Ecological and industrial aspects of the interaction of radionuclides with bacterial S-layer proteins

Radionuclides interact with biomass in various ways. In general, the different interaction mechanisms can be assigned to two physiological aspects, first a radiological toxicity and second a chemical toxicity. Bacteria and archaea developed very early in evolution a multifunctional cell envelope called surface-layer (S-layer) which protects bacteria in extreme environments. This layer is a para-crystalline protein lattice formed by the self-assembly of secreted proteins on the cell wall of bacteria and archaea. S-layers are often glycosylated and phosphorylated and their lattice formation depends often on bivalent cations such as calcium. In case of bacteria living in radionuclide and heavy metal contaminated environments, S-layers are able to act first of all as scavenger for reactive oxygen species (ROS) formed by either radiolysis of water or Fenton reaction. The inactivation of the radicals is achieved by intermolecular crosslinking of tyrosine residues of the protein monomers. Secondly, S-layer proteins are able to selectively bind several radionuclides such as uranium and curium but also other toxic elements such as arsenic. By restraining these metals on the surface of the cell a toxication of the organism is prevented. Interestingly, the mechanism and binding behavior is different for different elements and is highly dependent on pH. Whereas the hexavalent uranium is bound by several surface exposed functional groups and is easily released at acidic pH, the trivalent curium substitutes calcium and is only released at pH 2.5 or below. The stable and selective curium complexation is especially interesting as lanthanides are considered as chemical analogous for trivalent actinides. Thusly, S-layers bind also rare earth elements very effectively, being highly interesting for the development of metal selective filter materials for their enrichment and recovery.