Microbiology and Radioactivity Chapter 8. Diversity and Activity of Bacteria in Uranium Waste Piles

The pollution of the environment with toxic metals is one of the most severe problems of our industrial century. The uranium mining waste piles are a subject of particular attention, because in soils, sediments, and drain waters of these environments significant amounts of many hazardous metals such as uranium, caesium, strontium, selenium, molybdenum, arsenic, cadmium, chromium, mercury, lead, copper, nickel, zinc, etc. are present (Francis, 1990). In addition, significant amounts of thorium, radium, and polonium might also be present at the so called uranium "mill tailings" where the extraction of uranium from the ores was performed.
It was demonstrated that even in the most heavily polluted uranium wastes large numbers of bacteria are present (Cerda et al., 1993; Goebel & Stackebrandt, 1994; Shippers et al., 1994). Moreover, different groups of bacteria are interacting in various ways with metals and radionuclides. Some of the most important mechanisms by which bacteria can biotransform and influence the mobilisation and/or immobilisation of metals are listed below:
1) Direct oxidation and/or reduction of metals, which affect their solubility (DiSpirito & Tuovinen, 1982; Lovley, 1993; Nelson et al., 1999; Sharma et al. 2000; Wildung et al., 2000);
2) Direct or indirect oxidation of metal sulfides and solubilization of certain elements (Bosecker, 1997; Krebs & Brandl, 1997);
3) Indirect alteration of metal ionic states caused by bacterial induced pH and Eh changes in the medium (Bosecker, 1997; Bacelar-Nicolau & Johnson, 1999);
4) Bioaccumulation [biosorption by cell surface polymers (DiSpirito et al., 1983; Douglas & Beveridge, 1998; Macaskie et al., 1992; Panak et al., 1999; Selenska-Pobell et al., 1999; Valentive et al., 1996) and/or uptake of metals inside the cells (Klaus et al., 1999; Marques et al., 1991; Purchase et al., 1997)].
5) Bio-mineralization, which includes induction of metal precipitates by particular metabolic functions of some bacteria and consequent generation of minerals (Brown and Beveridge, 1998; Douglas and Beveridge, 1998; Francis, 1998);
6) Release of the biosorbed metals by chelation, alkylation, or decomposition (Bosecker, 1997; Francis, 1990; Francis et al., 1998).
It is clear that the above described bacterial activities are strongly influencing the fate and the migration of toxic metals in and outside of the sites where uranium mining was performed.
In addition to the living cells, significant amounts of different bacterial metabolites are present in the wastes which are also interacting with the heavy metals and influencing their behaviour.
For these reasons, the knowledge about the diversity and about the activity of the indigenous bacteria in the uranium waste piles is of basic importance for understanding the bio-geo-chemical processes occurring in these environments and especially for modelling the process of migration of the heavy metals and radionuclides in nature.