Geotimes — February 2005 — Trends and Innovation

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Trends and Innovations
Miracle Microbes at Work

Garbage-eating Geobacter
Mopping up Arsenic

Garbage-eating Geobacter

Science and serendipity have converged for microbiologist Derek Lovley. Seventeen years after the accidental discovery of an iron-breathing, garbage-eating, electron-seeding microbe in a Virginia swamp, he is testing and patenting the organism’s habits to tackle a suite of problems: toxic poisoning, landfill overflows and energy shortages.

In Rifle, Colo., researchers are injecting acetate (a vinegar) at the shooting gallery and shed shown here to boost the natural population of microbes that eat the acetate and clean up the uranium contamination at the site. Courtesy of University of Massachusetts, Amherst.

Lovley, who works at the University of Massachusetts, Amherst, first discovered the microbes in 1987. He was then working for the U.S. Geological Survey in the metal-loaded muck of the Potomac River outside Washington, D.C. “We were trying to see if there were organisms that would grow on metal,” Lovley recalls. “There was no way to know what we’d find.”

What Lovley and his colleagues found were organisms that thrive on organic waste matter and breathe iron oxide, or rust. The microbes, which he called Geobacteraceae (Geobacter), are single-celled bacteria, only about 1 to 2 microns long and 1.5 microns across (about 25,000 microns make an inch). Geobacter “uses iron the way we use oxygen,” Lovley says, to breathe and break down food. The microbes strip electrons from decayed matter in swamps and submerged soils and then use the electrons to oxidize organic compounds in the dissolved metals. In the process, they convert the compounds to carbon dioxide and transform iron, petroleum or even uranium from a dispersed liquid to a suspended solid.

Those powers attracted the attention of the U.S. Department of Energy, which is saddled with piles of uranium tailings left over from Cold War nuclear production. Lovley and scientists at the department’s Natural and Accelerated Bioremediation Research (NABIR) program began experimenting four years ago to see if Geobacter could confine the runaway groundwater pollution at these sites.

At Rifle, Colo., where uranium contamination has threatened the Colorado River, Lovley has spent the last three summers injecting acetate — essentially vinegar and “a very good food source for Geobacter” — into the groundwater. The acetate boosts natural and introduced populations of the microbes that then corral the uranium into a discrete pool. When Lovley stops the flow of acetate, the Geobacter colony shrinks back down and leaves the uranium in place. Geobacter cleaned up in “one month what we haven’t been able to do in 10 years,” says Robert T. Anderson of the NABIR program, and the microbes did the job without electricity. Next summer, the scientists will use electrodes to seed the organisms to try to stimulate them even more.

Even if the researchers needed electrical power, Geobacter could double as a microbial fuel cell, says Leonard Tender, an electrochemist at the Naval Research Laboratory in Washington, D.C., who has worked on microbial fuel cells since the mid-1990s. He and colleague Clare Reimers found microbes in seafloor sediment that oxidize fallen organic matter and bottle up electrons. The scientists created a battery by placing a graphite electrode in the sediment to attract the hungry microbes and then connecting it to an electrode in the overlying seawater with a copper wire that transfers the electrons. When they asked Lovley to identify the organisms powering the fuel cell, he discovered a cousin of the freshwater Geobacter.

Iron-breathing microbes “physically attach to sediment and pass electrons through their membrane to the electrode,” Tender says, while other microorganisms need “mediators” to close the deal. Geobacter is already powering 1-watt marine instruments such as weather sensors and deep-sea mapping devices that once relied on heavy, finite batteries.

Lovley does not expect Geobacter to light up cities, but he says that the microbes’ ability to convert organic waste to energy could turn garbage into electricity for developing nations. Some villages already collect methane gas from landfills. In the United States, conscious citizens could even power their lawn mowers with microbial fuel cells charged by grass clippings. Lovley is now patenting the microbial fuel cell and is trying to maximize electrical output of these “bacterial batteries.” He’s mapped the genome of Geobacter and hopes to incorporate the talents of other iron-breathing microbes that feed on sugars such as glucose from plant tissue.

Still, Lovley does not credit the progress strictly to lab homework. “It’s been one lucky circumstance after another,” he says.

