Radioactive material lies underground in old rusting nuclear waste containers like a ticking time bomb throughout the world. In other places, radioactive material from mining operations and other activities contaminates groundwater. New research on ancient microorganisms shows that they can help to decontaminate the dangerous radioactive substances. Greater knowledge about this can also ensure that nuclear waste does not leak out of their storage containers.
People usually associate radioactive threats with missiles or nuclear power plants. Nevertheless, buried under the Earth’s surface lies an even greater threat from metal containers filled with nuclear waste and from uranium left over from mining operations. This problem is hard to control because it requires constant monitoring and complex clean-up processes. Now, new research and knowledge on microorganisms can help to both control and eliminate the radioactive material.
“Our experiments show that a particular strain of archaea prokaryotes can inactivate radioactive uranium, which would otherwise percolate into the groundwater. The archaeon converts radioactive uranium to an insoluble non-radioactive form that stays where it is. The archaeon is a Methanosarcina methanogen, which follows up and finishes the job of a uranium-decontaminating bacterium, Geobacter. This means that Methanosarcina and Geobacter together help to clean up after a radioactive spill,” explains Amelia-Elena Rotaru, Assistant Professor, Department of Biology, University of Southern Denmark.
Archaea finish the job
Groundwater reservoirs in Colorado were one of the places the researchers studied. Uranium mining in the United States started in 1872, and Colorado has the country’s third highest reserves of uranium. Uranium mining was therefore an important source of income in Colorado. Unfortunately, this resulted in significant contamination, with uranium percolating into the groundwater.
"It was therefore a massive breakthrough when Geobacter were successfully induced to convert soluble uranium into insoluble uranium more than 10 years ago. These Geobacter bacteria usually live in small numbers in groundwater aquifers, but uranium-respiring Geobacter were successfully proliferated when scientists spiked the underground waters of Rifle, Colorado with their favourite food, acetate."
However, to their surprise, Geobacter disappeared a year later, hunted down by protozoans and viruses; instead, an archaea relative of Methanosarcina took over. Methanosarcina is extremely versatile and can live everywhere, from extreme environments such as in deep-sea vents to more common environments such as anaerobic digesters and soils.
"It was therefore not that surprising that they could not only survive but also convert uranium in the underground waters of Rifle, Colorado. They actually have a very special biochemistry that enables them to survive and thrive under extreme conditions."
Methanogens could rust nuclear-waste containers
Although uranium pollution is a major environmental problem in global terms, methanogenic archaea have been also linked to another environmental problem – rusting metal.
"Buried containers full of radioactive waste are made of steel that could rust, and researchers fear that the radioactive waste will leak out of these steel containers over time. Methanogenic archaea are one of the underlying causes of steel corrosion underground."
Normally metal only rusts when oxygen is present, but archaea can make metal rust, even in the oxygen-free conditions underground. The methanogenic archaea feed off the metal and respire carbon dioxide, not requiring oxygen for respiration. As a consequence the metal slowly rusts away even in the absence of oxygen if nothing is done.
"The oil and gas pipelines in the United States alone are long enough to circle the Earth a couple of hundred times, so containers with radioactive waste are definitely not the only problem. Huge sums of money could be saved if steel corrosion by anaerobic microorganisms could be prevented."
“Potential for Methanosarcina to contribute to uranium reduction during acetate-promoted groundwater bioremediation” was published in Microbial Ecology. The Novo Nordisk Foundation awarded Amelia-Elena Rotaru, a senior author, a grant in 2015 for the project Microbial Chemical Plants: Harnessing Microbial Metabolism for the Production of Valuable Chemicals.