Bio-oxidation of iron using Thiobacillus ferrooxidans

The effects of pH, ferrous and ferric ion concentrations on iron oxidation by Thiobacillus ferrooxidans were examined. The initial temperature and bacterial concentration were maintained at 37°C and 2±1×104cells/ml, respectively. The iron oxidation rate increased with increased initial ferrous iron concentration to 4g/l and thereafter decreased. The presence of iron(III) showed a negative effect on the bacterial iron oxidation rate. The increase of pH also showed an increase in the oxidation rate up to pH 1.75. The oxidation rate followed first order kinetics for the parameters studied. A rate equation has been developed.     

Bio-oxidation of iron using Thiobacillus ferrooxidans

The effects of pH, ferrous and ferric ion concentrations on iron oxidation by Thiobacillus ferrooxidans were examined. The initial temperature and bacterial concentration were maintained at 37°C and 2±1×104cells/ml, respectively. The iron oxidation rate increased with increased initial ferrous iron concentration to 4g/l and thereafter decreased. The presence of iron(III) showed a negative effect on the bacterial iron oxidation rate. The increase of pH also showed an increase in the oxidation rate up to pH 1.75. The oxidation rate followed first order kinetics for the parameters studied. A rate equation has been developed.     

High-rate acidophilic ferrous iron oxidation in a biofilm airlift reactor and the role of the carrier material

In this study, the feasibility and engineering aspects of acidophilic ferrous iron oxidation in a continuous biofilm airlift reactor inoculated with a mixed culture of Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans bacteria were investigated. Specific attention was paid to biofilm formation, competition between both types of bacteria, ferrous iron oxidation rate, and gas liquid mass transfer limitations. The reactor was operated at a constant temperature of 30 degrees C and at pH values of 0-1.8. Startup of the reactor was performed with basalt carrier material. During the experiments the basalt was slowly removed and the ferric iron precipitates formed served as a biofilm carrier. These precipitates have highly suitable characteristics as a carrier material for the immobilization of ferrous iron-oxidizing bacteria and dense conglomerates were observed. Lowering the pH (0.6-1) resulted in dissolution of the ferric precipitates and induced granular sludge formation. The maximum ferrous iron oxidation rate achieved in this study was about 145 molFe(2+)/m(3).h at a hydraulic residence time of 0.25 h. Optimal treatment performance was obtained at a loading rate of 100 mol/m(3).h at a conversion efficiency as high as 98%. Fluorescent in situ hybridization (FISH) studies showed that when the reactor was operated at high ferrous iron conversion (>85%) for 1 month, the desirable L. ferrooxidans species could out-compete A. ferrooxidans due to the low Fe(2+) and high Fe(3+) concentrations.     

Oxidation of high ferrous iron concentrations by chemolithotrophic Thiobacillus ferrooxidans in packed bed bioreactors

The bacterial oxidation of high ferrous iron concentrations in batch culture has been studied in a packed bed bioreactor. It has been found that aeration rates from 0.49 to 1.2 VVM did not influence the biofilm oxidation activity during the period of biofilm formation up to 30 g l⁻¹ initial ferrous iron concentration. The contribution of swimming, attached and fixed bacteria to the Fe²⁺ oxidation process has been evaluated. Kinetics data showed that the oxidation rate depends on the aeration rate, when the initial Fe²⁺ concentration exceeded 30 g l⁻¹. The maximum overall Fe²⁺ oxidation rate was 1.8 g l⁻¹ h⁻¹, when the initial ferrous iron concentration was in the range 30 to 45 g l⁻¹.     

Selection of Leptospirillum ferrooxidans SRPCBL and development for enhanced ferric regeneration in stirred tank and airlift column reactor

Presence of Leptospirillum ferrooxidans plays significant role in ferric sulphate generation during bioleaching process. Thus, an attempt was made to select L. ferrooxidans from the polymetallic concentrate leachate and further developed it for enhanced ferric iron regeneration from the leachate in shake flask, stirred tank and column reactor. When ferric to ferrous iron ratio in the shake flask reached to 20:1, L. ferrooxidans out competed Acidithiobacillus ferrooxidans and accounted for more than 99% of the total population. The isolate was confirmed by 16S rRNA genes sequence analysis and named as L. ferrooxidans SRPCBL. When the culture was exposure to UV dose and the oxidation-reduction potential of the inoculation medium was adjusted to 40 0mV by ferrous:ferric iron ratio, the IOR reached to as high as 1.2 g/L/h in shake flask, even with initial ferrous iron concentration of 200 g/L. The chalcopyrite concentrate leachate containing 12.8, 15.7, and 42.0 g/L ferrous iron, ferric iron and copper, respectively was studied for ferric iron regeneration with the developed polymetallic resistant L. ferrooxidans SRPCBL in stirred tank and a developed biofilm airlift column, the highest IOR achieved were 2.20 g/L/h and 3.1 g/L/h, respectively, with ferrous oxidation efficiency of 98%. The ferric regeneration ability of the developed isolate from the leachate proves useful for a two-stage metal extraction process.     

