The economically important metal gallium (Ga) is considered to be a critical raw material with a potentially high supply risk. The industrial semiconductor production process generates multiple waste streams that contain Ga in economically significant amounts. This study explores bioleaching for Ga recycling from a waste metal hydroxide sludge originating from industrial GaAs-wafer production. Previous studies have demonstrated that hydrometallurgical recycling of this sludge provides an effective approach for gallium recovery. Furthermore, it was shown that reducing the ferric iron in the leaching solution has a beneficial effect on downstream processing, enabling less consumption of chemicals. Based on these results, the present study investigates the potential of bioleaching using Acidithiobacillus thiooxidans with the addition of elemental sulfur for the recovery of gallium and the reduction of ferric iron from the metal hydroxide sludge. Shake flask experiments showed that 60% of the total iron in the leachate was present as ferrous iron after processing 3% (w/v) sludge after 8 days. Subsequent reactor experiments at 3% (w/v) solid load resulted in only 30% iron reduction after 14 days, while element recoveries reached 80% for Fe, 100% for Ga, and 99% for As. A further bioreactor experiment with a gradual addition of the sludge up to 5% (w/v) led to a 70% reduction of ferric iron after 14 days. The experiments revealed that microbially mediated ferric iron reduction decreased at pH values below 1.0. Fluoride was found to inhibit microbial activity during the experiments, which was mitigated by the addition of aluminum at an Al:Fe ratio of 1.4:1. Overall, this study demonstrates that bioleaching is a possible approach for the recycling of gallium-containing metal hydroxide sludge.

Exoelectrogens are capable of transferring electrons through a defined route outside of the cell, making them ideal for microbial fuel cells, electrosynthesis and biosensing applications. Engineering well characterized hosts and transferring exoelectrogenic capabilities can further our understanding of these extracellular electron transfer pathways and lead to new developments in their utilization for biotechnological applications. The extreme acidophile Acidithiobacillus ferrooxidans is capable of extracellular iron reduction under anaerobic conditions. In order to confirm the functionality of the suggested iron reduction pathway, we produced the c-type cytochromes CycA2, Cyc1A, Cyc2A and the copper redox protein rusticyanin in Escherichia coli C43(DE3), BL21(DE3), T7 Express and Vibrio natriegens Vmax X2. Both Vmax X2 and C43(DE3) produced all four redox proteins in a redox-active state in the correct cell compartment. T7 Express was unable to produce holo-CycA and BL21(DE3) only reached low relative expression levels per OD600 of holo-Cyc2. Introducing the At. ferrooxidans iron reduction pathway into E. coli lead to a 2-fold and 1.4-fold increased Fe(III)-citrate reduction per OD600 in C43(DE3) and BL21(DE3) respectively. The inner membrane cytochrome CycA was necessary for efficient iron reduction in the E. coli strains. While Vmax X2 was able to produce the entire iron reduction pathway, it was unable to utilize it for Fe(III)-citrate reduction. The recombinant iron reduction pathway of C43(DE3) was redox active under turnover conditions and enabled 5-fold increased current production in an electrochemical H-cell while respiring with a graphite felt electrode. Overall, this study provides the first experimental validation of the suggested minimal iron reduction pathway of At. ferrooxidans consisting of CycA, Cyc1, Rus, and Cyc2.

