A COMPREHENSIVE REVIEW ON THE USE OF MICROBES IN MINERAL RECOVERY AND ACID MINES

Biomining is successful on the commercial scale for the recovery of various metals such as copper and ores from their ores. The methodology involved in this is primarily chemistry-driven and is a combination of ferric and hydrogen ions which varies on the type involved. Hydrogen ions present here are produced by the activity of chemolithotrophic bacteria growing in a highly acidic environment. Bioleaching reactions, on the other hand, the role of microorganisms involved, and whether the reaction carried out are direct or indirect are discussed below. In places where the availability of oxidants to sulfide mineral surfaces is exposed due to mining, the acid mine drainage tends to contaminate the surroundings as well. Microorganisms that mainly consist of autotrophic and heterotrophic archaea and bacteria, take part in catalyzing iron and sulfur oxidation which determines the release of metals and sulfur into the surrounding. Indication towards the physiological synergy in sulfur, iron, and carbon flow in the microbial community is assessed. With this, the development, and future aspects of this with the challenges in mind are described as well.

Active microbial ecosystem in glacier basal ice fuelled by iron and silicate comminution-derived hydrogen

The basal zone of glaciers is characterised by physicochemical properties that are distinct from firnified ice because of strong interactions with underlying substrate. Basal ice ecology and the roles that the microbiota play in biogeochemical cycling, weathering, and proglacial soil formation, remains poorly known. We report bacterial diversity and potential ecological roles at three temperate Icelandic glaciers. We sampled three physically distinct basal ice facies (stratified, dispersed, debris bands) and found biological similarities and differences between them; basal ice character is therefore an important sampling consideration in future studies. High abundance of silicates and Fe-containing minerals could sustain the basal ice ecosystem, in which chemolithotrophic bacteria (~23%), especially Fe-oxidisers and hydrogenotrophs, can fix C, which can be utilised by heterotrophs. Methanogenic-affiliated detected sequences showed that silicate comminution-derived hydrogen can also be utilised for methanogenesis. Metabolism predicted by 16S rRNA diversity revealed that methane metabolism and C-fixation are the most common pathways, indicating the importance of these metabolic routes. Carbon concentrations were low compared to other ecosystems, but we report the highest carbon concentration in basal ice to date. Carbon release from melting basal ice may play an important role in promoting pioneering communities establishment and soil development in deglaciating forelands.

Cryptic roles of tetrathionate in the sulfur cycle of marine sediments: microbial drivers and indicators

To explore the potential role of tetrathionate in the sedimentary sulfur cycle, population ecology of microorganisms capable of metabolizing this polythionate was revealed at 15–30 cm resolution along two, ∼3 m long, cores collected from 530 and 580 m below the sea level, off India's west coast, within the oxygen minimum zone (OMZ) of the Arabian Sea. Metagenome analysis along the cores revealed widespread occurrence of genes involved in the formation, oxidation, and reduction of tetrathionate; high diversity and relative abundance were also detected for bacteria that are known to render these metabolisms in vitro. Results of slurry culture of the sediment samples in thiosulfate- or tetrathionate-containing microbial growth media, data obtained via pure-culture isolation, and finally metatranscriptome analyses corroborated the in situ functionality of the tetrathionate-forming, tetrathionate-oxidizing, and tetrathionate-reducing microorganisms. Ion chromatography of pore waters revealed the presence of up to 11.1 µM thiosulfate in the two cores, whereas tetrathionate remained undetected in spectroscopic assay based on its reaction with cyanide. While thiosulfate oxidation by chemolithotrophic bacteria prevalent in situ is the apparent source of tetrathionate in this ecosystem, high biochemical and geochemical reactivity of this polythionate could be instrumental in its cryptic status in the sulfur cycle. Potential abiotic origin of tetrathionate in the sediment horizon explored could neither be ruled out nor confirmed from the geochemical information available. On the other hand, tetrathionate potentially present in the system can be either oxidized to sulfate or reduced back to thiosulfate/sulfide via chemolithotrophic oxidation and respiration by native bacterial populations, respectively. Up to 2.01 mM sulfide present in the sediment cores may also reduce tetrathionate abiotically to thiosulfate and elemental sulfur. However, in the absence of measured data for O2 or other oxyanions having possibilities of serving as electron acceptors, the biogeochemical modalities of the oxidative half of the tetrathionate cycle remained unresolved.

