Designer dots could shield food chain

UC SANTA BARBARA (US) — Researchers have for the first time been able to demonstrate how nanoparticles are able to biomagnify in a simple microbial food chain.

“This was a simple scientific curiosity,” says Patricia Holden, professor of environmental science and management at University of California, Santa Barbara.

“But it is also of great importance to this new field of looking at the interface of nanotechnology and the environment.”

In an earlier study, researchers observed that nanoparticles formed from cadmium selenide were entering certain bacteria (called Pseudomonas) and accumulating in them.

“We already knew that the bacteria were internalizing these nanoparticles from our previous study,” Holden says.

Holden approached colleagues who were working with a protozoan called Tetrahymena and nanoparticles and asked if they would be interested in a collaboration to evaluate how the protozoan predator is affected by the accumulated nanoparticles inside a bacterial prey.

For the new study, the scientists repeated the growth of the bacteria with quantum dots and coupled it to a trophic transfer study—the study of the transfer of a compound from a lower to a higher level in a food chain by predation.


The concentration of quantum dots (black bars), as measured by cadmium, increased from bacterial prey into the protozoan predator — an outcome called biomagnification.

“We looked at the difference to the predator as it was growing at the expense of different prey types—’control’ prey without any metals, prey that had been grown with a dissolved cadmium salt, and prey that had been grown with cadmium selenide quantum dots,” Holden says.

What they discovered is that the concentration of cadmium increased in the transfer from bacteria to protozoa and, in the process of increasing concentration, the nanoparticles were substantially intact, with very little degradation.

“We were able to measure the ratio of the cadmium to the selenium in particles that were inside the protozoa and see that it was substantially the same as in the original nanoparticles that had been used to feed the bacteria,” says Eduardo Orias, research professor of genomics.

The fact that the ratio of cadmium and selenide was preserved throughout the course of the study indicates that the nanoparticles were themselves biomagnified.

“Biomagnification—the increase in concentration of cadmium as the tracer for nanoparticles from prey into predator—this is the first time this has been reported for nanomaterials in an aquatic environment, and furthermore involving microscopic life forms, which comprise the base of all food webs,” Holden says.

An implication is that nanoparticles inside the protozoa could then be available to the next level of predators in the food chain, which could lead to broader ecological effects.

“These protozoa are greatly enriched in nanoparticles because of feeding on quantum dot-laced bacteria,” Holden says.

“Because there were toxic effects on the protozoa in this study, there is a concern that there could also be toxic effects higher in the food chain, especially in aquatic environments.

One of the missions of UC CEIN is to try to understand the effects of nanomaterials in the environment, and how scientists can prevent any possible negative effects that might pose a threat to any form of life.

“In this context, one might argue that if you could ‘design out’ whatever property of the quantum dots causes them to enter bacteria, then we could avoid this potential consequence,” Holden says.

“That would be a positive way of viewing a study like this. Now scientists can look back and say, ‘How do we prevent this from happening?’ ”

Researchers from University of California, Davis, University of California, Riverside, and Columbia University contributed to the research, that was funded in part by Environmental Protection Agency and by the UC Center for the Environmental Implications of Nanotechnology (UC CEIN).

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