Nature
PUBLISHED
In the heady days of the Cold War, the USSR extracted vast amounts of uranium from a mine in East Germany, much of it destined for nuclear warheads. Unbeknownst to the bigwigs back in Moscow, strange and potentially useful microbes were quietly lurking in the mine's backwaters.
Those microbes were discovered in untreated water flowing from the Wismut GmbH Schlema-Alberoda mine in Saxony, Germany. Between 1946 and 1990, when Germany reunified, around 80,000 tons of uranium were extracted from the site, one of the largest uranium ore deposits in the world.
Strange things live in the mine
Microbiologists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) recently collected water samples from an inlet trail from the mine and found that they were teeming with anaerobic bacteria.
Among them were some that can break down metals, like Geobacter and Shewanella, others that run on sulfur, like Desulfovibrio, and a supporting cast of microbes that ferment and break down nitrogen compounds, including Clostridium and Pseudomonas.
Just as their previous experiments had hinted, the team found this gang of bacteria was able to utilize the highly radioactive uranium, as long as they had glycerol, a basic building block of plant and animal fats, to feed on.
“There are bacteria that can metabolically utilize the heavy metal, uranium, which is toxic for humans,” Evelyn Krawczyk-Bärsch at HZDR’s Terrestrial Microbiology research group, and co-author of the study, said in a statement.
“Our group’s investigations had already revealed that bacteria can use uranium dissolved in water for their metabolism when they have access to glycerol as a food source,” she added.
The researchers decided to dig deeper into how this was happening and whether the bacteria could be used as a handy tool to clear up radioactive contamination.
They set up an experiment in the lab in which the bacteria were placed in conditions very similar to their natural low-oxygen environment deep below the uranium mine. After being left for 130 days, just 5 percent of the uranium dissolved in the water remained in the samples.
A really weird chemical state
Using advanced microscopic and spectroscopic techniques, the researchers found the bacteria had incorporated the uranium into their cell walls.
They first looked closely at the bacterial membranes to figure out exactly what chemical form the uranium had taken. In chemistry, atoms are often described by their "valency," a kind of shorthand for how many other atoms they can latch onto in a compound. Uranium typically shows up with a valency of 4 or 6.
There’s a rarer version with a valency of 5, but it's usually short-lived and unstable, appearing only fleetingly before changing into something else. Weirdly, the uranium embedded in the bacteria’s cell walls had taken on this unusual, rare “pentavalent” form.
"The findings of our study were extremely surprising because in the biomass we analyzed, an unusually large share of the uranium turned out to be this rare, five-valent form," explained Antonio M. Newman-Portela at the University of Granada in Spain.
Stranger still, the pentavalent uranium combines with iron and oxygen to form a currently unnamed and stable compound: FeU(V)O4.
“This uranium compound doesn’t have a name yet as it is comparatively new. It was first demonstrated in a study in 2020 in which soil samples from parts of Croatia contaminated by uranium ammunition were analyzed,” noted Krawczyk-Bärsch.
“It was found that even under the influence of atmospheric oxygen, this uranium compound had remained stable for more than 25 years. But until now, we didn’t know how this compound is formed in nature or that bacteria play a role in its formation,” she continued.
Clean up Chornobyl?
All of this raises the tantalizing possibility of using these bacteria to clean up uranium and other radioactive junk. Could they, for instance, be introduced into the contaminated waters of Chornobyl to lock its loose uranium into a stable, harmless form?
Whether that's a genuine possibility or just wishful thinking remains to be seen, but it's a pretty intriguing idea either way.
The new study is published in the journal Nature Communications.