Two pathways for thiosulfate oxidation in the alphaproteobacterial chemolithotrophParacoccus thiocyanatusSST

Chemolithotrophic bacteria oxidize various sulfur species for energy and electrons, thereby operationalizing biogeochemical sulfur cycles in nature. The best-studied pathway of bacterial sulfur-chemolithotrophy, involving direct oxidation of thiosulfate to sulfate (without any free intermediate) by the SoxXAYZBCD multienzyme system, is apparently the exclusive mechanism of thiosulfate oxidation in facultatively chemolithotrophic alphaproteobacteria. Here we explore the molecular mechanisms of sulfur oxidation in the thiosulfate- and tetrathionate-oxidizing alphaproteobacteriumParacoccus thiocyanatusSST, and compare them with the prototypical Sox process characterized inParacoccus pantotrophus. Our results revealed the unique case where, an alphaproteobacterium has Sox as its secondary pathway of thiosulfate oxidation, converting ∼10% of the thiosulfate supplied whilst 90% of the substrate is oxidized via a Tetrathionate-Intermediate pathway. Knock-out mutation, followed by the study of sulfur oxidation kinetics, showed that thiosulfate-to-tetrathionate conversion, in SST, is catalyzed by a thiosulfate dehydrogenase (TsdA) homolog that has far-higher substrate-affinity than the Sox system of this bacterium, which, remarkably, is also less efficient than theP. pantotrophusSox.soxB-deletion in SST abolished sulfate-formation from thiosulfate/tetrathionate while thiosulfate-to-tetrathionate conversion remained unperturbed. Physiological studies revealed the involvement of glutathione in SST tetrathionate oxidation. However, zero impact of the knock-out of a thiol dehydrotransferase (thdT) homolog, together with no production of sulfite as an intermediate, indicated that tetrathionate oxidation in SST is mechanistically novel, and distinct from its betaproteobacterial counterpart mediated by glutathione, ThdT, SoxBCD and sulfite:acceptor oxidoreductase. All the present findings collectively highlight extensive functional diversification of sulfur-oxidizing enzymes across phylogenetically close, as well as distant, bacteria.

Potential of Chemolitotrophic Bacteria From Gold Mining Area in Sulfur Oxidation Process

Gold in nature is covered by rocks which contain sulfide minerals such as pyrite, chalcopyrite, arsenopyrite, and others sulfide minerals. Chemolithotrophic bacteria have the ability to oxidize the sulfur compounds and can be used in the process of releasing gold from carrier rocks which contain sulfide minerals. This research aimed to explore and identify the chemolithotrophic bacteria from gold mining areas as well as determine their potential for sulfur oxidation. The methods used in this study were exploring the potential of bacteria in sulfur oxidation and describing the variety of bacteria that were isolated from gold mining areas by 16s rRNA identification. The results showed that there were six isolates from isolation with Starkey solid medium, i.e. Bl-1, B1-2, B1-3, B1-4, B1-5 and B1-6 that were similar to Paenibacillus sp., Enterobacter ludwigiis train E8-13, uncultured Burkholderia sp., Uncultured bacterium clone N4.5, Bacillus subtilis strain CICC 10023, and Bacterium enrichment culture clone 02 respectively. The B1-3 isolate showed the highest increase of sulfate compound in the medium (8.04 % at 649.55 ppm). This indigenous bacteria will be able to be used to release gold from rock which contains sulfide minerals and reduce the use of hazardous chemicals commonly used in gold mining.

Genetic variability in Acidithiobacillus spp. – a working horse of environmental biotechnologies

The genus Acidithiobacillus comprises 7 species of Gram-negative obligatory acidophilic chemolithotrophic bacteria that derive energy mainly from the oxidation of reduced sulphur compounds. Four of the species also catalyse the dissimilatory oxidation of ferrous iron while three (A. thiooxidans, A. albertensis, and A. caldus) do not. Bacteria from the genus Acidithiobacillus are often associated with mineral biotechnologies (biomining) and acid mine drainage. While acceleration of mineral solubilisation is a positive aspect in environmental biotechnologies, it is undesirable in acid mine drainage with strong negative ecological impact and there is profound interest in genetics and genomics of these bacteria. Representatives of Acidithiobacillus genus occur world-wide, however there are limited data on Acidithiobacillus spp. variability from Slovakia. In our work the variability of Acidithiobacillus spp., from Slovakia was analysed and the presence of A ferrooxidans was detected. In addition, for the first time we report here on the occurrence of A. albertensis as well. Comparative analyses confirmed pronounced genetic and genomic diversity within the genus, especially within A. ferrooxidans and A. thioxidans complexes. Based on data presented, several Acidithiobacillus species could be considered as a complex species and the description of several new species is very probable in the near future.