Nanoparticles kill friendly soil bacteria

Extremely tiny silver particles found in products like odour-killing socks are highly toxic to some beneficial Arctic soil bacteria, a Canadian study has found.
Queen's University's Niraj Kumar and Virginia Walker, co-authors of the study, pose with the piece of equipment they used to measure the respiration of microbe communities living in Arctic soil samples. ((Queen's University))

Extremely tiny silver particles found in products like odour-killing socks are highly toxic to some beneficial Arctic soil bacteria, a Canadian study has found.

Adding polluting quantities of silver nanoparticles to soil from the High Arctic, far from human settlements, reduces the population of many species of bacteria. It also makes one beneficial species disappear altogether, says the study, published this week in the Journal of Hazardous Materials.

Nanoparticle products

  • Silver nanoparticles: antimicrobial products, including textiles, medical devices, packaging.
  • Copper nanoparticles: coatings, lubricants, batteries, inks, plastics.
  • Silica nanoparticles: medical devices and electronics.

Virginia Walker, a Queen's University biologist who co-authored the study, said the findings suggest that people should reconsider how they use nanoparticles, which were introduced to consumer products in the 1990s.

"Maybe sometimes we should just stop, pause and reflect and wonder what's happening with these nanoparticles," she said. "Do we want to perturb the environment for frivolous reasons?"

Silver nanoparticles are added to many consumer products as an anti-microbial agent, including many brands of odour-resistant socks and T-shirts.

The Project on Emerging Nanotechnologies, a partnership between the Woodrow Wilson International Center for Scholars and the Pew Charitable Trusts, lists over 1,300 products reported by their manufacturers to contain nanoparticles, including non-stick cookware, fabric softeners, children's plush toys and even some foods and beverages.

Walker said millions of tonnes of nanoparticles are manufactured each year. Inevitably, some end up in the environment.

She wanted to find out how they would affect soil bacteria.

She and her research team collected soil samples at Alexandra Fjord on Ellsmere Island in Nunavut, as she believed there would be little nanoparticle contamination there from human activity.

They analyzed the fatty acids and DNA in the soil to figure out what kind of microbes are present. Then they mixed nanoparticles made of silver, copper or silica until they made up 0.066 per cent of the soil by weight.

After six months, they analyzed the fatty acids and DNA in the soil samples again, along with the respiration of the soil bacteria, and compared them to a control that did not contain nanoparticles.

They found that copper and silica nanoparticles did not have a big effect. 

Releasing nanoparticles

A 2008 study by Troy Benn and Paul Westerhoff at Arizona State University found that odour-resistant socks contained up to 31 mg of silver per sock, although most brands contain closer to 1 mg.

To achieve the level of silver nanoparticles in Virginia Walker's experiment, you would need to mix in all the silver in one the most silver-heavy brand of socks for every 50 grams of soil.

The ASU study found that some socks released up to 2 mg of silver after repeated washings, and a lot of it was dissolved in the water rather than in the form of nanoparticles. However, they estimated that most wastewater treatment plants could remove the extra silver from consumer products.

But in the soil with the silver nanoparticles, signs of a species of bacteria called Bradyrhizobium canariense, which is also found in soils far south of the Arctic, disappeared altogether. Most other species decreased, and the amount of bacterial DNA overall fell by 44 per cent. Only one group of bacteria, called Bacillales, increased. That kind is known to be very hardy and survive under extreme conditions.

The researchers were concerned about the disappearance of Bradyrhizobium because it is a nitrogen-fixing species — that is, it makes the nutrient nitrogen available in a form that plants can use.

In the lab experiments, they confirmed that Bradyrhizobium was far more susceptible to silver nanoparticles than other bacteria. It died when the concentrations of nanoparticles were even 10 times lower than the amount mixed into the soil.

"We wonder about the impact to biogeochemical cycles and to the Arctic," they wrote in the paper.

However, Walker acknowledged that Bradyrhizobium is not the only nitrogen-fixing bacteria in the Arctic and that other bacteria might be able to take their place in the soil ecosystem.

She and her colleagues are currently studying the effect of nanoparticles in soil samples from other places.