Sabrina Hedrich

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.

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 Fe ³ ⁺: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. Key points: • 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 Fe ³⁺ :F⁻ ratios (> 7.5:1) than Al3⁺:F⁻ ratios (> 1:1) Graphical

Microbial iron 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 MIRC 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, 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 O/R 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.

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.

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 V_max 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 V_max 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, V_max 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.