Microbial iron (Fe) redox cycling underpins key biogeochemical processes, yet the functional diversity, ecological roles, and trait architectures of iron-transforming microbes remain poorly synthesized across global environments. Here, we present a systematic review and trait-based meta-analysis of 387 microbial taxa spanning 314 studies and 76 years of research, integrating phenotypic, genomic, and environmental data to define ecologically coherent microbial iron redox cycle guilds. Rather than relying on taxonomy, our framework delineates first-order functional guilds—Fe(III) reducers, Fe(II) oxidizers, and dual-capacity Fe oxidizers/reducers—and resolves second-order guilds based on trait syndromes, such as acidophily, redox flexibility, or metabolic breadth. Trait profiling revealed that iron-cycling capacities frequently transcend phylogenetic boundaries, with multiple guilds converging in chemically stratified hotspots like hot springs, hydrothermal vents, and acid mine drainages. Dual-capacity Fe oxidizers/reducers (e.g., Acidithiobacillus ferrooxidans and Metallosphaera sedula) emerged as overlooked mediators of “cryptic” iron cycling, possessing genomic repertoires capable of toggling between oxidative and reductive modes in response to redox oscillations. Hierarchical clustering and kernel density analyses of ecophysiological traits highlighted niche partitioning along key environmental filters, including pH, iron availability, salinity, and temperature. Collectively, this work introduces the Guild Exploitation Pattern as a conceptual lens for understanding iron microbiome assembly, providing a data-driven foundation for predicting microbial contributions to iron cycling under changing environmental conditions. IMPORTANCE Iron redox reactions shape nutrient turnover, contaminant mobility, and primary productivity, yet the microbes driving these processes are often studied in isolation. By integrating decades of data into a trait-based guild framework, we reveal the ecophysiological diversity and niche differentiation of microbial iron redox cycling taxa across environments. Our synthesis exposes major gaps, such as limited trait data for >80% of dual-capacity Fe oxidizing/reducing species and highlights the need for functional trait surveys to complement metagenomics and cultivation efforts. The guild framework presented here advances predictive microbial ecology by linking metabolic traits with environmental gradients, offering a robust foundation for incorporating iron cycling into ecosystem models and biogeochemical forecasts.

Bioleaching is an established process for sulfidic ores and is increasingly applied to the recycling of industrial residues. However, unlike ores, many residues like sludge contain inhibitory elements, among which fluoride poses a major challenge due to its toxicity toward acidophilic microorganisms even at low concentrations. This study systematically investigated fluoride tolerance in pure and mixed cultures of various acidophilic sulfur- and iron-oxidizing bacteria commonly used for bioleaching, including Acidithiobacillus spp., Leptospirillum spp., and Sulfobacillus thermosulfidooxidans. Fluoride toxicity was found to be substrate-dependent. During sulfur oxidation, A. thiooxidans displayed the highest fluoride tolerance (0.5 mM F⁻), whereas S. thermosulfidooxidans showed complete inhibition. In contrast, iron-oxidizing bacteria demonstrated increased fluoride tolerance, with S. thermosulfidooxidans remaining active at 1.5 mM F⁻ when grown on ferrous iron. Mixed cultures showed enhanced fluoride tolerance during sulfur oxidation but reduced tolerance during iron oxidation. pH was identified as a critical factor influencing fluoride toxicity due to increased formation of undissociated HF at low pH. To mitigate fluoride inhibition, fluoride complexation with ferric iron or aluminum was evaluated. For A. ferrooxidans, iron oxidation resumed at Fe3⁺:F⁻ ratios of 7.5:1, while other cultures required ratios of at least 10:1. Aluminum complexation required Al:F⁻ ratios between 1:1 and 2:1, depending on the culture and growth conditions. Overall, fluoride inhibition during bioleaching is influenced by multiple factors, including pH, ferric iron concentration, and the fluoride dissolution rate. Early addition of aluminum is recommended to prevent microbial inhibition and ensure stable bioleaching performance. • Higher fluoride tolerance was observed during iron oxidation. • S. thermosulfidooxidans remained active up to 1.5 mM F⁻. • Fluoride toxicity is strongly pH dependent due to increased HF formation at low pH. • Effective fluoride complexation requires higher Fe3+:F⁻ ratios (> 7.5:1) than Al3⁺:F⁻ ratios (> 1:1)

