Special Issue "Geomicrobiology and Biogeochemistry of Precious Metals"

A special issue of Minerals (ISSN 2075-163X).

Deadline for manuscript submissions: closed (31 January 2018)

Printed Edition Available!
A printed edition of this Special Issue is available here.

Special Issue Editors

Guest Editor
Dr. Frank Reith

ARC Future Fellow, School of Biological Sciences, University of Adelaide, Adelaide, South Australia 5005, Australia
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Interests: geomicrobiology; biogeochemistry; microbial ecology; OMICS; forensics; modelling; biomineralisation; gold; platinum-group-elements; uranium; synchrotron applications; bacterial metal resistance
Guest Editor
Dr. Jeremiah Shuster

School of Biological Sciences, University of Adelaide, Adelaide, South Australia 5005, Australia
Website | E-Mail
Interests: geomicrobiology; biogeochemistry; gold; metal cycling; microbe–mineral interactions; electron microscopy; biomineralisation; microfossils

Special Issue Information

Dear Colleagues,

Precious metals continue to have economic and sociocultural importance as their usage evolves and diversifies over time. Since antiquity, gold, silver and later platinum-group-elements have been valued as currencies and stored-investment commodities. Today, the industrial application of precious metals is increasing with developing scientific and technological innovations. For example, the discovery of microbial resistance mechanisms to highly toxic mobile precious metals can be used in biosensors and bioindicators for gold, silver and platinum, thereby expanding biotechnology into mineral exploration and hydrometallurgy. From a geochemical perspective, the in situ cycling and transformation of precious metals is a dynamic balance between dissolution-, transport- and re-precitation processes that are catalysed by the biosphere. Microbial weathering mobilises precious metals by releasing metals ‘trapped’ within minerals and by solubilising these metals via oxidation-promoting complexation. Higher organisms such as plants mitigate precious metal mobility within soils as well as within hydrological regimes. The cycling of precious metals is completed as microbes destabilise soluble metal complexes through bioprecipitation and biomineralisation forming secondary colloids, crystalline microparticles, as well as macroscopic grains and even nuggets.

This Special Issue aims to bring together studies in the areas of geomicrobiology and biogeochemistry of precious metals. Contributions that demonstrate or enhance the fundamental understanding of how biogenic process affect precious metal cycling of or how organisms are effected by mobile precious metals are invited. We especially encourage studies highlighting biological applications for precious metal exploration, mineral/ore processing and bioremediation to encourage sustainable use of resources.

Dr. Frank Reith
Dr. Jeremiah Shuster
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Minerals is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1200 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Geomicrobiology
  • Biogeochemistry
  • Precious metals
  • Gold, silver, platinum-group-elements
  • Prokaryotes
  • Fungi
  • Plants
  • Biomineralisation
  • Metal-resistance
  • Exploration
  • Metallurgy
  • Bioremediation

Published Papers (11 papers)

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Editorial

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Open AccessEditorial Editorial for Special Issue “Geomicrobiology and Biogeochemistry of Precious Metals”
Minerals 2018, 8(10), 459; https://doi.org/10.3390/min8100459
Received: 11 October 2018 / Accepted: 15 October 2018 / Published: 17 October 2018
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Research

