Soybeans are the second largest crop harvested in the United States and a $30-billion-a-year industry. But soybean production could face a new long-term threat from large amounts of metallic nanoparticles in the environment. A new study reveals that one type of these nano-sized materials, found in everything from cosmetics to electronic devices, can be absorbed by soybeans and move throughout their tissues, while another type of nanoparticle likely stunts their growth. The findings, the authors say, raise concerns about the impact of nanoparticles on a host of other crops.
Metal contamination has long been a concern in agriculture. The U.S. Environmental Protection Agency currently requires companies that discharge potentially toxic metals into wastewater to remove the metals before the water is sent to wastewater treatment plants. Half of the organic material collected from such treatment plants eventually ends up on farmers' fields as fertilizer. However, the presence of metal nanoparticles in wastewater is neither regulated nor monitored. And, within a decade, industry could be producing millions of metric tons of some metal nanoparticles per year, raising concerns about their accumulation in the environment.
Such concerns have already prompted some initial studies looking for possible biological impacts. In 2010, researchers led by Jorge Gardea-Torresdey, a chemist at the University of Texas, El Paso, reported in Environmental Science & Technology that when soybeans were grown hydroponically and exposed to high levels of zinc oxide and cerium oxide nanoparticles, those particles could accumulate in plant tissues, with cerium oxide triggering genetic damage in the plants' roots. Another study published online last month in Applied and Environmental Microbiology found that high concentrations of zinc oxide and titanium dioxide nanoparticles disrupt soil bacterial communities and interfere with the ability of rhizobia bacteria that associate with soybeans to use nitrogen from the air to produce fertilizer for the plant. This ability of soybeans to "fix" nitrogen has a major impact on agriculture. Whereas 97% of corn farmed in the United States requires fertilizer, only 18.5% of the 77 million acres of soybeans farmed in the United States each year are fertilized.
Still, it was unclear whether compounds present in the dirt would lock up nanoparticles or whether the nanoparticles would be "bioavailable," allowing plants to take them up. So for the new study, University of California, Santa Barbara, environmental microbiologist Patricia Holden, who led the Applied and Environmental Microbiology study, teamed up with Gardea-Torresdey as well as colleagues at four other institutions in the United States and South Korea to test the effect of metal nanoparticles on soybeans grown the old-fashioned way. They mixed zinc oxide and cerium oxide nanoparticles into separate batches of soil from an organic farm in California. Then they split these mixtures into several lots, each time adding a different amount of pristine soil to give the lots concentrations of metal nanoparticles that previous hydroponic studies had flagged as potentially having a biological impact. That gave them three different soil concentrations of each of the two nanoparticles. For the zinc oxide, the concentration was either 0.005, 0.1, or 0.5 grams per kilogram of soil. For cerium oxide, the concentration was either 0.1, 0.5, or 1 gram per kilogram. The team then tracked the amount of nanoparticles taken up by different tissues of the plant and the impact on their growth.
The researchers report their results online this week in the Proceedings of the National Academy of Sciences. Only plant roots and the root nodules that serve as nurseries for rhizobia absorbed cerium oxide nanoparticles. But at the mid- and high-exposure levels the plants were essentially unable to fix nitrogen. Holden says that in these cases the root nodules that normally house nitrogen-fixing bacteria were "were vacant of bacteria." But she cautions that although it appears that the cerium oxide nanoparticles are the culprit, the researchers can't yet be sure.
As for the zinc oxide, Holden's team found that these metal particles didn't seem to have any significant biological effect, though soybean tissues with high levels of the metal tended to hold less water. Still, zinc levels rose throughout the plants' tissues, even in the beans that would be used as food for humans and livestock. This high level of zinc doesn't necessarily pose a threat to people, Holden says, since the zinc levels were essentially on par with the amount of zinc recommended for people's diets in the United States. But she adds that she and her colleagues aren't sure if the zinc in soybeans is transformed into zinc ions, the form of the metal that is normally present in our bodies, or remains zinc oxide nanoparticles, which could have different biological effects.
"This is a pioneering study" that's beginning to tease out potential agricultural impacts of nanomaterials, says Andre Nel, a nanomaterials toxicologist at the University of California, Los Angeles. He is quick to add that it is unlikely that soybeans and other plants are currently exposed to nanoparticle concentrations as high as those used in the new study, but researchers aren't sure. "In the world of agricultural exposure levels, nobody knows what the nanomaterial concentration is."
"Is this an indication we should be worried about the food supply? I don't know," Holden says. The take-home message, she says, is that "there is a potential for nanomaterials to be bioavailable in soil and affect agriculture." Whether they will eventually accumulate in the high concentrations shown to have an impact here is unclear. However, she says, "it's important that the scientific community asks these questions in advance."