Interaction of radio-metals with microorganisms

Metals interact in various ways with living organisms. This affects first of all the behavior of the metals in the environment as different metal species differ in their mobility in the geo- and the biosphere as well as in their bioavailability. Conversely, metals are essential for the vitality of cells. Many metals are an integral part of one or more enzymes involved in metabolic and biochemical processes. Beside essential metals there are also toxic and radioactive metals that can seriously damage an organism at least at higher concentrations. Figure 1 shows possible interaction mechanisms between microorganisms and radio-metals.
Furthermore, radio-metals may damage or even destroy cells by radiation. The latter excites or ionizes atoms or molecules causing the formation of radicals, changes of biomolecules or even the breakage of chemical bonds. But also for this kind of damage microorganisms have successfully developed effective strategies. Different spectroscopic methods and electron microscopy reveal that different groups of organisms such as bacteria, algae and fungi, differ in their interaction with radio-metals.
In case of bacterial uranium mining waste pile isolates belonging to the genera Lysinibacillus and Bacillus it was demonstrated that so-called S-layer proteins, forming a latticed protein envelope on many bacteria and almost all archaea, are able to effectively scavenge reactive oxygen species (ROS). The latter can be formed by either radiolysis of water or the Fenton reaction. These ROS react with tyrosine side chains of the proteins forming bityrosines and thereby causing an intramolecular crosslinking. Furthermore, these S-layers possess different functional groups on their surface such as carboxyl, hydroxyl, amino, phosphate, sulfoxide and sulfate groups. These groups mediate selective binding of different metals including uranyl(VI) [2]. As most S-layer proteins are also calcium binding proteins, these S-layers additionally possess at least two different Ca binding sites binding trivalent actinides such as Cm(III) with high affinity [3]. In case of the alga Chlorella vulgaris U(VI) concentrations up to 5 µM are bound via carboxyl and phosphate groups being located on the cell surface. This process is followed by desorption in which probably the secretion of complexing bio-ligands is involved [4]. At higher uranium concentrations of 100 µM the alga will die and no desorption can be observed. In comparison to this alga, the fungus Schizophyllum commune interacts with moderate concentrations of uranium (4.2 µM) via organic phosphates. At higher U(VI) concentrations (420 µM) the fungus stays alive and accumulates uranium additionally inside the cell by forming inorganic uranyl phosphates [5]. Due to their high uranium resistance and high accumulation rates different fungi were selected to be further investigated regarding their application potential for a fungal-based concept for the reliable immobilization of released radionuclides within the so called BioVeStRa project.

References
[1] Lloyd J.R. and Macaskie L. E. (2002), In Interactions of Microorganisms with Radionuclides, Ed. Keith-Roach & Livens, Elsevier, 313-342,
[2] Merroun M.L. et al. (2005), Appl. Environ. Microbiol. 71(9), 5532-5543.
[3] Moll H. et al. (2011), Curium(III) complexation with surface-layer (S-layer) proteins from a uranium mining waste pile isolate. Poster at Migration 2011, 18.-23.09.2011, Beijing, PR China.
[4] Vogel M. et al. (2010), Sci. Total Environ. 409, 384-395.
[5] Günther A. et al. (2014), Biometals 27,775-785.