Researchers from the Helmholtz Center Dresden-Rossendorf (HZDR), in collaboration with Wismut GmbH and scientists from the University of Granada in Spain, have demonstrated for the first time that bacteria can convert dissolved uranium in water into a stable chemical compound when provided with glycerol as a food source. In this process, uranium attains a chemical state previously recognized only as transitional. The study’s findings are published in the journal Nature Communications and are significant for future research on using bacteria for environmental restoration.
Bacteria found in the environment, whether in soil or water, play a crucial role in ecosystems, with some species specializing in breaking down harmful substances. “There are bacteria that can metabolically utilize the toxic heavy metal uranium,” said Dr. Evelyn Krawczyk-Bersch, a scientist from the terrestrial microbiology research group at HZDR and co-author of the study. She added, “Our group’s research has already shown that bacteria can use uranium dissolved in water for their metabolism when they have access to glycerol as a food source.” Glycerol is a key component of plant and animal fats and is produced in nature during processes like the decomposition of wood by fungi.
Bacteria accumulate uranium in cell walls
In this study, scientists aimed to determine how effectively bacteria can reduce the concentration of dissolved uranium in water and what chemical forms free uranium is converted into through bacterial metabolic processes. The experiments utilized mine water from a flooded uranium mine in the Ore Mountains, owned by Wismut GmbH. Laboratory studies were conducted in an oxygen-free environment with a specific amount of glycerol added to the water samples.
“We wanted to replicate the natural conditions for the bacterial community already present in the mine water, as there is typically little to no oxygen at a depth of about 2,000 meters (6,600 feet) in the mine,” explained Antonio M. Newman-Portela, a former doctoral student at HZDR and the Department of Microbiology at the University of Granada, and the study’s lead author. Under conditions conducive to bacterial growth, the bacteria began to utilize glycerol as a food source. “After 130 days, only about 5% of uranium remained dissolved in the water samples,” said Newman-Portela. “We suspected that the bacteria had accumulated uranium in their cell walls, a process we were already aware of from the literature.” The researchers confirmed the presence of uranium in the bacterial cell walls.
Formation of the stable compound FeU(V)O4
To identify the exact chemical compounds, the team employed advanced microscopic and spectroscopic methods. The research included experiments at the Rossendorf Beamline (ROBL), operated by HZDR at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, as well as additional studies at the University of Granada. In chemical terms, “valence” refers to the number of “hands” an atom has to bond with other atoms in a chemical compound. “Uranium is typically found with a valence of 4 or 6. While pentavalent uranium exists, it is rare or only transitional. It has previously been observed in an unstable oxidation state,” explained Newman-Portela. “Therefore, the results of our study were extremely surprising, as a notably high proportion of the identified uranium in the analyzed biomass from our experiments was also pentavalent uranium.”
The researchers discovered that pentavalent uranium formed a compound, FeU(V)O4, with iron and oxygen. “This uranium compound does not yet have a name because it is relatively new. It was first demonstrated in a 2020 study analyzing soil samples from areas in Croatia contaminated with uranium munitions,” said Krawczyk-Bersch. It was established that this uranium compound remained stable for over 25 years even when exposed to atmospheric oxygen. “However, until now, we did not know how this compound forms in nature or that bacteria play a role in its formation,” she added. In further experiments, the HZDR research group observed that the quantity of FeU(V)O4 increased when the dried biomass was exposed to oxygen.
“Our study has shown for the first time that bacteria, which receive glycerol as a carbon source, can convert toxic uranium dissolved in water into a stable chemical compound,” summarized Krawczyk-Bersch. “We still need to investigate how effectively bacteria can help neutralize uranium for environmental restoration purposes.” In future research, the HZDR team aims to gather new data on uranium-binding bacteria and gain a deeper understanding of the underlying biochemical and geochemical processes.
Source: Phys.org