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Open AccessArticle Surface Chemical Characterisation of Pyrite Exposed to Acidithiobacillus ferrooxidans and Associated Extracellular Polymeric Substances
Minerals 2018, 8(4), 132; https://doi.org/10.3390/min8040132
Received: 31 January 2018 / Revised: 14 March 2018 / Accepted: 20 March 2018 / Published: 24 March 2018
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Abstract
A. ferrooxidans and their metabolic products have previously been explored as a viable alternative depressant of pyrite for froth flotation; however, the mechanism by which separation is achieved is not completely understood. Scanning electron microscopy (SEM), photoemission electron microscopy (PEEM), time-of-flight secondary ion
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A. ferrooxidans and their metabolic products have previously been explored as a viable alternative depressant of pyrite for froth flotation; however, the mechanism by which separation is achieved is not completely understood. Scanning electron microscopy (SEM), photoemission electron microscopy (PEEM), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and captive bubble contact angle measurements have been used to examine the surface physicochemical properties of pyrite upon exposure to A. ferrooxidans grown in HH medium at pH 1.8. C K-edge near edge X-ray absorption fine structure (NEXAFS) spectra collected from PEEM images indicate hydrophilic lipids, fatty acids and biopolymers are formed at the mineral surface during early exposure. After 168 h, the spectra indicate a shift towards protein and DNA, corresponding to an increase in cell population and biofilm formation on the surface, as observed by SEM. The Fe L-edge NEXAFS show gradual oxidation of the mineral surface from Fe(II) sulfide to Fe(III) oxyhydroxides. The oxidation of the iron species at the pyrite surface is accelerated in the presence of A. ferrooxidans and extracellular polymeric substances (EPS) as compared to HH medium controls. The surface chemical changes induced by the interaction with A. ferrooxidans show a significant decrease in surface hydrophobicity within the first 2 h of exposure. The implications of these findings are the potential use of EPS produced during early attachment of A. ferrooxidans, as a depressant for bioflotation. Full article
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Open AccessArticle Enhanced Tolerance to Cadmium in Bacterial-Fungal Co-Cultures as a Strategy for Metal Biorecovery from e-Waste
Minerals 2018, 8(4), 121; https://doi.org/10.3390/min8040121
Received: 31 January 2018 / Revised: 6 March 2018 / Accepted: 14 March 2018 / Published: 21 March 2018
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Abstract
We investigated a microbe-based approach to be used for the biorecovery of valuable metals from e-waste. E-waste is a heterogeneous matrix at the microbial scale. Therefore, this study aims at taking advantage of bacterial-fungal (BF) interactions in order to mobilize and immobilize a
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We investigated a microbe-based approach to be used for the biorecovery of valuable metals from e-waste. E-waste is a heterogeneous matrix at the microbial scale. Therefore, this study aims at taking advantage of bacterial-fungal (BF) interactions in order to mobilize and immobilize a selected metal present in e-waste. We used cadmium (Cd) and a selection of Cd-tolerant microorganisms from our culture collection or isolated from a naturally cadmium-contaminated soil. Several experiments were designed in order to use the synergistic bioremediation capabilities of BF couples to mobilize and immobilize Cd from a culture medium. Initial results showed that the selected synergistic BF couples are more tolerant to Cd concentrations than the organisms alone. However, setting the conditions leading to effective immobilization of this toxic metal still need further work. Using microbial consortia rather than single species represents an innovative alternative to traditional bioremediation approaches for the development of new biotechnological approaches in urban mining. Full article
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Open AccessArticle C14–22 n-Alkanes in Soil from the Freetown Layered Intrusion, Sierra Leone: Products of Pt Catalytic Breakdown of Natural Longer Chain n-Alkanes?
Minerals 2018, 8(3), 105; https://doi.org/10.3390/min8030105
Received: 26 January 2018 / Revised: 26 February 2018 / Accepted: 27 February 2018 / Published: 6 March 2018
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Abstract
Soil above a platinum-group element (PGE)-bearing horizon within the Freetown Layered Intrusion, Sierra Leone, contains anomalous concentrations of n-alkanes (CnH2n+2) in the range C14 to C22 not readily attributable to an algal or lacustrine origin. Longer chain
