Multi-walled carbon nanotubes appear to have long-term effects on biological communities in freshwater sediment, even at low concentrations. An experiment under natural field conditions has shown that the composition of the aquatic community changes following long-term exposure to these miniscule particles. This study was conducted by Ilona Velzeboer, researcher at Wageningen University and the IMARES research institute of Wageningen UR. She published her research in the leading journal Environmental Science & Technology.
“It was already known that nanoparticles can cause adverse effects on organisms”, explains Velzeboer. “However, until now these effects were studied primarily in the laboratory with single species toxicity tests, and the effects were seen only with much higher concentrations of nanoparticles than those that are expected to be found in the environment. No research had been conducted under realistic, ecologically relevant conditions – as they occur in the field”. According to Velzeboer, no researchers had yet examined the long-term effects at such low concentrations. “The fact that these effects indeed exist comes as a surprise; they are possibly related to the reproduction and life cycle of the organisms. With shorter experiments, these aspects usually do not play a role”.
Estimating the risks
The EU and the Dutch government have acknowledged the potential problem of nanoparticles and have commissioned research into their potential risks. The new results can be very important, not only for estimating ecologically relevant risks, but also for establishing priorities for future research. A growing body of research has shown that it is especially the sediments in rivers and lakes where nanoparticles accumulate. “Consequently, it would now appear to be advisable to conduct long-term studies with multiple generations of organisms”, adds Bart Koelmans, leader of the research team to which Velzeboer belongs. For example, the researchers believe that more field studies are required to determine the effects of other types of nanoparticles, such as metallic ones. Furthermore, they argue that it is important to determine the mechanism of action of nanoparticles in living organisms.
Koelmans' team applied a method that had previously been used in the field to ascertain the ecological effects of activated carbon, which was being used for the remediation of contaminated sediment. In the present study authored by Velzeboer, various concentrations of carbon nanotubes were added to aquariums with clean sediment and then buried in a drainage ditch in the field. After periods of 3 and 15 months, the researchers determined whether organisms from the environment had begun to live in the aquariums, and they identified and counted these organisms. Statistical techniques were then used to determine the significance of these results and their possible causes. Besides nanoparticles, other factors could have affected the animals’ abundance and diversity, such as the presence of water plants, spatial variation or the duration of exposure. It is probable that all these factors play a role, and that each of them could explain part of the effect. “In this case, the carbon nanoparticles explained slightly less than 10% of the total change in the composition of the biological community in the sediment, i.e. the benthic community”, explains Bart Koelmans. “The magnitude of this effect is comparable to some natural factors. The effect was the same as that of activated carbon in a previous experiment, but in that case the concentration was 50 times higher. This could mean that the aquatic organisms are much more sensitive to nanotubes than to activated charcoal”.