The apparent harmlessness of carbon nanotubes has so far kept them outside the growing debate about the potential hazards of nanotechnology. Scientists believed that the structures were insoluble in water, and they appeared to offer no threat to biological organisms. But that picture seems to be changing. Recent research has suggested that nanotubes may in fact have some solubility in water, and a recent paper in Nature Nanotechnology suggests that nanotubes clearly interact with certain protozoa. Unfortunately, the protozoa in question are not just laboratory curiosities, but rather organisms that are vital to biological treatment of waste water and possibly necessary for controlling bacteria populations in nature.

Researchers at the University of Waterloo exposed the protozoa tetrahymena thermophila—a rather voracious bacteriophage—to single-walled carbon nanotubes. The organisms exhibited a range of responses, depending on the concentration of nanotubes, varying from reduced mobility to clumping into clusters to dying outright. In most cases, the protozoa ingested the nanotube particles, which was of concern to researchers.

This taste for the unusual suggests that the protozoa could concentrate nanotube fragments from the environment and pass them up the food chain, with completely unknown consequences. Another serious issue is that, in most cases, the protozoa’s ability to ingest bacteria decreased after they came in contact with the nanotubes. In, for example, sewage-treatment plants that use tetrahymena, the consequences are obviously negative. But the researchers pointed out that, if nanotube fragments somehow entered a natural environment, the result might be to eliminate a major control on bacterial populations, again with unknown results.

These results do not predict an ecological catastrophe. Rather, as the researchers point out, they turn a spotlight on our near-total ignorance of the interaction between nanoparticles in general and biological systems in general. And they show one instance in which scientists could build a disaster scenario upon an interaction they demonstrated in the lab. Given this data and the absence of almost any other knowledge, it might be just as well to treat nanoparticles as ecologically dangerous and handle them accordingly.