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Soil above a platinum-group element (PGE)-bearing horizon within the Freetown Layered Intrusion, Sierra Leone, contains anomalous concentrations of n-alkanes (CnH2n+2) in the range C14 to C22 not readily attributable to an algal or lacustrine origin. Longer chain n-alkanes (C23 to C31) in the soil were derived from the breakdown of leaf litter beneath the closed canopy humid tropical forest. Spontaneous breakdown of the longer chain n-alkanes to form C14–22 n-alkanes without biogenic or abiogenic catalysts is unlikely as the n-alkanes are stable. In the Freetown soil, the catalytic properties of the PGE (Pt in particular) may lower the temperature at which oxidation of the longer chain n-alkanes can occur. Reaction between these n-alkanes and Pt species, such as Pt2+(H2O)2(OH)2 and Pt4+(H2O)2(OH)4 can bend and twist the alkanes, and significantly lower the Heat of Formation. Microbial catalysis is a possibility. Since a direct organic geochemical source of the lighter n-alkanes has not yet been identified, this paper explores the theoretical potential for abiogenic Pt species catalysis as a mechanism of breakdown of the longer n-alkanes to form C14–22 alkanes. This novel mechanism could offer additional evidence for the presence of the PGE in solution, as predicted by soil geochemistry. Full article
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Open AccessArticle Interaction of Freshwater Diatom with Gold Nanoparticles: Adsorption, Assimilation, and Stabilization by Cell Exometabolites
Minerals 2018, 8(3), 99; https://doi.org/10.3390/min8030099
Received: 22 January 2018 / Revised: 27 February 2018 / Accepted: 1 March 2018 / Published: 5 March 2018
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Abstract
The rising concern about the potential toxicity of synthetic gold nanoparticles (AuNPs) in aquatic environments requires a rigorous estimation of physico-chemical parameters of reactions between AuNPs and major freshwater microorganisms. This study addresses the interaction of 10-nm size, positively charged AuNPs
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The rising concern about the potential toxicity of synthetic gold nanoparticles (AuNPs) in aquatic environments requires a rigorous estimation of physico-chemical parameters of reactions between AuNPs and major freshwater microorganisms. This study addresses the interaction of 10-nm size, positively charged AuNPs with periphytic freshwater diatoms (Eolimna minima). The adsorption experiments on viable cells were performed in 10 mM NaCl and 5 mM NaCl + 5 mM NaHCO3 solution at a variable pH (3–10), at an AuNPs concentration from 1 µg/L to 10,000 µg/L, and an exposure time from a few minutes to 55 days. Three types of experiments, adsorption as a function of time (kinetics), pH-dependent adsorption edge, and constant-pH “Langmuirian” type isotherms, were conducted. In addition, long-term interactions (days to weeks) of live diatoms (under light and in the darkness) were performed. The adsorption was maximal at a pH from 3 to 6 and sizably decreased at a pH of 6 to 10. Results of adsorption experiments were modeled using a second order kinetic model, a Linear Programming Model, Freundlich isotherm, and a ligand binding equation for one site competition. The adsorption of AuNPs(+) most likely occurred on negatively-charged surface sites of diatom cell walls such as carboxylates or phosphorylates, similar to previously studied metal cations. Under light exposure, the AuNPs were stabilized in aqueous solution in the presence of live cells, probably due to the production of exometabolites by diatoms. The adsorbed amount of AuNPs decreased after several days of reaction, suggesting some AuNPs desorption. In the darkness, the adsorption and assimilation were stronger than under light. Overall, the behavior of positively charged AuNPs at the diatom–aqueous solution interface is similar to that of metal cations, but the affinity of aqueous AuNPs to cell exometabolites is higher, which leads to the stabilization of nanoparticles in solution in the presence of diatoms and their exudates. During photosynthetic activity and the pH rising above 9 in the vicinity of diatom cells, the adsorption of AuNPs strongly decreases, which indicates a decreasing potential toxicity of AuNPs for photosynthesizing cells. The present study demonstrates the efficiency of a thermodynamic and kinetic approach for understanding gold nanoparticles interaction with aquatic freshwater peryphytic microorganisms. Full article
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Open AccessArticle Size-Controlled Production of Gold Bionanoparticles Using the Extremely Acidophilic Fe(III)-Reducing Bacterium, Acidocella aromatica
Minerals 2018, 8(3), 81; https://doi.org/10.3390/min8030081
Received: 24 January 2018 / Revised: 14 February 2018 / Accepted: 22 February 2018 / Published: 26 February 2018
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Abstract
Recycling of gold-bearing “urban mine” resources, such as waste printed circuit boards (PCBs), is attracting an increasing interest. Some of the gold leaching techniques utilize acidic lixiviants and in order to eventually target such acidic leachates, the utility of the acidophilic Fe(III)-reducing heterotrophic
