In 130 days, deep beneath the Ore Mountains bacteria turned uranium-contaminated mine water into a chemical trap, removing 95% of the dissolved metal and forming a rare compound that can remain stable for decades

Deep underground, where sunlight never reaches and oxygen is scarce, ordinary bacteria may be carrying out an unexpected chemical trick. Give the microbes a simple food source called glycerol, and they can help pull dissolved uranium out of contaminated mine water. In laboratory experiments using real mine water, researchers found that roughly 90 to 95 percent of dissolved uranium was removed over several weeks, while part of the uranium became locked into a rare pentavalent form that can remain remarkably stable.

The finding matters because uranium does not always remain safely buried inside rocks. Mining can expose uranium-bearing minerals to water and alter the metal's chemical state.

In its oxidized U(VI) form, uranium can become relatively mobile, allowing it to move through groundwater and contaminated environments. That mobility is one reason uranium pollution can persist long after mining has stopped. The new study suggests that microbial chemistry may provide another route for immobilizing it.

Why dissolved uranium is difficult to control

Uranium is both radioactive and chemically toxic, but its environmental behavior depends strongly on its oxidation state. Uranium in the hexavalent state, written as U(VI), is particularly important in contaminated groundwater because it can form soluble complexes with carbonate and other molecules. Instead of remaining fixed to a mineral surface, it can travel with flowing water and potentially reach ecosystems or water supplies.

A common picture of uranium bioremediation has involved microbes reducing U(VI) to U(IV). U(IV) is much less soluble under many oxygen-poor conditions and can precipitate as uraninite, a uranium dioxide mineral. That transformation can effectively take uranium out of the dissolved phase. But the new research shows that the chemistry does not necessarily stop there. Bacteria can also help produce U(V), an oxidation state traditionally regarded as a fleeting intermediate.

That is the unusual part of the discovery. Pentavalent uranium sits chemically between U(VI) and U(IV), and scientists have often treated it as an unstable stage during uranium reduction. The 2026 study shows that under realistic mine-water conditions, U(V) can persist and become incorporated into solid phases rather than immediately disappearing into another oxidation state.

Glycerol gives the underground microbes energy

The researchers worked with water from the flooded Schlema-Alberoda uranium mine in Germany's Ore Mountains, a site associated with the country's long uranium-mining history. Instead of introducing a laboratory strain designed specifically for uranium removal, they studied the microbial community already present in the mine water. That distinction is important because natural mine environments contain mixed populations of microorganisms living under complex chemical conditions.

The team supplied glycerol as an electron donor and food source while maintaining oxygen-free conditions. Glycerol is a simple compound associated with fats and is also produced as a byproduct of biodiesel manufacturing. For microbes, it can provide the chemical energy needed to drive metabolic reactions. In the mine-water experiments, glycerol proved particularly effective at stimulating microbial uranium reduction compared with some other tested electron donors.

As the microorganisms became active, dissolved U(VI) concentrations declined. The researchers observed the development of dark precipitates containing uranium, indicating that the metal was leaving the water and becoming associated with solid material. By the later stages of the experiment, roughly 90 percent of the dissolved uranium had been removed, with the broader experimental result reaching about 95 percent by around 130 days.

The bacteria do more than simply trap uranium

At first glance, it might seem that the bacteria simply absorbed uranium into their cell walls. There was evidence for uranium accumulation on bacterial surfaces, but microscopy and spectroscopy revealed a more complicated process. The microorganisms were changing uranium chemically while creating conditions in which new uranium-bearing nanoparticles could form.

The researchers used several advanced techniques to determine what was happening at the atomic level. These included high-energy-resolution X-ray absorption spectroscopy, extended X-ray absorption fine structure measurements and high-resolution transmission electron microscopy. Experiments were also performed at the Rossendorf Beamline at the European Synchrotron Radiation Facility in Grenoble. Together, these methods allowed the scientists to distinguish different uranium oxidation states and identify the structures surrounding the uranium atoms.

The results showed that U(VI) was being reduced along more than one chemical pathway. Some uranium became U(IV), forming tiny particles of uraninite. But a significant fraction also became U(V). In samples collected after substantial uranium removal, the researchers detected U(V) alongside the dominant U(IV), with the U(V) fraction accounting for roughly 20 to 30 percent of the uranium in the analyzed black precipitates.

