Uranium, a radioactive heavy metal, is typically locked inside minerals in soil. However, mining and other environmental processes can change uranium into forms that dissolve in water. Once it becomes mobile in this way, it can spread through the environment and create problems because of its toxicity.
Researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), working with Wismut GmbH and scientists from the University of Granada, have now shown for the first time that bacteria can transform uranium dissolved in water into a stable chemical compound when glycerol is available as a food source. During this process, the uranium enters a chemical state that had previously been considered only temporary.
The findings, published in Nature Communications, could contribute to future research into using bacteria to help clean up environments contaminated with uranium.
How Bacteria Interact With Uranium
Bacteria found in soil and water are essential parts of natural ecosystems, and some species are capable of processing substances that can be harmful to humans and other organisms.
“There are bacteria that can metabolically utilize the heavy metal, uranium, which is toxic for humans,” says Dr. Evelyn Krawczyk-Bärsch, scientist in HZDR’s Terrestrial Microbiology research group and co-author of the study. “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.”
Glycerol is a basic component of plant and animal fats. It can also form naturally, for example, when fungi break down wood.
The researchers wanted to determine how effectively bacteria could reduce the amount of uranium dissolved in water and identify the chemical forms created as the microbes processed the uranium.
Uranium Accumulates in Bacterial Cell Walls
To investigate, the team collected mine water from a flooded uranium mine in the Ore Mountains belonging to Wismut GmbH. In the laboratory, they added a controlled amount of glycerol to samples of the water and kept them in an environment without oxygen.
“We wanted to create natural conditions for the bacterial community already existing in the mine water because at a depth of approximately 2,000 meters there is usually little or no oxygen in the mine,” explains Dr. Antonio M. Newman-Portela, former doctoral candidate at both HZDR and the Microbiology Department at the University of Granada (Spain), and the lead author of the study.
With conditions suitable for bacterial growth, the microorganisms began using glycerol as a food source. Over time, the amount of uranium remaining dissolved in the water dropped dramatically.
“After 130 days, only around five percent of the uranium dissolved in the water remained in the samples,” says Newman-Portela. “We suspected that the bacteria had incorporated the uranium in their cell walls. We already knew about accumulation processes from the literature.”
The researchers were then able to confirm that uranium had indeed accumulated within the bacteria’s cell walls.
A Rare Form of Uranium Appears
The next question was exactly what type of uranium compound had formed. To find out, the researchers turned to advanced microscopy and spectroscopy.
Their investigation included experiments at the Rossendorf Beamline (ROBL), operated by HZDR at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, along with complementary studies carried out at the University of Granada.
The scientists examined the bacterial membrane to determine the chemical states in which the uranium was present. Chemists use the term “valency” to describe how many “hands” an atom effectively has available to connect with other atoms in a chemical compound.
“Uranium usually occurs with a valency of 4 or 6. Pentavalent uranium does exist, but it is rare or only transient. Until now, it had been seen in an unstable oxidation state,” explains Newman-Portela. “So, the findings of our study were extremely surprising because in the biomass analyzed from our experimental runs, an unusually high proportion of the uranium identified was also pentavalent uranium.”
The discovery was unexpected because this pentavalent state of uranium had generally been regarded as unstable and short-lived.
A Uranium Compound That Remains Stable
The researchers also determined that the pentavalent uranium combined with iron and oxygen to form 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,” explains 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.”
Additional experiments produced another surprising result. When the researchers exposed dried bacterial biomass to oxygen, the amount of FeU(V)O4 increased rather than decreasing.
That finding suggests the compound can remain stable even in the presence of oxygen, adding to evidence that bacterial activity may help convert mobile uranium into a form that is much less likely to move through water.
Potential for Uranium Cleanup
“Our study has revealed for the first time that bacteria supplied with glycerol as a carbon source can convert toxic uranium dissolved in water into a stable chemical compound,” says Krawczyk-Bärsch. “We still have to investigate to what extent bacteria might help to render uranium harmless for remediation purposes.”
The researchers now plan to study uranium-binding bacteria in greater detail and investigate the biochemical and geochemical processes that make the transformation possible. A better understanding of those mechanisms could eventually help determine whether bacteria can be used effectively in efforts to remediate uranium-contaminated environments.
