Diatoms shown to absorb and store uranium inside cells

Uranium, a toxic heavy metal, is present in soils, dissolves in mining water, and can enter farmland via phosphate fertilizers. In Germany, it is especially prevalent in Saxony and Thuringia, though also found in the south. Researchers at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), collaborating with French scientists, have clarified how uranium chemically interacts with diatoms, a key phytoplankton group. The findings enhance understanding of uranium's impact on ecological cycles.

"Naturally occurring uranium has a long half-life and is thus not only radiotoxic but also chemotoxic," said Dr. Susanne Sachs of HZDR's Institute of Resource Ecology. She noted that uranium's chemical form affects its uptake, with dissolved uranium more easily absorbed by living organisms.

Although uranium transfer through the food chain has been known, detailed mechanisms remain unclear. "Are there crop plants that accumulate uranium more than others and thus pose higher risks?" asked microbiologist Dr. Johannes Raff. "We are investigating how uranium behaves in the environment and interacts with various organisms."

A French doctoral student, supervised jointly by HZDR and Subatech at the French National Centre for Scientific Research (CNRS), studied uranium's interaction with the freshwater diatom Achnanthidium saprophilum. In nature, these algae coexist with bacteria, potentially influencing uranium binding when it enters aquatic systems.

The team sought to determine whether uranium binds only to algae surfaces or also enters cells, and to identify the chemical bonds formed. Algae were grown in culture, transferred to uranium-rich solution, and periodically sampled to measure uranium uptake. Electron microscopy and x-ray spectroscopy pinpointed accumulation sites.

Results showed uranium binding both on surfaces and within cells, especially where phosphorus compounds were present. Initially, uranium appeared to bind mainly externally, later penetrating cells to form new bonds. Infrared spectroscopy revealed associations with carboxylic groups and phosphate-containing structures.

"Our study demonstrates that diatoms are not only able to bind uranium but also to incorporate it into the cell interior," Sachs said. The stability of these bonds and the role of associated bacteria remain to be clarified, requiring long-term monitoring.

As diatoms form the base of aquatic food webs, they could pass uranium to higher species such as copepods, fish, birds, and amphibians. This has implications for legacy uranium mining and agricultural practices, given that an estimated 167 tons of uranium are deposited annually on German fields via phosphate fertilizers. Raff warned that radionuclides often accompany rare earths and other useful metals, posing ecological risks without proper mitigation.

Research Report:A multiscale investigation of uranium (VI) interaction with a freshwater diatom species