Researchers Measure Nanomaterials’ Trek Through Food Chain
A NIST study shows that although engineered nanomaterials can climb from the food chain's lowest rungs -- from single-celled organisms to multicelled ones -- the actual quantity of nanoparticles transferred is relatively small, and there is no indication that nanomaterials concentrate in the higher organisms, progressively accumulating.
Alexander E. Braun, Senior Editor -- Semiconductor International, 9/16/2008 10:59:00 AM
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The very properties that make engineered nanoparticles attractive for numerous applications — such as their biological and environmental stability, small size, solubility in aqueous solutions, and lack of toxicity to whole organisms — also raise concerns about their long-term environmental effects. This was the NIST effort’s driver. According to R. David Holbrook, the lead investigator, the group wanted to determine if nanoparticles can be passed up a model food chain and, if so, whether the transfer leads to a significant amount of bioaccumulation (the increase in concentration of a substance in an organism over time) and biomagnification (the progressive buildup of a substance in a predator organism after ingesting contaminated prey).
There is conclusive evidence that many man-made chemicals and heavy metals when released into the environment begin building up in the food chain. That is why we limit our consumption of things like canned tuna, because of the mercury that makes its way up through the food chain and gets accumulated in tuna, swordfish and other higher-level organisms.
“We wondered if the same thing could happen with nanomaterials,” Holbrook said. “So we chose simple systems consisting of very low trophic (nutrition) level organisms that form the foundation of some food chains. We wanted to find out if we had a very simple system and worked with a very well-known, well-characterized nanomaterial, whether we could get uptake from the liquid phase into the organism, and then transfer to a larger organism in a typical predator-prey ingestion process.” The research’s goal was to determine whether the quantum dots would begin to accumulate in the predator (the higher trophic organisms).
The researchers investigated the dietary accumulation, elimination and toxicity of two types of fluorescent quantum dots by establishing a simple, laboratory-based food chain using two microscopic aquatic organisms: Tetrahymena pyriformis, a single-celled ciliate protozoan, and the rotifer Brachionus calyciflorus that preys on it. The quantum dots were standard nanoparticles engineered to strongly fluoresce at specific wavelengths, thus revealing their presence in the two organisms.
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| The rotifer B. calyciflorus (seen whole in upper left), with the quantum dots (in red) it assimilated from the ciliates it ate. (Source: NIST) |
“There was uptake from the liquid phase of both types of quantum dots by T. pyriformis and they maintained their fluorescence even after the contaminated ciliates were ingested by the higher trophic level rotifers; in other words, the quantum dots were transferred intact across the food chain,” Holbrook said. Although the group determined that there is transfer during ingestion of the nanoparticle from the ciliate into the rotifer, which is the predator, no biomagnification, or any significant accumulation, of the particles was found. “It appears that when the ciliate and the rotifer take these things in they do end in their body cavity, but they are excreted fairly quickly, so there’s really no chance for a buildup.” Significantly, no toxicity was detected, which led Holbrook to conclude that at least with this simple system, toxicity does not occur with these organisms and these nanomaterials. “We did not detect any gross toxicity either. In other words, the organisms didn’t die.”
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| The rotifer’s prey, a ciliate T. pyriformis seen during cell division, with the accumulated quantum dots appearing red. (Source: NIST) |
Holbrook indicated that the results obtained were considered logical for the organisms used. “We’re talking about colloidal nanoparticles,” he said. “The way that these digestive guts have evolved has made them very good at taking up soluble ions, but not so good with colloidal phase, or nanoparticles. So the fact that they were excreted pretty quickly made sense, once we started putting all the evidence together.”
Although important, the results cannot be extrapolated to higher species. Because of the different kinds of digestive systems and absorption mechanisms, the outcome could be very different. “We’re looking for partners to collaborate with these issues, to observe higher-level organisms,” Holbrook said. “The nanomaterials we used are sensitive to pH changes, so if you have a stomach that is a very acidic environment for several hours, the kind of quantum dots we used might begin to degrade. Then they would no longer be nanoparticles, but soluble elements. In such a case, the risk from nanoparticles per se, goes down; however, the nanoparticles we worked with contain fadmium at their core, which would mean that dissolved cadmium, which is toxic, would be released.” So it may be that the nanoparticle itself is not an issue, but what happens if it degrades, releasing toxic ions, is. These are matters that the researchers will address once they find an adequate partner. The plan is to revisit the process using fish.
As Holbrook put it, “We know a little bit more than we did before, and what we’ve learned doesn’t look bad.”
