In previous studies, cerium dioxide nanoparticles showed toxic effects towards environmental organisms. Size-dependent toxicity of cerium dioxide nanoparticles, meaning the smaller particles exhibited the stronger effects, was only found occasionally.
Microorganisms show different reactions towards cerium dioxide nanoparticles. Whereas ceria nanoparticles (CeO2) are not toxic for yeast or bacteria from sewage sludge, they cause toxic reactions or growth inhibition in various bacteria e.g. occurring in soil or those associated with plants or biogas production. However, the mechanism of particle-mediated toxicity is not yet fully understood [7,8,11-19].
Cerium dioxide nanoparticles disturb the growth and metabolic activity of algae size-dependently. This is either due to the cell wall being damaged by the nanoparticles or by impairing nutrient uptake. Polymer surface coating of ceria nanoparticles can minimise the nanoparticles' toxic effects and also the binding to natural organic material lowers the toxicity by reducing the particle binding on the cell surface [3,11,17,19-27].
Water fleas swimming in water containing cerium dioxide nanoparticles show no adverse effects with respect to mortality or mobility but there exist species-specific variations in sensitivity. In general, cerium dioxide nanoparticles attach themselves to the outer surface of the animals and are taken up via the gut. However, during development water fleas molt several times leading to a loss of adherent nanoparticles. Chronic exposure with ceria nanoparticles over 21 days caused mortality and thus a reduced survival rate of the water fleas. However, this was caused indirectly by the particles, as their presence in the gut restricted the food intake [2,3,7-10]. Brine shrimp were unaffected by cerium dioxide nanoparticles at environmentally relevant concentrations .
Mussels display no toxic effects when exposed to environmentally relevant concentrations (PEC value) of cerium dioxide nanoparticles present in the water and excrete them without showing any impact on their viability. However, some studies found indications for changes in digestive glands, the blood composition as well as immune cells in mussels and sea urchin, respectively [29-31].
Ceria nanoparticles caused oxidative damage leading to a reduced life expectancy in roundworms (nematodes). The growth during development was also inhibited. Upon coating with positively charged materials, toxicity of cerium dioxide nanoparticles increases due to enhanced uptake. Earthworms show alterations in gut and skin tissue after exposure to cerium dioxide nanoparticles. Humic acids, however, as present in soils, can reduce the toxicity [32-35].
After uptake from the water, cerium dioxide nanoparticles end up in the liver of zebrafish. Ingested cerium dioxide nanoparticles induced growth inhibition, impaired development and physiological functions in zebrafish and goldfish. However, neither isolated rainbow trout liver cells nor zebrafish embryos showed any adverse effects after exposure with ceria nanoparticles. Equally, all observed toxic effect caused by cerium dioxide particles occurred size-independent [1-7].
The effects of cerium dioxide nanoparticles on various plants were assessed by parameters such as germination, root growth and fruit growth. Many plants internalised the cerium dioxide nanoparticles into roots and shoots. Despite particle incorporation, most plants showed normal germination but an increase in root growth. In some plants, exposure to cerium dioxide nanoparticles leads to a reduction in fruit number. Notably, cerium dioxide nanoparticles influence the composition of plants (e.g. of seeds). Probably, a change in the activity of various metabolic enzymes is responsible for this effect. Further, it has been shown that cerium dioxide can be transferred and bioaccumulate along the terrestrial food chain: zucchini-crickets-spiders [36-51].
According to the European Union legislation cerium dioxide has been classified as potentially chronically harmful to crustaceans, and potentially chronically toxic to algae, while they were not recognised as very toxic .
In conclusion, the toxicity of cerium oxide nanoparticles towards environmental test organisms is considered to be low. However, in some cases the adsorption of cerium dioxide nanoparticles on the surface of the organism was found to be high which could potentially lead to some adverse sub-lethal effects.
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