Metal-tolerant thermophiles: metals as electron donors and acceptors, toxicity, tolerance and industrial applications

Abstract

Metal-tolerant thermophiles are inhabitants of a wide range of extreme habitats like solfatara fields, hot springs, mud holes, hydrothermal vents oozing out from metal-rich ores, hypersaline pools and soil crusts enriched with metals and other elements. The ability to withstand adverse environmental conditions, like high temperature, high metal concentration and sometimes high pH in their niche, makes them an interesting subject for understanding mechanisms behind their ability to deal with multiple duress simultaneously. Metals are essential for biological systems, as they participate in biochemistries that cannot be achieved only by organic molecules. However, the excess concentration of metals can disrupt natural biogeochemical processes and can impose toxicity. Thermophiles counteract metal toxicity via their unique cell wall, metabolic factors and enzymes that carry out metal-based redox transformations, metal sequestration by metallothioneins and metallochaperones as well as metal efflux. Thermophilic metal resistance is heterogeneous at both genetic and physiology levels and may be chromosomally, plasmid or transposon encoded with one or more genes being involved. These effective response mechanisms either individually or synergistically make proliferation of thermophiles in metal-rich habitats possibly. This article presents the state of the art and future perspectives of responses of thermophiles to metals at genetic as well as physiological levels.

Identification of a New Heavy-Metal-Resistant Strain of Geobacillus stearothermophilus Isolated from a Hydrothermally Active Volcanic Area in Southern Italy

“…The new isolate was also found to be Cd(II) tolerant, and in this case the MIC value determined was similar to that measured in other Geobacilli (ranging from 0.4 to 3.2 mM) [71]. For the majority of these microorganisms Cd(II) resistance was ascribed to biosorption, i.e., a phenomenon of metal binding to the microbial cell wall, which does not involve energy consumption [72,73]. Interestingly, the Pisciarelli site is enriched in arsenic and vanadium but lacks cadmium (see Section 3.1).…”

Metal-Tolerant Thermophiles: From the Analysis of Resistance Mechanisms to their Biotechnological Exploitation

“…Microorganisms able to tolerate high levels of heavy metal ions have evolved in ore deposits, hydrothermal vents, geothermal sites, as well as in different polluted sites [23]. Metal tolerance of thermophilic Bacteria/Archaea is due to several mechanisms, many also found in mesophilic counterparts, such as: extracellular barrier, metal ion transport into and outside the cell, the utilization of toxic metal ions in metabolism or the presence of metal resistance genes with different genomic localization (chromosome, plasmid or transposon) [24].…”

“…Many thermophilic microorganisms employ intracellular enzymatic conversions combined with efflux systems to obtain heavy metal resistance. Enzymatic reduction of metal ions can result in the formation of less toxic forms like Hg(II) reduced to Hg(0), Cr(V) converted into Cr(III) [23] or, as in the case of As(V), reduction in the more toxic As(III) which is the only form extruded by the cell.…”

“…Another common mechanism to inactivate toxic metal ions is the intracellular sequestration or the complexation of metal ions by various compounds in the cytoplasm. The metallothioneins and phytochelatins are two classes of peptides rich in cysteine residues which bind metal ions through the sulfhydrylic groups [59].…”

“…In this contest, among the heavy metals resistance systems developed by microorganisms, and in particular by thermophiles, biosorption and bioaccumulation are emerging as promising lowcost methodologies for bioremediation [84,85]. Biosorption and bioaccumulation are two processes that consist into the ability of microorganisms to accumulate heavy metals from wastewater through metabolic pathways or physicalchemical uptake; but while biosorption is a passive process depending on the composition of the cellular surface and following a kinetic equilibrium, bioaccumulation is an energy driven process and requires an active metabolism [59].…”