Research indicates that silver nanoparticles, which are used as an antibacterial agent in a wide range of products, from odour-free socks to wound-healing bandages, have more of an impact on the environment than was thought. These effects include raising levels of greenhouse gases.
These antibacterial nanoparticles can find their way into wastewater, and have been shown to reduce the activity of bacteria used to remove ammonia when the water is treated.
So far most of the research on the environmental impact of nanoparticles has been carried out on single microbe or plant species within the laboratory.
To try to pin down their action in a more realistic setting, Benjamin Colman, a chemist at Duke University in Durham, North Carolina, and colleagues added a high dose of silver nanoparticles - 1.25 milligrams per gram of water - to microbes in a sample of stream water and soil kept within their laboratory.
They also set up two outdoor tubs of plants.
Treated sludge known to be free of nanoparticles was added to the soil in both tubs, while one tub was also dosed with 55 micrograms of silver nanoparticles per gram of sludge, a concentration similar to levels often found in waste water.
"We are trying to find out what happens when these silver nanoparticles get into the real environment. These particles are developed with the express purpose of killing things," New Scientist quoted Colman as saying.
Two months on, the microbial population in the outdoor tub containing silver had significantly declined relative to the lab sample measured after one week.
In addition, the activity of the enzymes they produce to break down organic matter was 34 per cent lower in the tub that had been dosed with nanoparticles than in the tub to which only sludge had been added.
Given that the outdoor tub containing nanoparticles had a much lower concentration of silver than the lab samples, the drop in its microbial activity is so large that it suggests the lab samples are not a good guide to real-world behaviour, said Colman.
The team also used a gas chromatograph to measure the gases produced by the microbes.
They found that the level of nitrous oxide, a greenhouse gas, given off in the tub containing nanoparticles was four times that in the tub in which only sludge was used.
Donald Wuebbles, an atmospheric scientist at the University of Illinois at Urbana-Champaign, said that if the results were replicated on a large scale, it could "further contribute to concerns about global changes in climate".
He pointed out that nitrous oxide could also damage the ozone layer if it gets into the stratosphere.
Colman presented the research at the annual meeting of the Ecological Society of America in Pittsburgh, Pennsylvania.