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. 2005 May 6;4(1):13.
doi: 10.1186/1475-2859-4-13.

Characteristics and adaptability of iron- and sulfur-oxidizing microorganisms used for the recovery of metals from minerals and their concentrates

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Characteristics and adaptability of iron- and sulfur-oxidizing microorganisms used for the recovery of metals from minerals and their concentrates

Douglas E Rawlings. Microb Cell Fact. .

Abstract

Microorganisms are used in large-scale heap or tank aeration processes for the commercial extraction of a variety of metals from their ores or concentrates. These include copper, cobalt, gold and, in the past, uranium. The metal solubilization processes are considered to be largely chemical with the microorganisms providing the chemicals and the space (exopolysaccharide layer) where the mineral dissolution reactions occur. Temperatures at which these processes are carried out can vary from ambient to 80 degrees C and the types of organisms present depends to a large extent on the process temperature used. Irrespective of the operation temperature, biomining microbes have several characteristics in common. One shared characteristic is their ability to produce the ferric iron and sulfuric acid required to degrade the mineral and facilitate metal recovery. Other characteristics are their ability to grow autotrophically, their acid-tolerance and their inherent metal resistance or ability to acquire metal resistance. Although the microorganisms that drive the process have the above properties in common, biomining microbes usually occur in consortia in which cross-feeding may occur such that a combination of microbes including some with heterotrophic tendencies may contribute to the efficiency of the process. The remarkable adaptability of these organisms is assisted by several of the processes being continuous-flow systems that enable the continual selection of microorganisms that are more efficient at mineral degradation. Adaptability is also assisted by the processes being open and non-sterile thereby permitting new organisms to enter. This openness allows for the possibility of new genes that improve cell fitness to be selected from the horizontal gene pool. Characteristics that biomining microorganisms have in common and examples of their remarkable adaptability are described.

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Figures

Figure 1
Figure 1
Model of the iron oxidation electron transport pathway of At. ferrooxidans based partly on references [10, 75]. Electrons are transferred from the membrane-located cytochrome c 2 [97] to rusticyanin and then along one of two paths. The downhill path is via cytochrome c4 (Cyt1) to cytochrome aa3 [2] or the uphill, reverse electron transport path via cytochrome c4 (CytA1) to a bc1 I complex and a NADH-Q oxidoreductase [28]. At. ferrooxidans has up to twelve cytochromes c [98] and a variety of cytochrome oxidases some of which appear to play different roles depending on whether iron or sulfur is being oxidized [10]. The NADH is responsible for mercury reduction using a MerA mercuric reductase and the cytochrome aa3 is required to reduce mercury via the unique iron dependent mechanism discovered in At. ferrooxidans [84].
Figure 2
Figure 2
A composite model of sulfur oxidation electron transport pathway of At. ferrooxidans based on references [10, 76, 96]. Thiol groups of outer membrane proteins are believed to transport the sulfur to the periplasm where it is oxidized by a periplasmic sulfur dioxygenase (SDO) to sulfite and a sulfite acceptor oxidoreductase (SOR) to sulfate [76]. Although other cytochrome oxidases are present, a ba3 cytochrome oxidase and a bc1 II complex together with a bd-type ubiquinol oxidase are believed to play the major roles during sulfur oxidation [10, 96]. Rusticyanin and an iron oxidizing protein (not shown) might also be involved during sulfur oxidation but their exact role is still to be determined [96].
Figure 3
Figure 3
The arsenic resistance gene containing transposon, TnAtcArs, present in highly arsenic resistant strains of At. caldus [90]. The arsenic resistance genes are located between the inverted repeat sequences (IR), resolvase (tnpR) and transposase (tnpA) genes of the Tn21-like transposon. R, arsenic resistance regulator; C, arsenate reductase; D, upper-limit arsenic regulator; A, arsenite efflux-dependent ATPase; 7, ORF with a NADH oxidoreductase domain; 8, ORF with a CBS-like domain; B, membrane arsenite efflux transporter.

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