Cerium oxide nanoparticles can be taken up from the water into the livers of zebra fish [1]. However, only a very small part of the CeO2 present in the water reaches internal organs. Hence it was concluded that fish receive only few nanoparticles from the water.
Water fleas swimming for 48 or 96 h in water containing CeO2 showed no effect with respect to mortality or mobility [2,3,4,7]. A chronic test with a duration of 21 days led to a reduced survival of the water fleas. The mortality was, however, caused indirectly by the particles, as their presence in the gut restricted the food intake of the fleas. Isolated liver cells of rainbow trout [2] and zebrafish embryos [4] exposed to these nanoparticles showed no negative effects. Here, nano-scaled CeO2 particles showed no higher toxicity than micro-scale particles of the same material.
CeO2 can bind to the outer membrane of bacteria [5]. Two different types of bacteria and sewage sludge bacteria exposed to CeO2 particles showed no signs of toxicity [3,6,7]. For some bacterial species a toxic effect was observed, if the particles were chemically modified by unknown compounds released by the bacteria.
However, toxicity occurred only in water, whereas bacteria cultivated in normal growth medium were protected. The mechanism of particle-mediated toxicity is not fully understood [5,8]. Experiments with cyanobacteria, which were protected by their very thick cell wall, confirmed that for a toxic effect a direct contact between particles and bacterial membranes is necessary [8].
Likewise, in algae a direct contact of particles and cells leads to a toxic effect. A reduction of algae growth, caused by membrane damage was observed. This damage is probably based on the photocatalytic activity of the particles, which is stronger for nanoparticles than for larger particles. A further explanation could be the depletion of phosphate, which is vital for the algae, by the nanoparticles. This hypothesis is supported by the observation that nanoparticles have a much higher binding capacity than coarser particles [9]. Another study found no toxicity for micrometer CeO2 particles, but a growth reduction with three different sizes of nanoparticles (14, 20 and 29 nm). Growth reduction correlated with the size of the particles, with the higher the reduction the smaller the particles and the higher the particle surface area [4]. Another study, albeit with a very short exposure period, found no effects on the growth of algae [7].
Seeds of the soybean showed a normal germination in the presence of CeO2 particles. They even showed an increased root growth and were able to incorporate particles into their roots [10]. Very high concentrations of CeO2 can induce genotoxic effects (see Glossary) in the plants.
Within the studies discussed here the question
to which extent toxic effects of nanoparticles are related to their large surface area is answered quite differently. A higher toxicity of smaller particles with a high particle surface is likewise described [9] as no effects of the particle surface [3,4]. In conclusion, the toxicity of cerium oxide nanoparticles towards environmental organisms is considered to be low; a size effect, or a dependency of the toxic effects of the particle surface is occasionally described.
Literature
1. Johnston, B.D. et al, 2010, Environ. Sci. & Technol, 44, 1144-51.
2. Gaiser B.K. et al., 2009, Environmental Health, 8 (Suppl I), S2.
3. Park B. et al., 2007, PFT, 4, 12.
4. van Hoecke, K. et al., 2009, Environ. Sci. & Technol, 43, 4537-4546.
5. Thill A. et al., 2006, Environ Sci & Technol, 40, 6151-6156.
6. Limbach L.K. et al., 2008, Environ. Sci. & Technol, 42, 5828-5833.
7. Velzeboer I. et al., 2008, ETC, 27, 1942.1947.
8. Zeyons O. et al., 2009, Nanotoxicology, 3, 284-295.
9. Rogers, N.J. et al.,2009, Environ Chem, 7, 50-60.
10. Lopez-Moreno M.L. et al., 2010, Environ Sci Technol, 44, 7315-7320.