The increased accumulation of spent lithium iron phosphate (LFP) batteries promotes the need for efficient and sustainable recycling strategies to recover lithium, graphite and phosphorus. This study explores a spent medium bioleaching approach for the recycling of LFP black mass to extract lithium. Experiments were conducted using floated and unfloated LFP black mass. In a first step, spent medium was generated by the sulfur-oxidizer Acidithiobacillus thiooxidans in a 1 L-stirred tank reactor until a pH value of 1.0 was achieved after six days. In the second process step, the produced spent medium was used to leach LFP black mass. Various process parameters were optimized to enhance metal leaching, including the addition of H2O2 to enable selective leaching. The reproducibility of spent medium production and subsequent leaching was validated through multiple tests. Optimal parameters were determined as follows: 7% (w/v) LFP black mass, 1.5 M H2O2, 60 degrees C and 60 min reaction time. Lithium leaching efficiencies of 85% were achieved in 1 L scale using unfloated LFP black mass. Using floated LFP black mass improved selective leaching, yielding an 88% lithium recovery. The XRD analysis of residues confirmed the successful precipitation of FePO4. Additionally, a hybrid leaching approach through combination of spent medium and sulfuric acid was tested at 10% (w/v) solid load, with a final concentration of 0.25 M H2SO4, resulting in a 91% lithium recovery. This study highlights the potential of bioleaching as a viable method for recycling LFP black mass at higher solid loads which requires low sulfuric acid concentrations and short process time.

C-type cytochromes fulfil many essential roles in both aerobic and anaerobic respiration. Their characterization requires large quantities of protein which can be obtained through heterologous production. Heterologous production of c-type cytochromes in Escherichia coli is hindered since the ccmABCDEFGH genes necessary for incorporation of heme c are only expressed under anaerobic conditions. Different strategies were devised to bypass this obstacle, such as co-expressing the ccm genes from the pEC86 vector. However, co-expression methods restrict the choice of expression host and vector. Here we describe the first use of Vibrio natriegens Vmax X2 for the recombinant production of difficult-to-express redox proteins from the extreme acidophile Acidithiobacillus ferrooxidans CCM4253, including three c-type cytochromes. Co-expression of the ccm genes was not required to produce holo-c-type cytochromes in Vmax X2. E. coli T7 Express only produced holo-c-type cytochromes during co-expression of the ccm genes and was not able to produce the inner membrane cytochrome CycA. Additionally, Vmax X2 cell extracts contained higher portions of recombinant holo-proteins than T7 Express cell extracts. All redox proteins were translocated to the intended cell compartment in both hosts. In conclusion, V. natriegens represents a promising alternative for the production of c-type cytochromes and difficult-to-express redox proteins.

GaAs wafers are essential for the production of GaAs semiconductor chips, which have a wide range of applications in manufacturing electronic devices, including power electronics, photovoltaic cells, sensors, and detectors. The demand for GaAs wafers is steadily increasing due to technological advances and growing environmental awareness, posing new challenges for the industry. During the production of GaAs wafers alone, 15

Lithium iron phosphate (LFP) batteries are an important source of critical raw materials due to their high content of lithium, graphite, and phosphorus. The overall aim of this study is to develop a recycling process for LFP-batteries combining hydrometallurgical and biohydrometallurgical approaches for efficient recycling of lithium. As part of the biological approach, various autotrophic bioleaching strategies were tested to selectively leach lithium and to enhance the chemical leaching step of the LFP black mass in the process. Direct and indirect bioleaching approaches using biologically produced lixiviants were examined. First experiments showed leaching yields for lithium of nearly 80

The winning of critical raw materials from secondary resources, e.g., from abandoned mines, mining residues, electronic waste or low-grade ores, is a potential source with promising outcomes due to innovative and more efficient extraction methods. The research and education mine “Reiche Zeche” at the TU Bergakademie Freiberg, Germany offers a scientific lab in a real application environment for an in-situ bioleaching section from a low-grade sulfide ore vein with on-site associated membrane downstream processing. The evaluation and resumption of previous research activities showed both the feasibility and the potential for further up-scaling. But there was also potential evaluated for improving the effectiveness, especially in terms of individual process elements within the leaching and membrane cycle as well as regarding microbiology. Based on this, further optimization was carried out and effectiveness was evaluated and compared to the prior state. The results regarding the leaching outcome proved that process optimization leads to stable and continuous leaching process operations as well as to improved and more favorable conditions for the microorganisms due to the implementation of a bioreactor and changing the whole leaching operation from a direct into an indirect process. Furthermore, cleaning in place (CIP) resulted in the maintenance of selectivity despite discontinuous membrane process operation.

