The geomicrobiology of gold

Abstract

Microorganisms capable of actively solubilizing and precipitating gold appear to play a larger role in the biogeochemical cycling of gold than previously believed. Recent research suggests that bacteria and archaea are involved in every step of the biogeochemical cycle of gold, from the formation of primary mineralization in hydrothermal and deep subsurface systems to its solubilization, dispersion and re-concentration as secondary gold under surface conditions. Enzymatically catalysed precipitation of gold has been observed in thermophilic and hyperthermophilic bacteria and archaea (for example, Thermotoga maritime, Pyrobaculum islandicum), and their activity led to the formation of gold-and silver-bearing sinters in New Zealand's hot spring systems. Sulphatereducing bacteria (SRB), for example, Desulfovibrio sp., may be involved in the formation of goldbearing sulphide minerals in deep subsurface environments; over geological timescales this may contribute to the formation of economic deposits. Iron-and sulphur-oxidizing bacteria (for example, Acidothiobacillus ferrooxidans, A. thiooxidans) are known to breakdown gold-hosting sulphide minerals in zones of primary mineralization, and release associated gold in the process. These and other bacteria (for example, actinobacteria) produce thiosulphate, which is known to oxidize gold and form stable, transportable complexes. Other microbial processes, for example, excretion of amino acids and cyanide, may control gold solubilization in auriferous top-and rhizosphere soils. A number of bacteria and archaea are capable of actively catalysing the precipitation of toxic gold(I/ III) complexes. Reductive precipitation of these complexes may improve survival rates of bacterial populations that are capable of (1) detoxifying the immediate cell environment by detecting, excreting and reducing gold complexes, possibly using P-type ATPase efflux pumps as well as membrane vesicles (for example, Salmonella enterica, Cupriavidus (Ralstonia) metallidurans, Plectonema boryanum); (2) gaining metabolic energy by utilizing gold-complexing ligands (for example, thiosulphate by A. ferrooxidans) or (3) using gold as metal centre in enzymes (Micrococcus luteus). C. metallidurans containing biofilms were detected on gold grains from two Australian sites, indicating that gold bioaccumulation may lead to gold biomineralization by forming secondary 'bacterioform' gold. Formation of secondary octahedral gold crystals from gold(III) chloride solution, was promoted by a cyanobacterium (P. boryanum) via an amorphous gold(I) sulphide intermediate. 'Bacterioform' gold and secondary gold crystals are common in quartz pebble conglomerates (QPC), where they are often associated with bituminous organic matter possibly derived from cyanobacteria. This may suggest that cyanobacteria have played a role in the formation of the Witwatersrand QPC, the world's largest gold deposit.

Geobiological Cycling of Gold: From Fundamental Process Understanding to Exploration Solutions

“…( Figure 1; [11,12]). In a recent study, Tomkins [16] suggested that microbial processes may have had a far greater influence on the formation of orogenic gold deposits as previously believed.…”

Immobilisation of Platinum by Cupriavidus metallidurans

“…The morphologies of bio-precipitated gold so closely resemble the gold found in contemporary (Australia) and paleoplacers, e.g., South Africa's Witwatersrand basin, that a biogenic origin for these auriferous deposits [8,9,18,19] is now accepted.…”

“…The biogeochemical cycling of precious metals and the formation of certain geologic deposits can be attributed to microbial activity [8][9][10][11][12]. In vitro studies have shown that bacteria are able to immobilize and/or precipitate gold from gold-rich solutions [13][14][15][16].…”

“…In Figure 4B, Pt-binding into the outer membrane (OM) appears to have caused a physical disruption to the OM packing order, resulting in vesicle formation. Shedding OM vesicles, as a consequence of membrane rearrangement to accommodate the Pt ions, has the benefit of keeping toxic material away from the cell [9,38]. These vesicles were important sites of Pt immobilisation.…”

“…This study employed the use of Cupriavidus metallidurans CH34: an aerobic, gram-negative, facultative chemolithoautotrophic, rod-shaped β-Proteobacterium known for being resistant to the toxic effects of a number of metallic cations, e.g., Cu, Pb, Zn, Cd, Ag and Au [9,14,[41][42][43]. Resistance is primarily facilitated by metal transporting ATPase efflux proteins in the cell envelope and cation reduction mechanisms in the cytoplasm, which have been shown to confer metal immobilisation [6,9,44]. This bacterium, which forms biofilms, was found to be living on the surface of gold grains obtained from auriferous soils in southern and northern Australia, and was shown to be able to precipitate aqueous gold-chloride species Reith et al [14,15].…”