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Uranium is found in minerals in the soil, dissolves in mining water and ends up in the fields together with phosphate fertilizer. In Germany, the heavy metal uranium is particularly common in Saxony and Thuringia although it also occurs in Southern Germany. In cooperation with French researchers, experts from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) have now elucidated how uranium interacts chemically with diatoms (DOI: 10.1038/s41598-025-93350-5). As algae can be the starting point for the accumulation of harmful substances in the food chain, the results help to better understand the impact of the release of uranium on natural cycles.
“Naturally occurring uranium has a long half-life and is thus not only radiotoxic but also chemotoxic,” says chemist Dr. Susanne Sachs from the Institute of Resource Ecology at HZDR. “The extent to which uranium can be absorbed by organisms depends on its chemical form: when it is dissolved, bacteria, plants, animals, and humans can absorb it more easily than when it isn’t.”
It has long been known that uranium can be passed on in the food chain. To date, however, there aren’t any fundamental insights into how these processes work in detail and how critical the consequences are for humans and the environment. “Are there crop plants, for example, that accumulate uranium more than others and do they then pose a particularly high risk?” asks microbiologist Dr. Johannes Raff. “There is still a lot we don’t know about this. So, at HZDR, we are trying to understand how uranium behaves in the environment and how it interacts with various organisms.”
Now, a doctoral dissertation that has been mentored by the scientists in cooperation with researchers from the Subatech laboratory at the French National Centre for Scientific Research (CNRS) has uncovered new knowledge. In order to explore how hexavalent uranium chemically interacts with the freshwater diatom Achnanthidium saprophilum, a French doctoral candidate conducted a series of experiments at HZDR. One major challenge was that in nature this algae is associated with various species of bacteria. So, potentially, both forms of life interact with uranium simultaneously when it enters a body of water.
Uranium is bound both on the surface and within the algae
“The first question to answer was whether uranium is only bound to the algae externally or enters the algae, too. We also wanted to discover which chemical bonds are formed and understand the underlying processes at molecular level,” explains Sachs.
Initially, the researchers grew the algae in a culture medium and then transferred them to a solution enriched with uranium. “At certain intervals – ranging from a few hours to weeks – our doctoral student removed the algae from the solution again and measured how much uranium remained in the solution,” says Sachs. “This meant we could see how much heavy metal the algae had absorbed in total and whether the intake had changed over time.” The team then prepared diatom samples to study them using electron microscopy and x-ray spectroscopy. They wanted to find out where precisely the uranium had accumulated.
Beyond phosphates: role of carboxyl groups
“Here we were able to observe that uranium really does interact with the algae. It occurs both on the surface and inside the algae – at the places where phosphorus compounds are present,” reports Sachs. With regard to the interaction of uranium with phosphorous-containing functional groups on the surface of the algae it was initially unclear onto which substructures the uranium had docked. Raff explains: “The units of phosphate on which the uranium is bound could be part of a sugar, a protein or even an inorganic substance on the surface of the algae. We can’t say for sure how stable these compounds are.”
Using fluorescence spectroscopy, the researchers checked whether different chemical bonds were created over time. And indeed: their analyses indicated that two different uranium species were present in the samples, whose proportions changed over time. The team suspected that, initially, uranium mainly bonds on the surface of the algae. Over time, it increasingly infiltrates the interior of the algae where a second type of bonding is formed. Infrared spectroscopy tests revealed that the uranium was bound on the one hand via so-called carboxylic functional groups, i.e., molecule parts containing oxygen that easily bond with metals, and on the other, via phosphate-containing groups.
Long-term perspective on ecosystem threats
“Our study demonstrates that diatoms are not only able to bind uranium but also to incorporate it into the cell interior,” summarizes Sachs. “Moreover, we were able to acquire some initial insights into the chemical bonds that are created.” However, it is still uncertain to what extent the associated bacteria might have contributed to the bonding – and whether the chemical bonds would change significantly over time. To find this out, the algae samples that were in contact with uranium would have to be regularly tested over a longer period – for example, over six months or a whole year. “It would be important to be able to differentiate so as not to over- or underestimate effects.”
