A PATCHWORK OF SMALL LAKES, forests and marshes surrounded by farms and suburbs, the Assunpink Wildlife Management Area in central New Jersey is an ideal place to track the effects of agricultural nitrogen runoff on wetland soil chemistry. Princeton professor Peter Jaffe and his students arrived at the study site in 2004, along with an ecology research group from Rutgers University.
But a drought made their intended experiments impossible; they couldn’t study the wetlands when they had dried out. Instead, Jaffe, a professor of civil and environmental engineering, asked his students to take soil samples back to the lab, where they added different sources of nitrogen to the samples and analyzed changes in their chemistry.
In some of the samples, the researchers observed something unexpected: in the absence of oxygen, a polluting form of nitrogen called ammonium was transformed into harmless nitrogen gas. In these samples, electrons — charged particles — were moving from ammonium, which comes from fertilizer runoff, to iron, a plentiful metal in the soil. Jaffe’s group documented this reaction for the first time.
Although Jaffe’s original goal was to examine how nitrogen runoff from fertilizer affects metal transport in wetland ecosystems, he wondered if this newly discovered natural phenomenon could be useful for cleaning up nitrogen pollution. To investigate it further, he secured funding from the National Science Foundation and Princeton’s Project X, a program that funds risky but promising projects, and returned to the site in 2011 with postdoctoral research associate Shan Huang. “My first fear was that we wouldn’t see anything,” Jaffe said. “But our findings were reproducible.”
Huang and Jaffe brought fresh soil samples back to the lab. To mimic the underground environment at the field site, Huang placed the small bottles of mud in a chamber without oxygen and bathed them in water spiked with ammonium. She monitored levels of ammonium and forms of iron in the samples and found that the ammonium was indeed being converted into nitrite, a precursor of nitrogen gas.
As Huang monitored the reaction, she noticed something intriguing. “After six months, the reaction became more and more efficient,” she said. “We thought there should be some bacteria responsible for the reaction.” When Huang sterilized the soil by heating it under high pressure, the ammonium removal stopped, suggesting that microorganisms were the force behind the chemical transformations.
Those microorganisms were carrying out a reaction, now known as Feammox, that oxidizes ammonium — takes the electrons from it — and transfers the electrons to iron (Fe is the symbol for the element iron). The oxidized ammonium became nitrite, and other bacteria converted the nitrite to nitrogen gas.
Since the Jaffe team’s initial discovery of Feammox, two other groups have observed the process, in a laboratory sewage treatment reactor and in soil from a Puerto Rican rainforest. But Jaffe and Huang were the first to identify the bacterium that carries out the reaction. Huang isolated this bacterial species from the others present in the soil. “We tried hundreds of different growth conditions, and finally got a pure culture,” she said. “It was 50 percent hard work and 50 percent luck.” The winning recipe had an acidic pH, similar to the soil environment where the bacteria were found.
The bug, which belongs to a family of soil microbes called Acidimicrobiaceae, functions without oxygen and at a relatively low temperature — about 68 degrees Fahrenheit, compared with 86 degrees for other microbes that oxidize ammonium in the absence of oxygen. “This is really good when you think about applying this in wastewater treatment plants,” said Melany Ruiz-Urigüen, a graduate student who is working to optimize the Feammox reaction. “It means you wouldn’t have to heat the water, and that saves a lot of energy.”
Ruiz-Urigüen is also testing whether the bacteria can use inexpensive sources of iron, such as scrap metal. The Feammox reaction requires iron oxide, which is found on rusty steel — a plentiful industrial waste. For Ruiz-Urigüen’s latest experiments, she uses steel wool pads purchased from a home improvement store. She sprays the metal mesh with a salty solution and lets it rust for two weeks before adding it to a reactor.
In the wetland environment, Feammox bacteria likely depend on plants to supply oxidized iron for the reaction. Oxygen leaks out of plant roots, forming orange nodules where it reacts with iron in the soil; this natural rust can seep into areas without oxygen, where Feammox bacteria thrive. To investigate how the Feammox process might work in an engineered wetland, graduate student Zheyun Zhang is monitoring the reaction in greenhouse pots planted with cattails and bulrushes. “We want to use the plants to recycle the iron,” Zhang said. “Plants are very cheap. And it’s a natural process.”
Jaffe and his team hope to improve the Feammox process and harness it to clean up pollution — not only of nitrogen runoff, but perhaps also of certain toxic metals that may act as stand-ins for iron. The success of this fortuitous discovery gives the researchers confidence as they strive to turn Feammox into a useful technology.
–By Molly Sharlach
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