The various biosorption mechanisms mentioned above can take place simultaneously. An alternative was to synthesize an in vitro gene that codes for proteins similar to PCs, whose general structure corresponded to (Glu-Cys) nGly (ECs) with all amino acids linked chemically by type α (Figure 5-B). For example, the alginates of marine algae occur as salts of K+, Na+, Ca2+, and Mg2+. The biosorption of copper by Ganoderma lucidium (Muraleedharan & Venkobachr, 1990) and Aspergillus niger fungi was also taken up by the ion exchange mechanism. Perhaps, because of the toxicity of these compounds, microorganisms have not evolved appropriate pathways to bioaccumulate them; populations of microorganisms responsible for this bioaccumulation are not large or active enough to remove these compounds completely, or complex mixtures of pollutants resist removal by existing pathways. This occurs, for example, with arsenate, which competes with phosphate, and cadmium, which competes with zinc. Thus, in general it appears that the potential impact of introduction of GMOs on native microbial populations is not uniform and therefore must be evaluated on a case-by-case basis. al., 1990). These PCs are present in plants, fungi, nematodes, parasites and algae, including cyanobacteria. For example a genetically engineered E.coli, which expresses the Hg2+ transport system and metallothionein (a metal binding protein), was able to selectively accumulate 8 µmole Hg2+/g cell dry weight. The metal transport systems may become confused by the presence of heavy metal ions of the same charge and ionic radius associated with essential ions. This kind of mechanism is not associated with metabolic activity. al., 2000; 2001; Xu et. This facilitates the transport and translocation of insoluble substrates, in turn facilitating biosorption. The toxicity and interesting solution chemistry of elements such as chromium, arsenic and selenium make them good candidates for study. In contrast, the presence of Fe2+ and Zn2+ was found to influence uranium uptake by Rhizopus arrhizus (Tsezos & Volesky, 1982) and cobalt uptake by different microorganisms seemed to be completely inhibited by the presence of uranium, lead, mercury and copper (Sakaguchi & Nakajima, 1991). Introduction. al., 2007). There are two major reasons for this: first, the efficiency of GMOs is dependent on their ability to carry the genetic material in a stable manner; second, the transfer of genetic material to the indigenous organisms is perceived to be a negative attribute, despite the fact that this transfer is a common phenomenon among native organisms. These organic acids help in the solubilization of metal compounds and leaching from their surfaces. al., 2010; Kuroda & Ueda, 2011). Genetic engineering has the potential to improve or redesign microorganisms, where biological metal-sequestering systems will have a higher intrinsic capability as well as specificity and greater resistance to environmental conditions (Bae et. In general, metal ions can be classified as: 1) Essential and important for metabolism (Na, K, Mg, Ca, V, Mn, Fe, Co, Ni, Cu, Zn, Mo and W); 2) Toxic heavy metals (Hg, Cr, Pb, Cd, As, Sr, Ag, Si, Al, Tl), which have no biological function (in ecotoxicology terms, hexavalent forms of Hg, Cr, Pb and Cd ions are the most dangerous); 3) Radionuclides (U, Rn, Th, Ra, Am, Tc), which are radioactive isotopes and, although toxic to cells, they are nonetheless important in nuclear medicine procedures; 4) Semi-metals or metalloids (B, Si, Ge, As, Sb, Te, Po, At, Se), which exert distinct biological effects on metals. A typical example includes creating a repetitive metal-binding motif, consisting of (Glu-Cys)nGly (Bae et. Transport across cell membranes: Heavy metal transportation across microbial cell membranes may be mediated by the same mechanism used to convey metabolically important ions such as potassium, magnesium and sodium. In the process of heavy metal ion bioremediation, a complex non-toxic biosurfactant/metallic ion structure is formed, resulting in emulsification and solubilization of the ions, and, consequently, their physical sequestration. (1995) demonstrated that uptake by recombinant E. coli (expressing the Neurospora crassa metallothionein gene within the periplasmic space) was rapid. al., 2001), respectively. Since then, there has been an intensive effort to investigate the binding properties of heavy metals to different types of biomass (Chen & Wang, 2008). al., 2001). Complexation: An extracellular complexation or coordination is the result of electrostatic attraction between a metallic ion chelating agent and a polymer that can be excreted by a microorganism that is viable or not. al., 1988). As a consequence, the rate of biodegradation/bioremediation can be increased. The synthetic phytochelatin EC20 has greater capacity for binding to heavy metal ions than natural PCs. In eight cases the operon was plasmid-borne, and in one case it had integrated into the host chromosome. al., 2000). The complex structure of microorganisms implies that there are many ways for the metal to be taken up by the microbial cell. Brief introduction to this section that descibes Open Access especially from an IntechOpen perspective, Want to get in touch? al., 2004). Bioremediation of polluants utilizing biodegradation abilities of microorganisms include the natural attenuation, although it may be enhanced by engineered techniques, either by addition of selected microorganisms (bioaugmentation) or by biostimulation, where nutrients are added. Natural phytochelatins (PCs) and synthetic phytochelatin (EC20): PCs are small cysteine-rich peptides with the general structure (Glu-Cys) nGly (n = 2-11) (Figure 5-A) (Grill et. al., 1999; Selatnia et. With molecular methods allowing the characterization of microbial community structure and activities, the performance of microorganisms under in situ conditions and in concert with the indigenous microflora will become predictable. The process continues until equilibrium is established between the amount of solid-bound sorbate species and its portion remaining in the solution. al. The mitochondria and the nucleus have their own GSH reservation, which is critical or instrumental in protecting these structures against ROS action (Penninckx, 2002; Inouhe, 2005; Mendoza -Cózatl et. The conditions during the composting process include elevated temperatures (as high as 80–90º C), decreases in pH due to organic acid production, and the production of toxic metabolites that can greatly decrease microbial populations. Electrostatic interactions have been proven to be responsible for copper biosorption by the Zoogloea ramigera bacterium and the Chiarella vulgaris alga (Asku et. Some of these elements are extremely toxic to human beings, even at very low concentrations (Roane & Pepper, 2000; Wang & Chen, 2006). As discussed above, there is concern that GMOs introduced into polluted sites to enhance bioremediation may have adverse environmental effects because of horizontal transfer of recombinant DNA (Davison, 1999). With molecular methods enabling the characterization of microbial community structure, metabolic pathways and enzyme activities, the performance of microorganisms under in situ conditions can be improved by making heavy metal bioremediation a much more efficient process.