响应面法优化Fe~(2+)的微生物氧化条件

氧化亚铁钩端螺旋菌(Leptospirillum ferrooxidans,L.f)是一种极端嗜酸,专性自养氧化铁的细菌,能够耐受较低pH和较高的温度,被广泛应用于生物浸矿和环境治理。氧化亚铁钩端螺旋体菌的生物浸矿效率与其对Fe~(2+)氧化速率相关,因此,本文采用响应面法,通过建立二次多项式回归方程考察pH、温度、Fe~(2+)浓度及转速四个培养因素对Fe~(2+)氧化速率的影响。结果显示在pH为2.25、温度为32℃、初始Fe~(2+)浓度为175.36 mmol/L、转数为165 r/min时,Fe~(2+)最高氧化速率为0.2911 g/Lh。     

Kinetics of iron oxidation by Leptospirillum ferriphilum dominated culture at pH below one

The kinetics of ferrous iron oxidation by Leptospirillum ferriphilum (L. ferriphilum) dominated culture was studied in the concentration range of 0.1-20 g Fe(2+)/L and the effect of ferric iron (0-60 g Fe(3+)/L) on Fe(2+) oxidation was investigated at pH below one. Denaturing gradient gel electrophoresis of PCR amplified 16S rRNA genes followed by partial sequencing confirmed that the bacterial community was dominated by L. ferriphilum. In batch assays, Fe(2+) oxidation started without lag phase and the oxidation was completed within 1 to 60 h depending on the initial Fe(2+) concentration. The specific Fe(2+) oxidation rates increased up to around 4 g/L and started to decrease at above 4 g/L. This implies substrate inhibition of Fe(2+) oxidation at higher concentrations. Haldane equation fitted the experimental data reasonably well (R(2) = 0.90). The maximum specific oxidation rate (q(m)) was 2.4 mg/mg VS . h, and the values of the half saturation (K(s)) and self inhibition constants (K(i)) were 413 and 8,650 mg/L, respectively. Fe(2+) oxidation was competitively inhibited by Fe(3+) and the competitive inhibition constant (K(ii)) was 830 mg/L. The time required to reach threshold Fe(2+) concentration was around 1 day and 2.3 days with initial Fe(3+) concentration of 5 and 60 g/L, respectively. The threshold Fe(2+) concentration, below which no further Fe(2+) oxidation occurred, linearly increased with increasing initial Fe(2+) and Fe(3+) concentrations. Fe(2+) oxidation proceeds by L. ferriphilum dominated culture at pH below 1 even in the presence of 60 g Fe(3+)/L. This indicates potential of using and biologically regenerating concentrated Fe(3+) sulfate solutions required, for example, in indirect tank leaching of ore concentrates.     

A kinetic model for biological oxidation of ferrous iron by Thiobacillus ferrooxidans

The kinetics of bacterial oxidation of ferrous iron in the presence of Thiobacillus ferrooxidans cells were studied using an initial-rate method. Measurements of the redox potential of the solution during the oxidation of ferrous iron were used to assess the initial rate of the reaction. Effects on the rate of reaction were determined for ferrous iron concentration in the range 0.25 to 30 kg m(-3), bacterial concentration in the range 3.25 x 10(7) to 4.47 x 10(8) cells mL(-1), and temperature in the range 20 to 35 degrees C. Using these experimental results and an approach based on Michaelis-Menten kinetics, a model for biological oxidation of ferrous iron was developed. The model, which incorporates terms for the effect of temperature and substrate and cell inhibition, was successfully used to simulate the full range of experimental data obtained. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 53: 478-486, 1997.     

固定化氧化亚铁硫杆菌培养条件对黄铁矾沉淀的影响

以聚乙烯醇-海藻酸钠复合材料为载体,Ca(NO3)2为交联剂对氧化亚铁硫杆菌进行包埋固定化。该固定化细胞的连续培养技术可以用于处理H2S、SO2,为了减少减少固定化细胞培养过程中带来许多不利效应的黄铁矾沉淀 (NH4Fe3(SO4)2(OH)6）,采取了改变初始pH值和目前普遍采用的9K培养基中的(NH4)2SO4浓度,K2HPO4浓度三种方法。结果显示：在三种方法中,降低(NH4)2SO4浓度是比较可行的一种方法,当(NH4)2SO4从3.0 g/L降低到0.5g/L,Fe2+氧化速率几乎没有受到影响,沉淀形成速率却减少了45%。在固定化细胞连续运行时,降低9K培养基中(NH4)2SO4的含量,当稀释率为0.4 h-1,运行时间为96 h,Fe2+氧化速率高达3.75 g/L.H,结果显示反应柱内沉淀明显减少,同时Fe2+氧化速率并没有明显变化。     

Relative contributions of biological and chemical reactions to the overall rate of pyrite oxidation at temperatures between 30 degrees C and 70 degrees C

