the organisms. From 40% to 100% of the nanoceria taken up by D. pulex was not release after the depuration process. However, the authors demonstrated that the shedding of the chitinous exoskeleton was the crucial mechanism governing the released of nanoceria regardless of the feeding regime during exposure.Moreover, interspecific toxic effects of nanoceria toward daphnia were explained by morphological differences such as the presence of reliefs on the cuticle and a longer distal spine in D. similis acting as traps for the nanoceria aggregates. Acute ecotoxicity testings showed that D. similis was 350 times more sensitive to nanoceria than D. pulex with 48 h EC50 for D. similis about of 0.3 mg l−1 . 100 In addition, D. similis has a mean swimming velocity twice as fast as D. pulex and thus initially collide with twice more nanoceria aggregates. The effect of the exposure methods, direct and through sorption to phytoplankton was tested on the mussel Mytilus galloprovincialis. 101 Ce uptake was enhanced by the ingestion via the phytoplankton in the first 5 days of exposure but was similar to a direct exposure after 2 weeks. The authors showed that with increasing nanoceria concentration, mussels increased their clearance rates as well as the pseudofeces production in order to prevent the ingestion of nanoceria.