The Gram-negative acidophile Acidithiobacillus ferrooxidans is capable of exocellular iron reduction through an electron transfer chain that spans both membranes and the periplasm. Four redox proteins are required to transfer electrons to Fe(III): (i) the inner membrane-anchored c-type cytochrome CycA, (ii) the periplasmic c-type cytochrome Cyc1, (iii) the periplasmic blue copper protein rusticyanin (Rus), and (iv) the outer membrane c-type cytochrome Cyc2. To investigate the Fe(III) reduction functionality of this pathway in neutrophilic hosts, it was reconstructed in three different Escherichia coli strains and Vibrio natriegens Vmax X2. The periplasmic proteins Rus and Cyc1 were only produced as membrane-associated proteins in E. coli and no holo-CycA was detected in E. coli cell extracts. V. natriegens produced all four holo-proteins simultaneously while the periplasmic proteins Rus and Cyc1 remained soluble. UV/Vis spectra of soluble cell extracts of V. natriegens showed typical absorbance maxima for heme c. The c-type cytochromes produced by V. natriegens could also be reduced and oxidized. Therefore, V. natriegens seems to be a very promising host for the production of soluble periplasmic as well as membrane-anchored proteins from an extreme acidophile.

Die Biohydrometallurgie als Teilgebiet der Hydrometallurgie macht sich spezielle Stoffwechselleistungen von Mikroorganismen zur Metallgewinnung zu nutze. Biomining ist die angewandte Biolaugung zur Metallgewinnung aus sulfidischen Erzen. Für das Recycling, also die Metallgewinnung aus Abfall und Reststoffen, gibt es bisher noch keine angewandten biohydrometallurgischen Verfahren, aber vielversprechende Laborversuche zur Metallextraktion aus Feststoffen. In diesem Übersichtsartikel werden diese zusammenfassend dargestellt und Perspektiven aufgezeigt.

Sands containing placers enriched with heavy minerals such as zircon, garnet, ilmenite, rutile, magnetite and monazite are recovered as construction material from the German Baltic Sea coast. The heavy minerals could be concentrated by gravity and magnetic separation. Rare earth elements (REE) were enriched in zircon concen-trates as well as in concentrates of the zirconium-containing mineral eudialyte from a deposit in Greenland. In order to test bioleaching for REE extraction from these concentrates, experiments with cultures of mesophilic as well as moderately thermophilic, acidophilic, lithoautotrophic bacteria and gluconic acid-producing organo-heterotrophic bacteria were run in shake flasks or pH-controlled 2 L bioreactors at 30 degrees C or 42 degrees C. Elemental sulfur and glucose served as substrate for the autotrophs and heterotrophs, respectively. Chemical leaching experiments as controls for bioleaching by the autotrophs were run without bacteria and adjusted over time to the same pH by sulfuric acid addition as previously measured in the inoculated bioreactors, in order to estimate bioleaching versus chemical leaching at the same pH. Chemical leaching experiments as controls for bioleaching by the heterotrophs were run with added gluconic acid as well as other organic acids. The results showed no significant differences in REE extraction between chemical and biological runs under same conditions (pH, temperature or gluconic acid concentration), indicating that bioleaching was in fact acid leaching based on the enzymatic oxidation of elemental sulfur to sulfuric acid in case of the autotrophs and gluconic acid production from glucose in case of the heterotrophs. The amount of extracted REEs increased with decreasing pH. Meso-philes versus moderate thermophiles did not show remarkable differences. The extraction efficiency for the single REEs was maximum 35% for zircon concentrates but up to about 70% for the eudialyte concentrate. This study allows for a comparison of chemical leaching vs. bioleaching and shows the limitation of bioleaching for pro-cessing heavy minerals for REE extraction.

