6. Metal Bioremediation by Thermophilic Microorganisms

Environmental pollution with toxic metals is a severe threat to biota and human health. Microbe-mediated bioremediation of such contaminants has emerged as a potential alternative to conventional treatment methods. Thermophilic microorganisms, owing to their natural ability to survive and flourish under elevated temperatures along with other stressful environmental conditions including high concentrations of heavy metals, have developed various adaptation strategies to cope with harsh environments, which may offer enormous opportunities for bioremediation of heavy metals at higher temperatures. Thermophilic microorganisms, being common in geological and anthropogenic thermal environments with high concentrations of dissolved metal, possess unique cell wall structures and metabolic and enzymatic properties that may contribute in metals–thermophiles interactions. Biosorption/bioaccumulation of metals is most effective and widely used approach for the bioremediation. The nature and extent of metal biosorption onto thermophilic bacteria may differ greatly from the mesophilic organisms. Microbial transformation of metal through oxidation/reduction reactions plays a critical role in metal speciation, distribution, and thus altered toxicity in the ecosystems, which may be implemented in metal recovery and remediation. Both sulfate- and metal-reducing bacteria have profound application in metal bioremediation. Thermophilic bacteria with higher metal tolerance and metabolic characteristics at high temperature may exhibit enhanced metal solubilization through sulfur- or iron-oxidizing processes. Thermophilic microbial community can perform both degradative and productive functions through coupling of metal reduction with oxidation of a variety of organic and inorganic substrates. Thermophilic bacteria are also able to reduce a wide spectrum of metals including Mn (IV), Cr (VI), U (VI), Tc (VII), Co (III), Mo (VI), Au (I, III), and Hg (II) which can be used for the immobilization of toxic metals/radionuclides during bioremediation of hot wastewater of disposal sites of radioactive wastes having temperature range favorable for thermophiles for a long period of time. This chapter discusses various modes of metal-microbe interactions in thermophilic bacteria for their promising application in bioremediation.