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Recycling of gold-bearing “urban mine” resources, such as waste printed circuit boards (PCBs), is attracting an increasing interest. Some of the gold leaching techniques utilize acidic lixiviants and in order to eventually target such acidic leachates, the utility of the acidophilic Fe(III)-reducing heterotrophic bacterium, Acidocella (Ac.) aromatica PFBC was evaluated for production of Au(0) bionanoparticles (bio-AuNPs). Au(III) ions (as AuCl4, initially 10 mg/L), were readily adsorbed onto the slightly-positively charged Ac. aromatica cell surface and transported into cytoplasm to successfully form intracellular bio-AuNPs in a simple one-step microbiological reaction. Generally, increasing the initial concentration of formate as e-donor corresponded to faster Au(III) bioreduction and a greater number of Au(0) nucleation sites with less crystal growth within 40–60 h: i.e., use of 1, 5, 10, or 20 mM formate led to production of bio-AuNPs of 48, 24, 13, or 12 nm in mean particle size with 2.3, 17, 62, and 97 particles/cell, respectively. Addition of Cu2+ as an enzymatic inhibitor significantly decreased the number of Au(0) nucleation sites but enhanced crystal growth of individual particles. As a result, the manipulation of the e-donor concentration combined with an enzyme inhibitor enabled the 3-grade size-control of bio-AuNPs (nearly within a normal distribution) at 48, 26 or 13 nm by use of 1 mM formate, 20 mM formate (+Cu2+) or 10 mM formate, respectively, from highly acidic, dilute Au(III) solutions. Full article
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Open AccessArticle Biological and Geochemical Development of Placer Gold Deposits at Rich Hill, Arizona, USA
Minerals 2018, 8(2), 56; https://doi.org/10.3390/min8020056
Received: 18 January 2018 / Revised: 2 February 2018 / Accepted: 5 February 2018 / Published: 8 February 2018
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Abstract
Placer gold from the Devils Nest deposits at Rich Hill, Arizona, USA, was studied using a range of micro-analytical and microbiological techniques to assess if differences in (paleo)-environmental conditions of three stratigraphically-adjacent placer units are recorded by the gold particles themselves. High-angle basin
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Placer gold from the Devils Nest deposits at Rich Hill, Arizona, USA, was studied using a range of micro-analytical and microbiological techniques to assess if differences in (paleo)-environmental conditions of three stratigraphically-adjacent placer units are recorded by the gold particles themselves. High-angle basin and range faulting at 5–17 Ma produced a shallow basin that preserved three placer units. The stratigraphically-oldest unit is thin gold-rich gravel within bedrock gravity traps, hosting elongated and flattened placer gold particles coated with manganese-, iron-, barium- (Mn-Fe-Ba) oxide crusts. These crusts host abundant nano-particulate and microcrystalline secondary gold, as well as thick biomats. Gold surfaces display unusual plumate-dendritic structures of putative secondary gold. A new micro-aerophilic Betaproteobacterium, identified as a strain of Comamonas testosteroni, was isolated from these biomats. Significantly, this ‘black’ placer gold is the radiogenically youngest of the gold from the three placer units. The middle unit has well-rounded gold nuggets with deep chemical weathering rims, which likely recorded chemical weathering during a wetter period in Arizona’s history. Biomats, nano-particulate gold and secondary gold growths were not observed here. The uppermost unit is a pulse placer deposited by debris flows during a recent drier period. Deep cracks and pits in the rough and angular gold from this unit host biomats and nano-particulate gold. During this late arid period, and continuing to the present, microbial communities established within the wet, oxygen-poor bedrock traps of the lowermost placer unit, which resulted in biological modification of placer gold chemistry, and production of Mn-Fe-Ba oxide biomats, which have coated and cemented both gold and sediments. Similarly, deep cracks and pits in gold from the uppermost unit provided a moist and sheltered micro-environment for additional gold-tolerant biological communities. In conclusion, placer gold from the Devils Nest deposits at Rich Hill, Arizona, USA, preserves a detailed record of physical, chemical and biological modifications. Full article
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Open AccessArticle Immobilisation of Platinum by Cupriavidus metallidurans
Minerals 2018, 8(1), 10; https://doi.org/10.3390/min8010010
Received: 1 December 2017 / Revised: 20 December 2017 / Accepted: 26 December 2017 / Published: 5 January 2018
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Abstract
The metal resistant bacterium Cupriavidus metallidurans CH34, challenged with aqueous platinous and platinic chloride, rapidly immobilized platinum. XANES/EXAFS analysis of these reaction systems demonstrated that platinum binding shifted from chloride to carboxyl functional groups within the bacteria. Pt(IV) was more toxic than Pt(II),
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The metal resistant bacterium Cupriavidus metallidurans CH34, challenged with aqueous platinous and platinic chloride, rapidly immobilized platinum. XANES/EXAFS analysis of these reaction systems demonstrated that platinum binding shifted from chloride to carboxyl functional groups within the bacteria. Pt(IV) was more toxic than Pt(II), presumably due to the oxidative stress imparted by the platinic form. Platinum immobilisation increased with time and with increasing concentrations of platinum. From a bacterial perspective, intracellular platinum concentrations were two to three orders of magnitude greater than the fluid phase, and became saturated at almost molar concentrations in both reaction systems. TEM revealed that C. metallidurans was also able to precipitate nm-scale colloidal platinum, primarily along the cell envelope where energy generation/electron transport occurs. Cells enriched in platinum shed outer membrane vesicles that were enriched in metallic, colloidal platinum, likely representing an important detoxification strategy. The formation of organo-platinum compounds and membrane encapsulated nanophase platinum, supports a role for bacteria in the formation and transport of platinum in natural systems, forming dispersion halos important to metal exploration. Full article