The demand on especially clean energy metals is growing exponentially creating a clear need to diversify the metals supply. Metal recovery from various waste deposits could be one solution to provide needed metals for the society and at the same time reduce the environmental effects of waste. Bioleaching has been commercially applied for the recovery of valuable metals from sulfidic ores, tailings and metallurgical side streams. Bioleaching approaches are still at laboratory scale for other waste streams, such as waste incineration ash, metallurgical slags and electronic waste. Adapted bioleaching strategies have been suggested for industrial and consumer metal-containing waste streams to overcome the challenges with alkalinity, heterogeneity, toxic contents, and the lack of sulfur and iron source. Integration of bioleaching with other technologies as well as integration of target waste with other material streams is one means to improve the process performance to reach higher metal yields and kinetics. Bioleaching has shown potential to decrease energy consumption and carbon footprint compared to more conventional technologies in commercial scale, which increases its attractiveness as a future technology choice.

Chemie Ingenieur TechnikVolume 95, Issue 11 p. 1866-1866 VorschauFree Access Vorschau: Chem. Ing. Tech. 12/2023 First published: 24 October 2023 https://doi.org/10.1002/cite.202371108AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume95, Issue11Special Issue: Prof. Dr. rer. nat. Jörg Kärger zum 80. Geburtstag gewidmetNovember 2023Pages 1866-1866 RelatedInformation

Granulated blast furnace slag (GGBFS) is a potential resource of rare earth elements (REEs), and due to the complex mineralogy, extraction by conventional hydrometallurgical process makes it an acid-consuming method. Bioleaching is thus investigated using a chemo-organotrophic bacterium Gluconobacter oxydans (DSMZ 46616) for REE extraction from GGBFS containing 157 ppm Ce, 90 ppm La, 71 ppm Nd and 40 ppm Er, hosted in a Ca-Al-Si matrix. The gluconic acid generation by G. oxydans was assessed for its role in REE extraction from GGBFS. With 5% (w/v) GGBFS using a mixture of a non-adapted and a GGBFS-adapted culture, a maximum solubilization of 67% and 88% Nd was observed after 12 and 40 days of incubation, respectively. The total amount of gluconic acid excreted by the bacteria increased with leaching duration, which contributed to a rise in metal extraction. Scanning electron microscope-energy dispersive analysis (SEM-EDAX) analysis of the solid residue showed bacterial cells in corrosion pits, and thereby assisting in metal solubilization.

The research and education mine “Reiche Zeche” in Freiberg (Saxony, Germany) represents one of the most famous mining facilities reminiscent to the century-long history of silver production in the Ore Mountains. The mine was set up at the end of the fourteenth century and became part of the “Bergakademie Freiberg” in 1919. Galena, pyrite, sphalerite, arsenopyrite, and chalcopyrite are the most common minerals found in the mine. As acid mine drainage is generated from the dissolution of sulfidic ores, the microbial habitats within the adits and galleries are characterized by low pH and high concentrations of metal(loid)s. The community composition was investigated at locations characterized by biofilm formation and iron-rich bottom pools. Amplicon libraries were sequenced on a MiSeq instrument. The taxonomic survey yielded an unexpected diversity of 25 bacterial phyla including ten genera of iron-oxidizing taxa. The community composition in the snottites and biofilms only slightly differed from the communities found in acidic bottom pools regarding the diversity of iron oxidizers, the key players in most investigated habitats. Sequences of the Candidate Phyla Radiation as, e.g., Dojkabacteria and Eremiobacterota were found in almost all samples. Archaea of the classes Thermoplasmata and Nitrososphaeria were detected in some biofilm communities.

