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Metal-eating microbe Geobacter metallireducens swims
  

 

Miles underground, microbes survive without oxygen or sunlight by feeding on metals like iron and manganese. One of these microorganisms, Geobacter metallireducens, has an unusual survival tactic for life in the underworld: It uses a sensor to 'sniff out' metals. If metal is not nearby, G. metallireducens can spontaneously grow flagella—whip-like cellular propellers—to find new energy sources.


Expression of flagella and pili by Geobacter metallireducens.

Previously, researchers believed G. metallireducens was immotile. By looking at the genome sequence of another metal-eating microbe of the same genus, scientists have now discovered that G. metallireducens contains genes for flagella.

The flagella genes surprised the scientists who had never seen G. metallireducens swim. Looking back, the researchers realized that in past motility experiments the organism had only been grown on soluble metals, which are easy to work with in the laboratory. After performing experiments with insoluble metals, such as iron oxide, the team found that G. metallireducens indeed grows flagella and swims.

"It's basically out of luck if it doesn't get up and move," says Derek Lovley, who led the research team at the University of Massachusetts in Amherst. He added that this adaptation allows the microorganism to find fresh sources of food.

Lovley's team also discovered genes that direct the organism's movement towards chemicals in the environment. In experiments, G. metallireducens can purposely follow a chemical trail leading to a metal source. This is the first demonstration that microorganisms have this type of sensor, the researcher report in Nature.

"That is the whole power of genomics—the ability to go in and look at a sequence and make predictions about ecological and physical attributes," Lovley says.

The microbe's metal diet has made it intriguing to researchers. In addition to using iron, the organism will use metals such as plutonium and uranium to metabolize food. Geobacter metallireducens consumes these radioactive elements and essentially eats away at the contaminants. In the case of uranium, it changes the metal from a soluble to an insoluble form. The insoluble uranium drops out of the groundwater, thus decontaminating streams and drinking water. It remains in the soil and could then be extracted.


Uranium pile at a mine site in Ship Rock, NM.

Field studies are currently underway to test the bioremediation potential of G. metallireducens at uranium sites. More research still needs to be done to fully understand how the organism's behavior in the laboratory translates to life underground, Lovley says.

The US Department of Energy's (DOE) interest in the organism's potential for environmental clean up led to funding for the initial sequencing of G. metallireducens. In 1995, the DOE created the Natural and Accelerated Bioremediation Research Program to develop strategies for bioremediation of radionuclides and metals. The microbe's appetite for uranium shows promise for bioremediation of old uranium mines. These remnants of the Cold War contain low, although not radioactive, levels of uranium, which are still creeping into the groundwater.

"We still don't know enough about the diversity of the microbial world to say which one is best," says Daniel Drell of the DOE Office of Biological and Environmental Research. "At the moment, they [Geobacter species] look awfully good," he added.

The success and predominance of G. metallireducens in the subsurface is probably due to its unique swimming adaptation, says Lovley. The organism grows the flagella and pili—hair-like anchors that help it attach to a metal—only when necessary.

"It's a very energy-efficient strategy," Lovley says, adding that this is an important survival technique in underground environments without much organic matter. "They are unlike anything else that has been seen," he says. "I think that explains why they do so well in these kind-of strange environments—by living in an unusual way."

Last fall, The Institute for Genomic Research (TIGR) in Rockville, Maryland finished the sequence of Geobacter sulfurreducens, a cousin of G. metallireducens. It was by comparing both sequences that Lovley's team discovered the existence of genes for flagella.

Research on G. sulfurreducens at TIGR continues. Barbara Methe, who began her work on Geobacter species at the University of Massachusetts under Lovley, now leads a research team at TIGR. Her team is developing microarrays to study gene expression in G. sulfurreducens and to find out what genes are turned 'on' and 'off' under certain conditions. This could someday help to better understand what inhibits and promotes the growth of this organism.

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Childers, S. et al. Geobacter metallireducens accesses insoluble Fe(III) oxide by chemotaxis. Nature 416, 767-769 (April 18, 2002).
 

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