Besides other phytoplankton, diatoms are the first link in the food chain that create the basis for feeding higher organisms such as copepods which, in turn, are food for fish. At this point, the algae become the gateway for uranium: if it enters water the question arises as to what extent the heavy metal is passed on to other species via diatoms and accumulates in the food chain – for example, in fish, birds and amphibians. The researchers hope to acquire more knowledge on this in follow-up experiments. Knowledge of this kind would not only be relevant in the context of the legacy of uranium ore mining or the long-term safety of final repositories. “According to the Federal Agricultural Research Centre, every year, 167 tons of uranium are deposited on fields in Germany together with phosphate fertilizers,” says Raff. “And rare earths and other useful metals are often associated with radionuclides. There is a risk that these dangerous substances get into the ecosystem and cause damage. This can be mitigated by careful action, appropriate risk assessments and protective measures.”
Publication:
Y. He, V. Sushko, R. Hübner, H. Foerstendorf, R. Steudtner, J. Raff, C. Mallet, A.-H. Le Jeune, A. Beauger, V. Breton, O. Péron, T. Stumpf, S. Sachs, G. Montavon, A multiscale investigation of uranium (VI) interaction with a freshwater diatom species, in Scientific Reports (2025). (DOI: 10.1038/s41598-025-93350-5)
Additional information:
Dr. Susanne Sachs | Dr. Johannes Raff
Institute of Resource Ecology at HZDR
Phone: +49 351 260 2436 | +49 351 260 2951
Email: s.sachs@hzdr.de | j.raff@hzdr.de
Media contact:
Simon Schmitt | Head
Communications and Media Relations at HZDR
Phone: +49 351 260 3400 | Mob.: +49 175 874 2865 | Email: s.schmitt@hzdr.de
The Helmholtz-Zentrum Dresden-Rossendorf (HZDR) performs – as an independent German research center – research in the fields of energy, health, and matter. We focus on answering the following questions:
• How can energy and resources be utilized in an efficient, safe, and sustainable way?
• How can malignant tumors be more precisely visualized, characterized, and more effectively treated?
• How do matter and materials behave under the influence of strong fields and in smallest dimensions?
To help answer these research questions, HZDR operates large-scale facilities, which are also used by visiting researchers: the Ion Beam Center, the Dresden High Magnetic Field Laboratory and the ELBE Center for High-Power Radiation Sources.
HZDR is a member of the Helmholtz Association and has six sites (Dresden, Freiberg, Görlitz, Grenoble, Leipzig, Schenefeld near Hamburg) with almost 1,500 members of staff, of whom about 680 are scientists, including 200 Ph.D. candidates.
Dr. Susanne Sachs | Dr. Johannes Raff
Institute of Resource Ecology at HZDR
Phone: +49 351 260 2436 | +49 351 260 2951
Email: s.sachs@hzdr.de | j.raff@hzdr.de
Y. He, V. Sushko, R. Hübner, H. Foerstendorf, R. Steudtner, J. Raff, C. Mallet, A.-H. Le Jeune, A. Beauger, V. Breton, O. Péron, T. Stumpf, S. Sachs, G. Montavon, A multiscale investigation of uranium (VI) interaction with a freshwater diatom species, in Scientific Reports (2025). (DOI: 10.1038/s41598-025-93350-5)
https://www.hzdr.de/presse/diatom-uranium
Artistic impression of the interaction of uranium with the freshwater diatom Achnanthidium saprophil ...
Quelle: B. Schröder
Copyright: B. Schröder/HZDR
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Artistic impression of the interaction of uranium with the freshwater diatom Achnanthidium saprophil ...
Quelle: B. Schröder
Copyright: B. Schröder/HZDR
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