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3 1.1. Introduction
Uranium mining and related activities (milling, ore processing, fuel fabrication, etc.) that result in the generation of large volume of heavy metal (Cu, Cd, Co, Zn, Ni, Cr, Pb, etc.) and radionuclide (U, Th, etc.) containing wastes is of intense environmental and public health concern (Lloyd and Macaskie, 2000; Gavrilescu et al., 2009). Once released, environmental fate of these metallic contaminants is significantly controlled by microbial activity. Bacteria owing to their small size and high surface to volume ratio represent a significant fraction of the reactive surface area for interaction with dissolved molecules/nano minerals and thus potentially affecting metal and radionuclide migration (Francis et al., 2004; Merroun et al., 2006). Microbial properties that can affect changes in metal speciation, toxicity and mobility constitutes important components of natural biogeochemical cycles for metals as well as associated elements in biomass, soil, rocks and minerals (e.g. sulfur and phosphorus, metals and metalloids, actinides, and radionuclides) (Gadd, 2010). Inhabiting microorganisms often exploit a range of interactions with heavy metals and radionuclides that include (i) binding to membrane, cell wall anionic ligands, intracellular accumulation/biosorption, (ii) precipitation and mineralization (iii) complexation by extracelluar polymeric substances and other metal chelating microbial products and (iv) redox transformations (Gadd, 2004; Merroun and Selenska-Pobell, 2008). These metal-microbe interaction mechanisms provide valuable insight into the role of inhabiting microbes in complex biogeochemical reactions and are of great importance for the development of microbe based remediation strategies and other biotechnological applications (Lloyd and Macaskie, 2000).
In recent years considerable efforts have been made to explore the microbial community structure and composition using both culture -dependent and -independent approaches at various uranium-mine contaminated habitats. The results indicated that these environments harbor a large variety of very diverse microorganisms organized in site-specific complex communities (Merroun and Selenska-Pobell, 2008). In a number of cases bacterial strains isolated from such sites have been well characterized in terms of their interaction mechanisms leading to metal/radionuclide biosorption/
bioaccumulation or precipitation (Selenska-Pobell et al., 1999; Panak et al., 2002;
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Suzuki and Banfield, 2004; Nedelkova et al., 2007; Choudhary and Sar, 2009). It was also observed that microorganisms present in U-contaminated sites are very well adapted to the local harsh conditions and play a major role in the mobility of U in the environment through complexation (Lovley et al., 1991; Merroun et al., 2005;
Martinez et al., 2007).
Along with the microbe metal interaction processes major thrust has also been given to characterize bacterial heavy metal resistance. Despite apparent toxicity, many microbes grow and even flourish in metal-polluted locations, and a variety of mechanisms, both active and incidental, contribute to metal resistance (Gadd, 2010).
It seems that most survival mechanisms depend on some change in metal speciation leading to decreased or increased mobility. These include redox transformations, the production of metal-binding peptides and proteins (e.g. metallothioneins, phytochelatins), organic and inorganic precipitation, active transport, efflux and intracellular compartmentalization, while cell walls and other structural components have significant metal-binding abilities (Silver, 1996; Robinson et al., 2001; Gadd, 2004, 2010). Presence of metal resistance determinants in indigenous microbes surviving in metal and radionuclide contaminated sites is not only necessary for their survival but also considered to be desirable as such traits facilitate and/or enhance microbial metabolism during subsequent bioremediation activities in these environments (Martinez et al., 2006). Deciphering heavy metal resistance mechanisms and identifying the putative genes conferring such resistance gained considerable research impetus owing to their importance in several biotechnological applications.
Particularly, the use of metal resistant bacteria in bioremediation of contaminated sites, and in biomining of expensive metals have been identified as the most promising areas (Barkay and Schaefer, 2001; Gadd, 2010). Addition of metal resistance properties to microbes for its use in other biotechnological processes, which may or may not be related to metal is also considered useful. In addition to these, microbe–metal–mineral transformations have also found applications in other areas of biotechnology and bioprocessing, including biosensors, biocatalysis, electricity generation and nanotechnology (Gadd, 2010). Along with such biotechnological significance, bacterial metal resistance or bacteria-metal interaction has created interest among geobiologists and geochemists as it provides an improved understanding of the biogeochemistry of metal cycling in the environment and plays a
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key role in affecting metal mobility and transfer between different biotic and abiotic locations (Nies, 1999; Gadd, 2004). In spite of considerable research activities oriented towards unraveling the mystery of microbial metal homeostasis, our knowledge on microbial potential and their role in dealing with such toxic elements (uranium and other metals) is not complete, particularly, in consideration with the vast metabolic and genetic diversity of microbial world along with their non culturablity.
Therefore there is strong demand for further investigations on isolating pure culture bacteria from highly contaminated habitats, deciphering their physiology and metabolic capabilities, and understanding the underlying mechanisms of microbe- metal interaction. From these perspectives, the present work is an attempt to address some of these important issues pertinent to interaction of bacteria from U- mine wastes with uranium and other metals
1.2. Review of literature
1.2.1 Uranium and other heavy metals in the environment
Heavy metals including uranium are naturally present in the Earth’s crust, and are continuously released into the environment from various geochemical activities including weathering of rocks and minerals. In addition to such natural release, anthropogenic sources such as, increased mining activities, and other industrial applications generate substantial quantities of heavy metal containing wastes (Lloyd and Macaskie, 2000). With reference to uranium in our environment a major localized source is U-mining and milling operations including nuclear fuel cycle, uranium conversion, enrichment, fuel fabrication, and use of phosphate fertilizers (that may contain 200 mg U Kg-1) (Meinrath et al., 2003; Gavrilescu et al., 2009). Along with U, other toxic co-contaminants released during U- mining activities are several heavy metals and metalloids (Ni, Cu, As, Mo, Se, Cr, Zn, Co and Cd) which pose a high risk of contamination for the surface and subsurface environments (Lloyd and Macaskie, 2000; Pollmann et al., 2006). Particularly, improper disposal and poor management of large amount of heavy metal and radioactive containing mixed wastes from uranium mining and milling cause enormous damage to the environment by affecting surface and ground water, soils, subsurface sediments and even the catchment areas of drinking water (Gavrilescu et al., 2009). Once released in to ecosystem uranium and