Silver is known to be toxic for bacteria and aquatic organisms this being the reason why it is one of the most studied nanomaterials. Although most studies compare the effects of silver ions (originating form silver salts) with those of silver nanoparticles, it is difficult to distinguish between the effects of the particles and the dissolved ions. The question has not yet fully been answered whether nanoparticles or silver ions are more toxic to environment organisms.


The specific effect of silver nanoparticles on bacteria is lower than expected since the inhibitory effect is caused by released silver ions. By controlling the ion release (for example, by varying the coating or particle size), the strength of growth inhibition can be controlled. Silver nanoparticles attach themselves to the bacterial cell wall and cause damage, which ultimately leads to the death of the bacteria. The type of surface modification affects the binding of the nanoparticles to the bacteria. Similarly, organic materials (such as those found in the environment) bind to the particles and subsequently alter their effect on bacteria [1-6].


Biofilms consist of bacteria and other organisms, which are embedded in a gel-like substance and cover e.g. stones in rivers. The gel-like substance protects the organisms from silver nanoparticles, but not from silver ions. Humic acids, which are found in different proportions in natural waters, on the one hand bind silver ions and reduce the toxicity of particle suspensions. On the other hand, they also stabilise the particles, thereby facilitating the penetration of individual particles into the biofilm. Although the uptake of the silver nanoparticles into the biofilm is increased, the silver toxicity is reduced [8,9].


With regard to algae, it has not been finally clarified whether nanosilver or silver ions exert a stronger toxic effect. For green algae, silver is known to be toxic, as they are very sensitive to silver and quickly absorb the silver ions. Experiments with green algae showed no differences with respect to the toxic effect of silver nanoparticles and silver ions. This behavior has been attributed to ion liberation from the particles. In addition, the coating has a decisive influence on the toxic outcome [10-14].


Silver in general disrupts the salt metabolism of water fleas affecting not only the behaviour but also the movement of the animals.beetle Whether the silver nanoparticles or silver ions are more toxic to water fleas, is not yet clear due to conflicting results. Size and shape had no influence on the toxicity of silver nanoparticles, even though the animals ingested more small nanoparticles compared to larger particles. In contrast, the type of surface coating significantly affected the effect and uptake of nano silver. In response to nano silver and silver ions, different genes of the water fleas were affected, which probably reflects the differences in the uptake of two substances [10-13,15-20,23]. To simulate realistic environmental conditions, laboratory experiments with silver nanoparticles were conducted in the presence of natural waters ingredients, resulting in a reduced silver toxicity. It is believed that the silver nanoparticles release less silver ions under natural conditions [21,22].


In earthworms, nano silver and silver ions exerted the same toxicity. WurmBoth silver ions and silver nanoparticles show a very limited uptake behaviour by earthworms form soil and are quickly excreted afterwards. The worms show an avoidance behaviour and preferably stay in soils without silver nanoparticles, when exposed to soil with and without nanoparticles. Nematodes show a retarded growth on silver nanoparticles, which can be attributed in part to the silver ions. However, very high concentrations were used, which are not environmentally relevant [24-28].


Silver nanoparticles and silver ions are toxic and genotoxic for mussel cells and the ions cause stronger effects compared to the nanoparticles [29].


fishSilver nanoparticles exert concentration-dependent toxic and genotoxic effects to fish and cultured fish cells. Nano silver taken up via the water accumulates in gills, intestines and liver and was shown to cause lesions in the liver. Silver nanoparticles present in tank water trigger stress response and damage the gills thereby impairing oxygen uptake. Silver ions and silver nanoparticles show different mechanisms of action in fish. Equally the particle shape (e.g., spherical, rod-shaped) influences the toxic effect, which can be traced back to the number of so-called surface defects. The more often these defects occur in the surface structure of the particles, the more toxic they act. This also explains the difference in effect strengths of various coated nanoparticles [10,12,13,30-32,36-39].

Silver nanoparticles can attach to the shell of fish eggs with the number of particles depending on the salinity of the surrounding water. In seawater, less silver nanoparticles bind to the eggs compared to freshwater. Nano silver exposure results in a delayed development of fish embryos, malformations of the skeleton and organs, as well as a slower heartbeat [34,35].

For a "realistic" estimate, silver concentrations from laboratory experiments (with various organisms) were set in the context of actually occurring environmental concentrations. Based on these data, the silver concentrations in two Taiwanese rivers were predicted. These calculated values were found to be far below the concentration range identified as critical for fish in the laboratory experiments. From the current point of view, silver nanoparticles present no real risk for environmental organisms [40].


BlumeA comparison of 11 plant species revealed different sensitivities for silver nanoparticles and silver ions, as well as for the various surface coatings of nano silver. In young grass plants the silver nanoparticles, but not the ions caused a delayed root growth. This effect was greater, the smaller the particles were. In a long-term experiment, a community of plants and bacteria was fertilised with nano silver-containing sludge. Biomass was not affected by the nanoparticle treatment and only one of the plant species displayed reduced growth in presence of nanoparticle -containing sludge. Similar effects were observed in communities treated with silver ions [41,42,7].


The sensitivity of various organisms to silver nanoparticles is different. Filter feeders (e.g. water fleas, mussels) were found to be more sensitive than fish. For some of the studied organisms, silver nanoparticles and silver ions had different mechanisms of action. That could be an explanation for the observed differences in the effects of the two silver forms.


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