Jeremiah Shuster

Gold biomineralization by a metallophore from a gold-associated microbe

Microorganisms produce and secrete secondary metabolites to assist in their survival. We report that the gold resident bacterium Delftia acidovorans produces a secondary metabolite that protects from soluble gold through the generation of solid gold forms. This finding is the first demonstration that a secreted metabolite can protect against toxic gold and cause gold biomineralization.

The preservation and degradation of filamentous bacteria and biomolecules within iron oxide deposits at Rio Tinto, Spain.

One of the keys to understanding and identifying life on other planets is to study the preservation of organic compounds and their precursor micro-organisms on Earth. Rio Tinto in southwestern Spain is a well documented site of microbial preservation within iron sulphates and iron oxides over a period of 2.1 Ma. This study has investigated the preservation of filamentous iron oxidising bacteria and organics through optical microscopy, scanning electron microscopy (SEM) and Fourier transform infra-red (FTIR) spectroscopy, from laboratory cultures of natural samples to contemporary natural materials to million-year old river terraces. Up to 40% elemental carbon and >7% nitrogen has been identified within microbial filaments and cell clusters in all samples through SEM EDS analyses. FTIR spectroscopy identified C-H(x) absorption bands between 2960 and 2800 cm(-1), Amide I and II absorption bands at 1656 and 1535 cm(-1), respectively and functional group vibrations from within nucleic acids at 917, 1016 and 1124 cm(-1). Absorption bands tracing the diagenetic transformation of jarosite to goethite to hematite through the samples are also identified. This combination of mineralogy, microbial morphology and biomolecular evidence allows us to further understand how organic fossils are created and preserved in iron-rich environments, and ultimately will aid in the search for the earliest life on Earth and potential organics on Mars.

The in-vitro “growth” of gold grains

Gold colloids, octahedral platelets, and foils, directly and indirectly formed from the reduction of soluble Au(I) thiosulfate and Au(III) chloride complexes by iron-oxidizing bacteria, cyanobacteria, and sulfate-reducing bacteria, were combined in an experimental system. This system represented simplified biogeochemical conditions occurring within a fluvial environment in which placer Au could occur. In this study, biofilm formation and physical aggregation (i.e., sedimentation processes) were critical for the accumulation of nanometer- to micrometer-sized Au particles into grains 4–5 mm in size. Characterization of grain surface textures by scanning electron microscopy in association with monitoring soluble Au concentrations over time suggested that dissolution and reprecipitation processes were occurring at the Au grain-fluid interface. This laboratory model demonstrates that the biogeochemical cycling of Au can contribute to the formation of anomalous enrichments such as placer Au deposits.

Bioconversion of coal: new insights from a core flooding study

A pressurized core flooding experiment was performed to better understand in situ coal bioconversion processes. The core flooding experiment was conducted using a biaxial core holder packed with subbituminous coal particles (250–150 μm grain size) obtained from the Highvale mine in Alberta, Canada. The coal pack was inoculated with a methanogenic microbial culture enriched from coal and was continuously flooded with mineral salt medium and an organic carbon/nitrogen nutrient supplement (tryptone). The changes in the physical properties of the coal pack during the core flooding suggested coal bioconversion to methane under the experimental conditions. Colonization and bioconversion of coal by microbes was evident from the change in core permeability and presence of metabolites and gas (CH4 and CO2) in the effluent. A total of 1.52 μmol of CH4 was produced per gram of coal during the 90 days experiment at 22 °C. Signature metabolites consistent with anaerobic biodegradation of hydrocarbons, e.g., carboxylic acids, were identified in effluent samples throughout incubation. The transient nature of metabolites in effluent samples supports fermentation of coal constituents and nutrient supplement to simple molecules such as acetic acid, which served as a substrate for methanogenesis during the bioconversion process. Accumulation of carboxylic acids such as succinic acid in the effluent also demonstrates that the coal bioconversion process may be used for extraction of other value-added products apart from CH4 generation. Importantly, results presented here suggest that coal bioconversion by biostimulation under reservoir conditions is a scalable technology with potential for energy generation and for overall reduction of greenhouse gas emissions.

