Biomineralization of Uranium in Natural Environments

Prokaryotic microorganisms (bacteria and archaea) are the most ubiquitous organisms in terrestrial and aquatic environments. They play a major role in deposition and weathering of a large variety of minerals enriched with or consisting mainly of different metals, such as iron, manganese, copper, gold, and even radionuclides (e.g. uranium). The structure of biologically synthesized minerals is strongly influenced by the metabolic properties of the bacterial or archaeal strains involved in their production and also by the different metal binding potential of their cell wall components.
The talk will focus on cell wall dependent accumulation and biomineralization of uranium by particular Gram-positive and Gram-negative bacteria recovered from uranium mining wastes. By using TEM, EXAFS and TRLF we were able to demonstrate that the Gram-negative and most of the Gram-positive bacteria inhabiting the oligotrophic uranium mining waste pile environments immobilize U(VI) at their cell walls or extracellularly in a form of uranyl phosphate compounds. Particular Gram-positive bacterial isolates, possessing highly ordered proteinaceous surface layers (S-layers), are immobilizing U(VI) by both phosphate groups of their thick peptidoglycan and of their phosphorylated S-layer and also by the carboxylic groups of the aspartate and glutamate stretches of their S-layers.
In contrast to bacteria, archaeal diversity is not very high in uranium polluted environments. In addition, these prokaryotic organisms interact with uranium in a significantly different way then bacteria. We were able to demonstrate that some archaeal organisms, indigenous for the uranium mining wastes, are not able to accumulate high amounts of uranium and that the mechanisms of uranium binding differ significantly from those of all studied bacteria. The latter is related to the unusual cell wall structure of the studied archaeal cells.