Given the recent uproar over cadmium in our local (British Columbia) oysters, this new study about nanoparticles formed from cadmium selenide and their possible absorption into the aquatic food chain caught my attention. From the Dec. 20, 2010 news item on physorg.com,
“We already knew that the bacteria were internalizing these nanoparticles from our previous study,” Holden [Patricia] said. “And we also knew that Ed (Orias) and Rebecca (Werlin) were working with a protozoan called Tetrahymena and nanoparticles. So we approached them 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.” Orias and Werlin credit their interest in nanoparticle toxicity to earlier funding from and participation in the University of California Toxic Substance Research & Training Program.
The scientists repeated the growth of the bacteria with quantum dots in the new study and 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. “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 said.
What they found was 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,” Orias said.
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 said.
The scientists involved with the study are also associated with the University of California Center for Environmental Implications of Nanotechnology (UC CEIN).
As for the local oyster/cadmium situation (from a Dec. 14, 2010 article by Larry Pynn, Postmedia News on Canada.com),
Bendell [Leah Bendell, professor specializing in ecotoxicology at Simon Fraser University, BC] said the Canadian consumption advice barely meets international standards and does not take into account that cadmium levels are higher in “hot spots” in popular B.C. oyster-growing areas such as Baynes Sound near Denman Island, Desolation Sound north of Powell River, and Effingham Inlet at Barkley Sound.
One study in 2000 found B.C. oysters reached levels as high as 4.9 parts per million, while a 2004-05 study found levels of up to 3.57 parts per million. A workshop sponsored by Simon Fraser University in May estimated current cadmium levels in B.C. oysters at one to four parts per million.
Europe does not allow the importation of oysters containing more than one part per million of cadmium; Hong Kong’s limit is two parts per million.
The BC Centre for Disease Control states that “levels of cadmium are much lower in oysters elsewhere in the world.”
They are advising local oyster lovers to exercise moderation with regard to eating them.
As for biomagnification and cadmium nanoparticles, here’s what the lead scientist suggested,
“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 said. “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?’ ” [emphasis mine]