In experiments mimicking a natural environment, Duke
University researchers have demonstrated that the silver nanoparticles used in
many consumer products can have an adverse effect on plants and microorganisms.
Fifty days after scientists applied a single low dose of
silver nanoparticles, the experimental environments produced about a third less
biomass in some plants and microbes.
These preliminary findings are important, the researchers
said, because little is known about the environmental effects of silver
nanoparticles, which are found in textiles, clothing, children's toys and
pacifiers, disinfectants and toothpaste.
conditions to be tested
"No one really knows what the effects of these
particles are in the environment," said Benjamin Colman, a post-doctoral
fellow in Duke's biology department and a member of the Center for the
Environmental Implications of Nanotechnology (CEINT).
"We're trying to come up with the data that can be used
to help regulators determine the risks to the environment from silver
nanoparticle exposures," Colman said. CEINT's research is funded by the
National Science Foundation and the Environmental Protection Agency
Previous studies have involved high concentrations of the
nanoparticles in a laboratory setting, which the researchers point out, doesn't
represent "real-world" conditions.
"Results from laboratory studies are difficult to
extrapolate to ecosystems, where exposures likely will be at low concentrations
and there is a diversity of organisms," Colman said.
Silver nanoparticles are used in consumer products because
they can kill bacteria, inhibiting unwanted odours. They work through a variety
of mechanisms, including generating free radicals of oxygen which can cause DNA
damage to microbial membranes without harming human cells.
The main route by which these particles enter the
environment is as a by-product of sewage treatment plants. The nanoparticles
are too small to be filtered out, so they and other materials end up in the
resulting wastewater treatment "sludge," which is then spread on the
land surface as a fertilizer.
For their studies, the researchers created mesocosms, which
are small, man-made structures containing different plants and microorganisms
meant to represent the environment. They applied sludge with low doses of
silver nanoparticles in some of the mesocosms, then compared plants and
microorganisms from treated and untreated mesocosms after 50 days.
The study appeared in the journal PLOS One.
What the study found
The researchers found that one of the plants studied, a
common annual grass known as Microstegium vimeneum, had 32 percent less biomass
in the mesocosms treated with the nanoparticles. Microbes were also affected by
the nanoparticles, Colman said. One enzyme associated with helping microbes
deal with external stresses was 52 percent less active, while another enzyme
that helps regulate processes within the cell was 27% less active. The overall
biomass of the microbes was also 35% lower, he said.
"Our field studies show adverse responses of plants and
microorganisms following a single low dose of silver nanoparticles applied by a
sewage biosolid," Colman said. "An estimated 60 percent of the
average 5.6 million tons of biosolids produced each year is applied to the land
for various reasons, and this practice represents an important and understudied
route of exposure of natural ecosystems to engineered nanoparticles."
"Our results show that silver nanoparticles in the
biosolids, added at concentrations that would be expected, caused
ecosystem-level impacts," Colman said. "Specifically, the
nanoparticles led to an increase in nitrous oxide fluxes, changes in microbial
community composition, biomass, and extracellular enzyme activity, as well as
species-specific effects on the above-ground vegetation."
The researchers plan to continue to study longer-term
effects of silver nanoparticles and to examine another ubiquitous nanoparticle
– titanium dioxide.