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Open AccessArticle Biogeochemical Cycling of Silver in Acidic, Weathering Environments
Minerals 2017, 7(11), 218; https://doi.org/10.3390/min7110218
Received: 15 September 2017 / Revised: 1 November 2017 / Accepted: 6 November 2017 / Published: 10 November 2017
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Abstract
Under acidic, weathering conditions, silver (Ag) is considered to be highly mobile and can be dispersed within near-surface environments. In this study, a range of regolith materials were sampled from three abandoned open pit mines located in the Iberian Pyrite Belt, Spain. Samples
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Under acidic, weathering conditions, silver (Ag) is considered to be highly mobile and can be dispersed within near-surface environments. In this study, a range of regolith materials were sampled from three abandoned open pit mines located in the Iberian Pyrite Belt, Spain. Samples were analyzed for Ag mineralogy, content, and distribution using micro-analytical techniques and high-resolution electron microscopy. While Ag concentrations were variable within these materials, elevated Ag concentrations occurred in gossans. The detection of Ag within younger regolith materials, i.e., terrace iron formations and mine soils, indicated that Ag cycling was a continuous process. Microbial microfossils were observed within crevices of gossan and their presence highlights the preservation of mineralized cells and the potential for biogeochemical processes contributing to metal mobility in the rock record. An acidophilic, iron-oxidizing microbial consortium was enriched from terrace iron formations. When the microbial consortium was exposed to dissolved Ag, more than 90% of Ag precipitated out of solution as argentojarosite. In terms of biogeochemical Ag cycling, this demonstrates that Ag re-precipitation processes may occur rapidly in comparison to Ag dissolution processes. The kinetics of Ag mobility was estimated for each type of regolith material. Gossans represented 0.6–146.7 years of biogeochemical Ag cycling while terrace iron formation and mine soils represented 1.9–42.7 years and 0.7–1.6 years of Ag biogeochemical cycling, respectively. Biogeochemical processes were interpreted from the chemical and structural characterization of regolith material and demonstrated that Ag can be highly dispersed throughout an acidic, weathering environment. Full article
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Open AccessArticle Mineralogy and Geochemistry of Biologically-Mediated Gold Mobilisation and Redeposition in a Semiarid Climate, Southern New Zealand
Minerals 2017, 7(8), 147; https://doi.org/10.3390/min7080147
Received: 21 July 2017 / Revised: 11 August 2017 / Accepted: 13 August 2017 / Published: 16 August 2017
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Abstract
Detrital gold in Late Pleistocene-Holocene placers has been chemically mobilised and redeposited at the micron scale by biologically-mediated reactions in groundwater. These processes have been occurring in a tectonically active semiarid rain shadow zone of southern New Zealand and are probably typical for
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Detrital gold in Late Pleistocene-Holocene placers has been chemically mobilised and redeposited at the micron scale by biologically-mediated reactions in groundwater. These processes have been occurring in a tectonically active semiarid rain shadow zone of southern New Zealand and are probably typical for this type of environment elsewhere in the world. The chemical system is dominated by sulfur, which has been derived from basement pyrite and marine aerosols in rain. Detrital and authigenic pyrite is common below the water table, and evaporative sulfate minerals are common above the fluctuating water table. Pyrite oxidation was common but any acid generated was neutralised on the large scale (tens of metres) by calcite, and pH remained circumneutral except on the small scale (centimetres) around pyritic material. Metastable thiosulfate ions were a temporary product of pyrite oxidation, enhanced by bacterial mediation, and similar bacterial mediation enhanced sulfate reduction to form authigenic pyrite below the water table. Deposition of mobilised gold resulted from localised variations in redox and/or pH, and this formed overgrowths on detrital gold of microparticulate and nanoparticulate gold that is locally crystalline. The redeposited gold is an incidental byproduct of the bacterially-enhanced sulfur reactions that have occurred near to the fluctuating sulfide-sulfate redox boundary. Full article
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Review