Structural and Chemical Characterization of Placer Gold Grains: Implications for Bacterial Contributions to Grain Formation

Gold grains collected from the Rio Saldaña River, Colombia were hundreds of micrometers in size and discoid-ellipse in shape. Fourteen of 63 grains contained an iron oxyhydroxide coating that occurred as ca. 50 to 100 nm thick lamina while thicker coatings were comprised of colloids 200 nm to 4 μm in diameter. Bacterial-size casts were observed throughout the thicker iron oxyhydroxide coating and intuitively represent relic impressions of bacterial cells. The surface textures of gold grains were generally smooth with surficial depressions or crevices containing detrital material colonized by bacteria. Focus Ion Beam (FIB) milled cross-sections demonstrated that the detrital material contained nanophase gold particles. Biofilm attached to this detrital material contained ca. 2 to 3 nm colloidal gold attached to exopolymeric substances. Cross sections of grains revealed solid cores with vesicular voids near the grain edge including a bacterial-size cast interpreted to be a permineralized bacterial cell. Synchrotron-based elemental mapping indicated that grains contained heterogenously distributed Ag and Cu. While strong Ag and Cu signals (relative to Au) were detected in the core, a stronger Au signal occurred at the edge of grains demonstrating enriched rims of secondary gold. The preservation of bacterial casts and biofilms associated with secondary gold structures at the surface of grains suggest that bacteria may contribute to gold enrichment and growth in this placer environment. Bacteria, occurring on the surface of 13 of 25 gold grains, were enriched by “inoculating” individual grains into separate test tubes containing R2B growth medium. Enriched growth of bacteria on gold grain surfaces demonstrated preferential attachment onto detrital material within creviced regions. The dominant bacteria from these enrichments were transferred to solid R2A medium to obtain pure isolates. The isolates were identified as one of four bacterial species: Nitrobacter sp. 263, Shewanella sp. YM-8, Sediminibacterium sp. B2-10-2 and sp. I-32 based on 16S ribosomal DNA sequencing.

The immobilization of gold from gold (III) chloride by a halophilic sulphate-reducing bacterial consortium

Abstract A consortium containing halophilic, dissimilatory sulphate-reducing bacteria was enriched from Basque Lake #1, located near Ashcroft, British Columbia, Canada to evaluate the role these bacteria have on the immobilization of soluble gold. The consortium immobilized increasing amounts of gold from gold (III) chloride solutions, under saline to hypersaline conditions, over time. Gold (III) chloride was reduced to elemental gold in all experimental systems. Salinity did not affect gold immobilization. Scanning electron microscopy and transmission electron microscopy demonstrated that reduced gold (III) chloride was immobilized as c. 3–10 nm gold colloids and c. 100 nm colloidal aggregates at the fluid–biofilm interface. The precipitation of gold at this organic interface protected cells within the biofilm from the ‘toxic effect’ of ionic gold. Analysis of these experimental systems using X-ray absorption near-edge spectroscopy confirmed that elemental gold with varying colloidal sizes formed within minutes. The immobilization of gold by halophilic sulphate-reducing bacteria highlights a possible role for the biosphere in ‘intercepting’ mobile gold complexes within natural, hydraulic flow paths. Based on the limited toxicity demonstrated in this experimental model, significant concentrations of elemental gold could accumulate over geological time in natural systems where soluble gold concentrations are more dilute and presumably ‘non-toxic’ to the biosphere.

Progressive biogeochemical transformation of placer gold particles drives compositional changes in associated biofilm communities

Biofilms on placer gold (Au)-particle surfaces drive Au solubilization and re-concentration thereby progressively transforming the particles. Gold solubilization induces Au-toxicity; however, Au-detoxifying community members ameliorates Au-toxicity by precipitating soluble Au to metallic Au. We hypothesize that Au-dissolution and re-concentration (precipitation) place selective pressures on associated microbial communities, leading to compositional changes and subsequent Au-particle transformation. We analyzed Au-particles from eight United Kingdom sites using next generation sequencing, electron microscopy and micro-analyses. Gold particles contained biofilms composed of prokaryotic cells and extracellular polymeric substances intermixed with (bio)minerals. Across all sites communities were dominated by Proteobacteria (689, 97% Operational Taxonomic Units, 59.3% of total reads), with β-Proteobacteria being the most abundant. A wide range of Au-morphotypes including nanoparticles, micro-crystals, sheet-like Au and secondary rims, indicated that dissolution and re-precipitation occurred, and from this transformation indices were calculated. Multivariate statistical analyses showed a significant relationship between the extent of Au-particle transformation and biofilm community composition, with putative metal-resistant Au-cycling taxa linked to progressive Au transformation. These included the genera Pseudomonas, Leptothrix and Acinetobacter. Additionally, putative exoelectrogenic genera Rhodoferax and Geobacter were highly abundant. In conclusion, biogeochemical Au-cycling and Au-particle transformation occurred at all sites and exerted a strong influence on biofilm community composition.