A rare uranium compound emerges

The most striking result was the identification of FeU(V)O₄, a uranium- and iron-containing compound in which uranium exists in the pentavalent state. The compound is unusual because U(V) has generally been considered difficult to stabilize under natural conditions. Yet FeU(V)O₄ appears capable of holding the uranium in place for long periods.

The researchers found FeU(V)O₄ nanoparticles alongside uraninite nanoparticles. Across five analyzed cells, they identified 231 nanoparticles, including 128 FeU(V)O₄ particles and 93 uraninite particles. Most of the uranium nanoparticles were extremely small, typically only about 2 to 3 nanometers across. At that scale, their surfaces and atomic structures can strongly influence how the particles interact with surrounding water and minerals.

The researchers also detected another U(V) form associated with carbonate. That means the microbial system was not producing a single uniform uranium compound. Instead, uranium was being distributed among several reduced chemical forms, with FeU(V)O₄ representing an especially important stable phase.

Why the oxygen experiment surprised researchers

The stability of FeU(V)O₄ became even more interesting when the researchers introduced oxygen. Conventional thinking might suggest that exposing a reduced uranium compound to oxygen would push uranium back toward a more oxidized and potentially mobile state. Instead, the amount of FeU(V)O₄ increased after dried bacterial biomass was exposed to oxygen.

The researchers subsequently detected U(V) after four weeks of oxygen exposure. The Nature Communications study therefore provides evidence that the pentavalent uranium phase is not simply an ultra-short-lived intermediate that exists for minutes or hours. Under the experimental conditions, U(V) persisted for at least 130 days without oxygen and remained detectable after weeks of oxygen exposure.

That observation changes the way scientists can think about uranium reduction underground. Rather than a simple sequence in which U(VI) is reduced directly to U(IV), microbial activity can produce a more complicated mixture involving U(VI), U(V) and U(IV). The intermediate state may itself contribute to long-term immobilization.

Microbes may be shaping uranium minerals

The microbial community also offers clues about how the chemistry develops. The researchers found enrichment of fermentative microorganisms and sulfate-reducing bacteria under the glycerol-stimulated conditions. These microbes can alter the surrounding redox environment and influence the availability of iron, sulfur and other elements that participate in uranium mineral formation.

In this setting, the bacteria are not necessarily acting like microscopic sponges that physically soak up uranium. Their larger role is chemical. By consuming glycerol and carrying out their metabolism, they change the surrounding environment in ways that favor uranium reduction and the formation of stable mineral phases. Uranium becomes part of a new solid structure rather than remaining freely dissolved in groundwater.

That distinction is crucial for remediation. Simply moving uranium from water onto a biological surface would not necessarily solve the problem if the metal could later detach and become mobile again. A stable mineral phase offers a stronger form of immobilization because the uranium is incorporated into a defined chemical structure.

What this could mean for uranium cleanup

The work does not mean that contaminated mines can immediately be treated by pouring glycerol underground. The experiments were controlled laboratory microcosms, and real contaminated sites vary enormously in their geology, groundwater chemistry, microbial populations and oxygen conditions. Long-term field applications would also have to consider how quickly an electron donor is consumed, how it spreads through groundwater and whether unwanted chemical changes occur elsewhere.

Still, glycerol presents an intriguing possibility. It is relatively inexpensive, and crude glycerol is generated in large quantities as a byproduct of biodiesel production. The study found it could stimulate microbial uranium reduction in mine water containing relatively low uranium concentrations, conditions much closer to environmental contamination than the highly concentrated solutions often used in laboratory experiments.

The deeper significance is not simply that bacteria removed uranium from water. It is that microorganisms appear capable of steering uranium into chemical forms scientists once considered too unstable to persist naturally. The discovery adds U(V) to the picture of uranium's environmental chemistry and suggests that the metal's fate underground can be more complicated, and potentially more controllable, than previously assumed.

For polluted groundwater, that could eventually matter enormously. The challenge is not only to remove uranium from water today, but to prevent it from becoming mobile again tomorrow. If microbial communities can transform dissolved uranium into stable mineral phases such as FeU(V)O₄, they may offer a biological route toward keeping a dangerous element where it can no longer travel freely through the environment.