The effects of titanium dioxide nanoparticles have been studied in many plants and animals. They are thus among the most extensively tested nanoparticles. Both in vivo and in vitro data are available. Experiments have been carried out in various media (water, soil) and with different routes of exposure (water, food, blood, soil).
However, the studies are not easily comparable because the particle manufacturers and, thus, usually also the properties of titanium dioxide particles differ from study to study .
The rainbow trout as aquatic test organism has been very well studied and was confronted with titanium dioxide nanoparticles via the water, food, and the bloodstream. In coarser form (microscale), titanium dioxide particles have long been used in nutritional studies in fish and are considered nontoxic. TiO2 nanoparticles ingested via the food can be detected in the gills, intestine, liver, brain, and spleen. Thus, there is a systemic distribution of the particles in the body, but this had no effect on the health of the animals . Titanium dioxide nanoparticles taken up directly from the water are only slightly absorbed into the fish's body .
In another study, rainbow trout were injected with TiO2 nanoparticles directly into the bloodstream and the distribution in the organs was observed. A non-environmentally relevant exposure, however, it serves to clarify mechanisms of action and effects on the uptake of very high doses, e.g. for a possible industrial accident . The particles were enriched in the kidney and liver without affecting the functions of these important organs.
Zebrafish can also take up TiO2 particles from the water. The egg of fish embryos is not permeable for particles. If embryos are exposed to the particles in the presence of strong lighting, occurrence of malformations and increased mortality of the embryos is observed . This effect does not occur under normal lighting conditions and is therefore due to the photocatalytic properties of the TiO2 particles. Adult zebrafish showed no effects after exposure to TiO2; the gills showed no morphological changes [6,7]. However, there were changes in the activity of certain genes; these changes were partly consistent with those observed after copper and silver nanoparticle exposure.
Water fleas (Daphnia magna) are among the most commonly used test organisms. In the often used two-day test, in which the daphnia are exposed for 48 h to the particles, no or only minimal effects (mobility, mortality) were observed in several studies [6,8,9,10]. However, when the observation period was extended to 3-21 days, effects on molting and reproductive ability became apparent, some leading to the death of all test organisms [8,9,10]. These indirect toxic effects are due, on the one hand, to the attachment of particles on the exoskeleton (carapace) of the animals, on the other hand, to particle absorption in the intestine. The latter may inhibit food intake in the chronic tests .
Water flea accumulate titanium dioxide nanoparticles (black coloured areas) in their gut. © Zhu et al., 2010.
An important question in risk research is the extent to which a transfer of nanoparticles through the food chain takes place. In a "small" food chain, consisting of water fleas and zebrafish, it was shown that transfer of nanoparticles from TiO2-fed daphnids to zebrafish occurs .
For other freshwater and saltwater organisms (mussels, snails) titanium dioxide nanoparticles were not acutely toxic [12,13], but the activities of certain enzymes showed a response to particle exposure [13,14].
Lugworms living in marine sediments did not internalize particles via the skin or the gut into the body tissue . At very high concentrations, the worms’ food intake was reduced, a typical response to contaminants in the sediment. Also in high concentrations, TiO2 nanoparticles induced DNA and cell damage.
As an example of soil-dwelling organisms, woodlice were fed with titanium dioxide-soaked leaves. The nanoparticles had little influence on the metabolism and no effect on feed intake, body weight or mortality [16,17], although the concentrations used were very high. Similar to water fleas, however, longer duration of exposure had an influence on the effect of TiO2, an indication that unlike the commonly used short-term tests also chronic tests with longer exposure times should be performed. After 7 days of TiO2 exposure via the soil, a worm species showed DNA damage and evidence of oxidative stress, also in very high concentrations . Similar observations were made for a nematode; here, also growth and number of offspring was reduced .
TiO2 nanoparticles were tested on various plants. In the onion and in willow trees, the toxicity was low and all growth parameters were unchanged [20,21]. Another study examined tobacco and onion plants; here, high, non-environmentally relevant concentrations caused genotoxic effects . For a freshwater green alga exposed to 3 different TiO2 nanoparticles, growth inhibitory effects were observed, but these do not only depend on differences in particle sizes, but also on other characteristics such as different crystal structures . Further, it is unclear, whether the nanoparticles hinder the necessary light and thereby inhibit algae growth.
In conclusion, from the studies available so far, a low toxicity of titanium dioxide nanoparticles to environmental organisms can be derived. Effects were always observed at concentrations well above the predicted environmental concentrations (PEC value).
The particles are taken up, however, without a doubt in organisms and cells, so we must consider for the future, that the effects of very low concentrations of these substances over a longer period, as it would comply with the conditions in the environment, have not yet been adequately examined (in Daphnia and woodlice).
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