Scientists know little about how releasing some of the more than 2 million tons of nanoparticles produced every year will affect organis in the environment. A new study reports that earthworms (Eisenia fetida) can ingest gold nanoparticles from their surrounding soil and accumulate them in their tissues—a finding with significant implications for food webs (Environ. Sci. Technol., DOI: 10.1021/es101885w).
Manufacturers add nanoparticles to coetics, clothing, and medical devices. Normal use of these products—such as washing your clothes—can release nanoparticles into wastewater, where they end up in sewage sludge. In the U.S. and Europe, farmers apply sewage sludge to their fields as fertilizer.
Environmental toxicologist Jason Unrine of the University of Kentucky, Lexington, and colleagues wanted to know if nanoparticles applied to these fields eventually made their way into the food chain. "Because we expected the nanomaterials to aggregate onto soil particles, we were initially very skeptical that organis could take them up from the soil," Unrine says.
To find out, Unrine's team mixed earthworms—organis near the bottom of the food chain—into artificial soil tainted with gold nanoparticles. "We used gold nanoparticles because they’re stable, insoluble, and easily detected," Unrine says. "They make a good tracer."
After 28 days, the researchers investigated the worms' tissues to look for uptake of nanoparticles. The scientists first performed laser ablation inductively coupled plaa mass spectrometry (LA-ICP-MS) to map the total gold distribution in earthworm tissues. Then they used a more time-consuming technique called synchrotron-based X-ray microspectroscopy on selected regions to confirm that the gold was in metal form and not ionic gold left over from the manufacturing process.
Unrine and his colleagues found 20- and 55-nm diameter gold nanoparticles distributed throughout the worms' bodies with the highest concentrations in their guts. But the exposed worms did not upregulate the gene for metallothionein, a metal-ion–sensing protein, further confirming that the gold detected in earthworm tissues was in nanoparticle form.
Although the gold nanoparticles didn't significantly affect earthworm mortality, exposed worms produced up to 90% fewer offspring. Unrine plans future studies to more closely examine the biological responses of earthworms to nanomaterials.
"This study provides the best evidence to date of nanoparticle uptake into tissue from soil," says Robert Hurt, an environmental engineer at Brown University. And he thinks the study could have significant implications: "Nanoparticles are typically found at low concentrations in the environment, but they will have a greater potential to harm human health if they biomagnify in the food chain."
