Abstract
Extreme thermoacidophiles (Topt > 65 °C, pHopt < 3.5) inhabit unique environments fraught with challenges, including extremely high temperatures, low pH, as well as high levels of soluble metal species. In fact, certain members of this group thrive by metabolizing heavy metals, creating a dynamic equilibrium between biooxidation to meet bioenergetic needs and mechanisms for tolerating and resisting the toxic effects of solubilized metals. Extremely thermoacidophilic archaea dominate bioleaching operations at elevated temperatures and have been considered for processing certain mineral types (e.g., chalcopyrite), some of which are recalcitrant to their mesophilic counterparts. A key issue to consider, in addition to temperature and pH, is the extent to which solid phase heavy metals are solubilized and the concomitant impact of these mobilized metals on the microorganism’s growth physiology. Here, extreme thermoacidophiles are examined from the perspectives of biodiversity, heavy metal biooxidation, metal resistance mechanisms, microbe-solid interactions, and application of these archaea in biomining operations.
Highlights
The commercial application of microorganisms for the extraction of metals from sulfide ores, concentrates, low-grade ores and tailings, often referred to as bioleaching and biomining, falls within the discipline biohydrometallurgy [1,2]
The CopA operon occurs as the gene cluster copRTA, encoding the copper-responsive regulator CopR [189], the copper-binding protein CopT containing the metal coordinating ligands within the so-called trafficking, resistance and sensing of heavy metals (TRASH) domain [190], and the Cu(I) transporting P1B-type ATPase, which are induced under the presence of excess copper and represent the general structure of the operon found in archaea [191,192]
These studies have largely focused on bacteria with the mechanisms of uranium accumulation and the resulting uranium complexes being poorly understood in archaea
Summary
The commercial application of microorganisms for the extraction of metals from sulfide ores, concentrates, low-grade ores and tailings, often referred to as bioleaching and biomining, falls within the discipline biohydrometallurgy [1,2]. Bioleaching leverages microbially-based conversion of insoluble metal sulfides (or oxides) to water-soluble metal sulfates. Conversion of insoluble chalcopyrite (CuFeS2) to a soluble copper sulfate has become the basis for technologically important processes. The development of bioleaching and biomining technologies has been ongoing for several decades, and more recently is finding increased interest in commercial application. The future of biomining was once declared to be “hot”, owing to the recalcitrance of metal sulfides such as chalcopyrite at moderate temperatures, thereby requiring thermal conditions (65–80 °C) to obtain increased solubilization rates [4,5,6]. The mechanisms by which metal biooxidation, metal resistance, and microbe-solid interactions take place in thermal, acidic environments are not well understood but, if elucidated, could provide valuable information necessary for the successful application and optimization of extremely thermoacidophilic bioleaching
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