Accumulation of Heavy Metals by Microorganisms: Biomineralization and Nanocluster Formation

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 metal binding potentials of their cell wall components.
This chapter is focused on cell wall dependent accumulation and biomineralization of iron and uranium. By using transmission electron microscopic analysis in combination with x-ray absorption and time-resolved laser-induced fluorescence spectroscopic analyses it is demonstrated that the Gram-negative and most of the Gram-positive bacteria as well as some archaea immobilize U(VI) at their cell walls or extracellularly in a form of uranyl phosphate compounds. However, some Gram-positive bacteria which possess highly ordered proteinaceous surface layers (S-layers), are immobilizing U(VI) not only by phosphate groups mainly from their peptidoglycan but also by the carboxylic groups of the aspartate and glutamate stretches of their S-layers.
In addition, the cell wall supported formation of metallic palladium nanoclusters by some bacteria is presented as well. Despite the different mechanisms of the biological deposition of Pd by Gram-negative and Gram-positive bacteria, the nanoparticles formed by both organisms have almost identical size and catalytic activity.