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.