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Open AccessReview Reflecting on Gold Geomicrobiology Research: Thoughts and Considerations for Future Endeavors
Minerals 2018, 8(9), 401; https://doi.org/10.3390/min8090401
Received: 13 August 2018 / Revised: 10 September 2018 / Accepted: 11 September 2018 / Published: 13 September 2018
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Abstract
Research in gold (Au) geomicrobiology has developed extensively over the last ten years, as more Au-bearing materials from around the world point towards a consistent story: That microbes interact with Au. In weathering environments, Au is mobile, taking the form of oxidized, soluble
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Research in gold (Au) geomicrobiology has developed extensively over the last ten years, as more Au-bearing materials from around the world point towards a consistent story: That microbes interact with Au. In weathering environments, Au is mobile, taking the form of oxidized, soluble complexes or reduced, elemental Au nanoparticles. The transition of Au between aqueous and solid states is attributed to varying geochemical conditions, catalyzed in part by the biosphere. Hence, a global Au-biogeochemical-cycle was proposed. The primary focus of this mini-review is to reflect upon the biogeochemical processes that contribute to what we currently know about Au cycling. In general, the global Au-biogeochemical-cycle begins with the liberation of gold-silver particles from a primary host rock, by physical weathering. Through oxidative-complexation, inorganic and organic soluble-Au complexes are produced. However, in the presence of microbes or other reductants—e.g., clays and Fe-oxides—these Au complexes can be destabilized. The reduction of soluble Au ultimately leads to the bioprecipitation and biomineralization of Au, the product of which can aggregate into larger structures, thereby completing the Au cycle. Evidence of these processes have been “recorded” in the preservation of secondary Au structures that have been observed on Au particles from around the world. These structures—i.e., nanometer-size to micrometer-size Au dissolution and reprecipitation features—are “snap shots” of biogeochemical influences on Au, during its journey in Earth-surface environments. Therefore, microbes can have a profound effect on the occurrence of Au in natural environments, given the nutrients necessary for microbial metabolism are sustained and Au is in the system. Full article
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