The radioactive heavy metal uranium is usually found in the soil in a mineral-bound form but can be converted into soluble forms by environmental influences or mining activities. If it enters the environment, this poses a problem due to its toxic properties. Now, researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), together with Wismut GmbH and Spanish scientists from the University of Granada, have demonstrated for the first time that bacteria can convert uranium dissolved in water into a stable chemical compound when they have access to glycerol as a food source. In the process, uranium assumes a chemical state that had only previously been known as a transient state. The results have been published in the journal Nature Communications (DOI: 10.1038/s41467-026-72560-z) and are relevant to future research on the use of bacteria for environmental remediation.
Bacteria in the environment, in soil or water, play an important role in ecosystems. Some of them specialize in breaking down harmful substances. "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. In nature, for example, it is formed when wood is decomposed by fungi. But to what extent can bacteria reduce the amount of dissolved uranium in the water? And into which chemical forms is free uranium converted by bacterial metabolic processes? These were the questions the researchers addressed in the new study.
Uranium in cell walls
For their experiments, they used mine water from a flooded uranium mine in the Ore Mountains belonging to Wismut GmbH. In laboratory experiments conducted in an oxygen-free environment, the research team added a specific amount of glycerol to the water samples. "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. Under conditions favorable for bacterial growth, the bacteria accepted glycerol as a source of food. "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." And, indeed, the researchers were able to prove the existence of uranium in the bacteria's cell walls.
Unusual chemical state
But precisely which chemical compounds were involved? In order to establish this, the team used advanced microscopic and spectroscopic methods. The studies comprised experiments at the Rossendorf Beamline (ROBL), operated by HZDR at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, as well as complementary studies at the University of Granada.
Initially, the scientists explored the bacterial membrane to establish in which chemical states the uranium was present. In chemical terminology, the term "valency" is used to describe how many "hands" an atom has to hold onto other atoms within 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."
Stable – even under the influence of oxygen
Furthermore, the researchers found that the pentavalent uranium formed the compound FeU(V)O4 with iron and oxygen. "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." In further experiments, the HZDR research team observed that the amount of FeU(V)O4 actually increased when the dried biomass was exposed to oxygen.
"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." In future work, the HZDR team aims to gain further insides into uranium-binding bacteria as well as to better understand the underlying biochemical and geochemical processes.
Publication:
A. M. Newman-Portela, K. O. Kvashnina, E. F. Bazarkina, A. Rossberg, F. Bok, S. Ting-Shyang Wei, A. Kassahun, T. Stumpf, J. Raff, M. L. Merroun, E. Krawczyk-Bärsch: Pentavalent and tetravalent uranium formation via glycerol-stimulated bacteria in mine water, in Nature Communications, 2026 (DOI: 10.1038/s41467-026-72560-z)
Further information:
Dr. Evelyn Krawczyk-Bärsch | Dr. Johannes Raff
Institute of Resource Ecology at HZDR
