Trends and Innovations
Miracle Microbes at Work
Mopping up Arsenic
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 organisms
habits to tackle a suite of problems: toxic poisoning, landfill overflows and
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 wed 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 departments 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 havent 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. Hes 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. Its
been one lucky circumstance after another, he says.
Geotimes contributing writer
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Arsenic is naturally present throughout
Earths 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. Weve 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. Its 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, its 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 teams 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 EPAs National Risk Management Research Laboratory
in Ada, Okla., says that research such as that of Bethke and co-workers is important,
but he shares Zhengs 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
In the meantime, the popularity of both biotic and abiotic methods has waxed and
waned, Wilkin says. Unfortunately, theres no alchemy. We cant
turn arsenic to gold.
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