Joshua Zaffos
Geotimes contributing writer

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Mopping up arsenic

Arsenic is naturally present throughout Earth’s crust, and although it is not always poisonous, at high enough levels it can cause skin lesions, as well as skin cancer and other health problems. In Bangladesh, India and elsewhere in Southeast Asia, an epidemic of arsenic poisoning has been under way for a decade from contaminated groundwater, and arsenic is also in the glacial sediments of the Midwest and in aquifers across the United States. Researchers have been looking for ways to mop up arsenic, using both biologic and non-biologic methods.

Since the 1990s, a variety of private companies, the U.S. Department of Energy and others have pursued bioremediation methods. “We’ve been doing this with [sites] where sulfuric acid has been dumped into the ground,” says Jim Saunders, a geochemist at Auburn University in Alabama. Saunders holds a patent on a method that uses injection wells to add microbes, sodium lactate (a sugary food source for the microbes), iron and iron sulfate to a contaminated site to encourage microbial activity. “It’s not easy,” he says, but at least one of his sulfate sources — blackstrap molasses — is readily available. Saunders says he is even pursuing Geobacter (see story above) as a possible microbe for arsenic bioremediation.

Just as humans breathe in oxygen and breathe out carbon dioxide, certain microbes breathe in sulfate and breathe out hydrogen sulfide — which can directly react with arsenate or arsenite, two natural forms of arsenic. With a little iron in the hydrogen sulfide mix, arsenic will precipitate out of contaminated water.

One team from the University of Illinois at Urbana-Champaign tested wells in Illinois’ Mahomet glacial aquifer, where arsenic levels vary from 0 micrograms per liter to greater than 50 micrograms per liter, which is above the levels acceptable to the U.S. Environmental Protection Agency (EPA) for drinking water. The researchers reported in the November Geology a correlation between the presence of sulfate and an absence of arsenic, which they suggest could make sulfate a potential indicator of relatively arsenic-free water.

Furthermore, they speculate that adding more sulfate to an arsenic-laden groundwater system would stimulate microbial communities there, says Craig Bethke, one of the co-investigators. “The bacteria are already down there,” Bethke says, and adding sulfate could jumpstart their processes.

But because groundwater moves so slowly, “it’s hard for me to imagine this could work on a large scale,” says Yan Zheng, a geochemist at Queens College in New York City and a lead investigator in a groundwater arsenic project in Bangladesh. While Zheng thinks that bioremediation is important, she says she remains unconvinced by the team’s results. “The fact that this water has arsenic may or may not have anything to do with” the presence of sulfate. The researchers “need to measure the sediments themselves and show that there is arsenic that is incorporated into sulfides in the sediments,” Zheng says. Plus, she says, even if the arsenic has been bound up in precipitates, with or without the help of microbes, it may be able to reenter the system.

Richard Wilkin, a geochemist at EPA’s National Risk Management Research Laboratory in Ada, Okla., says that research such as that of Bethke and co-workers is important, but he shares Zheng’s concerns. He says that the hypothesis “that arsenic forms a stable precipitate or co-precipitate is unproven at this point.”

Wilkin says that the bioremediation process is in some ways like a time-release drug: Instead of attacking a region with, for example, tablets of hydrogen sulfide (a toxic chemical that smells like rotten eggs), sulfate injected into an aquifer would slowly disperse and react with microbial populations.

But the key may be that microbes are a relatively low-cost way to treat some sites, says Wilkin, who generally works on inorganic methods for treating groundwater with arsenic and other metals, for example, using iron filings to catch arsenic and other compounds in plumes of contaminated groundwater. Wilkin says that an important goal now for in-place groundwater remediation is “to find inexpensive materials that are effective, long-lasting and that have no harmful side effects.” Those criteria could make microbes more attractive.

In several water sources, some researchers suspect that microbes might even be responsible for the presence of arsenic, though that has not yet been confirmed. All researchers say that further work is necessary to better understand how groundwater, arsenic, iron, sulfate, microbes and other ingredients interact in what is a very complex system.

In the meantime, the popularity of both biotic and abiotic methods has waxed and waned, Wilkin says. Unfortunately, “there’s no alchemy. We can’t turn arsenic to gold.”

Naomi Lubick

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