A novel thermoacidophilic archeaon, strain J1T (=DSM 112778T,=JCM 34702T), was isolated from a hot pool in a volcanic area of Java, Indonesia. Cells of the strain were irregular, motile cocci of 1.0-1.2 µm diameter. Aerobic, organoheterotrophic growth with casamino acids was observed at an optimum temperature of 70 °C in a range of 55-78 °C and at an optimum pH of 3 in a range of 1.5 to 5. Various organic compounds were utilized, including a greater variety of sugars than has been reported for growth of other species of the genus. Chemolithoautotrophic growth was observed with reduced sulphur compounds, including mineral sulphides. Ferric iron was reduced during anaerobic growth with elemental sulphur. Cellular lipids were calditoglycerocaldarchaeol and caldarchaeol with some derivates. The organism contained the respiratory quinone caldariellaquinone. On the basis of phylogenetic and chemotaxonomic comparison with its closest relatives, it was concluded that strain J1T represents a novel species, for which the name Metallosphaera javensis is proposed. Low DNA-DNA relatedness values (16S rRNA gene <98.4%, average nucleotide identity (ANI) <80.1%) distinguished J1T from other species of the genus Metallosphaera and the DNA G+C content of 47.3% is the highest among the known species of the genus.

Biochemical processes are a key element of natural cycles occurring in the environment and enabling life on earth. With regard to microbially catalyzed iron transformation, research predominantly has focused on iron oxidation in acidophiles, whereas iron reduction played a minor role. Microbial conversion of ferric to ferrous iron has however become more relevant in recent years. While there are several reviews on neutrophilic iron reducers, this article summarizes the research on extreme acidophilic iron reducers. After the first reports of dissimilatory iron reduction by acidophilic, chemolithoautotrophic Acidithiobacillus strains and heterotrophic Acidiphilium species, many other prokaryotes were shown to reduce iron as part of their metabolism. Still, little is known about the exact mechanisms of iron reduction in extreme acidophiles. Initially, hypotheses and postulations for the occurring mechanisms relied on observations of growth behavior or predictions based on the genome. By comparing genomes of well-studied neutrophilic with acidophilic iron reducers (e.g., Ferroglobus placidus and Sulfolobus spp.), it became clear that the electron transport for iron reduction proceeds differently in acidophiles. Moreover, transcriptomic investigations indicated an enzymatically-mediated process in Acidithiobacillus ferrooxidans using respiratory chain components of the iron oxidation in reverse. Depending on the strain of At. ferrooxidans, further mechanisms were postulated, e.g., indirect iron reduction by hydrogen sulfide, which may form by disproportionation of elemental sulfur. Alternative scenarios include Hip, a high potential iron-sulfur protein, and further cytochromes. Apart from the anaerobic iron reduction mechanisms, sulfur-oxidizing acidithiobacilli have been shown to mediate iron reduction at low pH (< 1.3) under aerobic conditions. This presumably non-enzymatic process may be attributed to intermediates formed during sulfur/tetrathionate and/or hydrogen oxidation and has already been successfully applied for the reductive bioleaching of laterites. The aim of this review is to provide an up-to-date overview on ferric iron reduction by acidophiles. The importance of this process in anaerobic habitats will be demonstrated as well as its potential for application.

A mixotrophic and acidophilic bacterial strain BGR 140T was isolated from mine tailings in the Harz Mountains near Goslar, Germany. Cells of BGR 140T were Gram-stain-positive, endospore-forming, motile and rod-shaped. BGR 140T grew aerobically at 25–55 °C (optimum 45 °C) and at pH 1.5–5.0 (optimum pH 3.0). The results of analysis of the 16S rRNA gene sequences indicated that BGR 140T was phylogenetically related to different members of the genus Sulfobacillus , and the sequence identities to Sulfobacillus acidophilus DSM 10332T, Sulfobacillus thermotolerans DSM 17362T, and Sulfobacillus benefaciens DSM 19468T were 94.8, 91.8 and 91.6 %, respectively. Its cell wall peptidoglycan is A1γ, composed of meso-diaminopimelic acid. The respiratory quinone is DMK-6. The major polar lipids were determined to be glycolipid, phospholipid and phosphatidylglycerol. The predominant fatty acid is 11-cycloheptanoyl-undecanoate. The genomic DNA G+C content is 58.2 mol%. On the basis of the results of phenotypic and genomic analyses, it is concluded that strain BGR 140T represents a novel species of the genus Sulfobacillus , for which the name Sulfobacillus harzensis sp. nov. is proposed because of its origin. Its type strain is BGR 140T (=DSM 109850T=JCM 39070T).