Silver nanoparticles, used for their potent antimicrobial properties in hospitals and consumer products, may negatively impact plant growth as they make their way into the environment, according to a new study. Whereas it may not spell the end of all flora as we know it, the findings suggest that the nanomaterial has environmental impacts worthy of further investigation.

The antimicrobial properties of silver in its ionized form have been recognized for centuries. When it is nanosize—between one and 100 nanometers, which is smaller than many viruses (a nanometer is one billionth of a meter)—silver is even more effective at killing microbes. This antimicrobial potency has prompted manufacturers to include silver nanoparticles in a wide variety of consumer products, such as odor-resistant clothing, hand sanitizers, water treatment systems and even microbe-proof teddy bears. (Currently, labels on products for sale in the U.S. are not required to disclose the presence of nanomaterials. Consumers can learn more about which products contain nanoparticles by visiting the Web site for The Project on Emerging Nanotechnologies.)

Although the microbicidal effects of silver nanoparticles are well documented, their impact on the environment is less understood.

"There have been a lot of lab studies looking at silver nanoparticles showing that they are highly toxic to bacteria, fungi, other microorganisms," explains Ben Colman, a postdoctoral researcher at Duke University who led the study. "Most of these studies have been conducted in very simple lab settings, [with] one species of bacteria—often the "lab rat" of the bacteria world, E [scherichia]. coli —[in] a test tube with very simple media and nanoparticles. So we wanted to move beyond this because it's really hard to extrapolate from these single-species studies in simple environments to what will inevitably happen when these particles enter the environment."

Nanoparticles likely enter the environment through wastewater, where they accumulate in biosolids (sewage sludge) at wastewater treatment plants. One of the ways in which the sludge is disposed of is through land application, because it is valuable as a fertilizer. Whereas fertilizers add nutrients to the soil that are essential for plant growth, plants also depend on soil bacteria and fungi to help mine nutrients from the air and soil. Therefore, the antimicrobial effects of silver nanoparticles could have impacts at the ecosystem level—for example, affecting plants whose growth is dependent on soil-dwelling microorganisms.

In order to examine silver nanoparticles' ecosystemic impact the researchers prepared series of outdoor "mesocosms"—intermediate-sized "fields" of plants growing in rubber tubs. They applied 0.2 kilograms of biosolid to each tub, amending the fertilizer with 11 milligrams of silver nanoparticles per tub. This concentration is within the range that the U.S. Environmental Protection Agency reported finding in a recent survey of biosolids from water treatment plants (pdf). The nanoparticles reduced the growth of one of the tested plant species by 22 percent as compared with silver-free biosolid treatment. Similarly, microbial biomass was reduced by 20 percent. Colman presented the findings August 4 at the 95th annual meeting of the Ecological Society of America.

Colman noted that in his previous in vitro studies of sediment he saw no effect on the amount of microbes present even though he used 1,250 milligrams of nano-silver per kilogram of sediment. 

"What we found was actually a little bit surprising," Colman says. "We added lower concentrations [of silver] to a more complex system, but rather than find no measurable effect, we found that the silver nanoparticles significantly altered the plant growth, microbial biomass and microbial activity."

As to how the nanoparticles impact plant growth, Colman says his guess it that "partly they are impacting the soil microorganisms directly, partly they are impacting the plants directly, and no doubt the microbes are having impacts on the plants…that could directly influence how the plants are growing. We are doing more work to try to discern that a little bit better."

Next, the team plans to investigate the effects of nanoparticles in a controlled wetland ecosystem, complete with algae, aquatic plants, microbes, insects, zooplankton and fish.

"The exciting part about this work is that we have a chance to get in early on studying what could be a potential problem. Typically, as ecologists we document the effects of things such as DDT after the fact, after it's had widespread effects."