The stoichiometry and kinetics of the spontaneous, chemical reaction between pyrite and ferric iron was studied at 30, 45, and 70 degrees C in shake flasks at pH 1.5 by monitoring the ferrous iron, total iron, elemental sulfur, and sulfate concentration profiles in time. It was found that the sulfur moiety of pyrite was oxidized completely to sulfate. Elemental sulfur was not produced in detectable amounts. The iron moiety of pyrite was released as ferrous iron. All observed initial reaction rates could be fitted into an empirical equation. This equation includes the concentrations of ferric iron and pyrite, and a constant which is dependent on the temperature and the nature of the main anion present. It was observed that ferrous iron formed during the reaction slowed down the oxidation of pyrite by ferric iron. The extent of this effect decreased with increasing temperature. With the aid of the empirical equation, the contribution of the chemical oxidation of pyrite by ferric iron to the overall oxidation in a hypothetical plug-flow reactor, in which biologically mediated oxdidation of pyrite and ferrous iron by oxygen also takes place, can be assessed. At 30, 45, and 70 degrees C, respectively, 2, 8-17, and 43% of the pyrite was oxidized chemically by ferric iron. Therefore, it is expected that only in reactors operating at high temperatures with extremely thermophilic bacteria, will chemical oxidation cause a significant deviation from the apparent first order overall kinetics of biological pyrite oxidation.     

Ferrous iron oxidation by foam immobilized Acidithiobacillus ferrooxidans: Experiments and modeling

Ferrous iron bio‐oxidation by Acidithiobacillus ferrooxidans immobilized on polyurethane foam was investigated. Cells were immobilized on foams by placing them in a growth environment and fully bacterially activated polyurethane foams (BAPUFs) were prepared by serial subculturing in batches with partially bacterially activated foam (pBAPUFs). The dependence of foam density on cell immobilization process, the effect of pH and BAPUF loading on ferrous oxidation were studied to choose operating parameters for continuous operations. With an objective to have high cell densities both in foam and the liquid phase, pretreated foams of density 50 kg/m³ as cell support and ferrous oxidation at pH 1.5 to moderate the ferric precipitation were preferred. A novel basket‐type bioreactor for continuous ferrous iron oxidation, which features a multiple effect of stirred tank in combination with recirculation, was designed and operated. The results were compared with that of a free cell and a sheet‐type foam immobilized reactors. A fivefold increase in ferric iron productivity at 33.02 g/h/L of free volume in foam was achieved using basket‐type bioreactor when compared to a free cell continuous system. A mathematical model for ferrous iron oxidation by Acidithiobacillus ferrooxidans cells immobilized on polyurethane foam was developed with cell growth in foam accounted by an effectiveness factor. The basic parameters of simulation were estimated using the experimental data on free cell growth as well as from cell attachment to foam under nongrowing conditions. The model predicted the phase of both oxidation of ferrous in shake flasks by pBAPUFs as well as by fully activated BAPUFs for different cell loadings in foam. Model for stirred tank basket bioreactor predicted within 5% both transient and steady state of the experiments closely for the simulated dilution rates. Bio‐oxidation at high Fe²⁺ concentrations were simulated with experiments when substrate and product inhibition coefficients were factored into cell growth kinetics. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009     

Isolation and study of two strains ofLeptospirillum-like bacteria from a natural mixed population cultured on a cobaltiferous pyrite substrate

Two strains ofLeptospirillum-like bacteria, L6 and L8, have been isolated from a mixed inoculum, also containingThiobacillus ferrooxidans andT. thiooxidans, cultured for one year with a colbaltiferous pyrite as energy substrate in a 100 I continuous bioleaching laboratory unit. Several physiological properties of the strains are described. The vibrio-shaped microorganisms grew at pH values lower than 1.3. Their growth rate was maximum between 2.5 and 8.0 g l¹ ferrous iron. The optimal growth temperature was 37.5° C. Ferric iron had a stimulative effect on bacterial development up to 8 g l^–1, and growth was as rapid at 14 g l^–1 ferric iron as at 8 g l^–1. The negative influence of cobalt on the final cell concentration was observed at 0.5 g l^–1, but the growth rate was not affected up to 2 g l^–1. The G + C content of strains L8 is 55.6 mol%.     

Oxidation of Ferrous Iron and Elemental Sulfur by Thiobacillus ferrooxidans

The oxidation of ferrous iron and elemental sulfur by Thiobacillus ferrooxidans that was absorbed and unabsorbed onto the surface of sulfur prills was studied. Unadsorbed sulfur-grown cells oxidized ferrous iron at a rate that was 3 to 7 times slower than that of ferrous iron-grown cells, but sulfur-grown cells were able to reach the oxidation rate of the ferrous iron-adapted cells after only 1.5 generations in a medium containing ferrous iron. Bacteria that were adsorbed to sulfur prills oxidized ferrous iron at a rate similar to that of unadsorbed sulfur-grown bacteria. They also showed the enhancement of ferrous iron oxidation activity in the presence of ferrous iron, even though sulfur continued to be available to the bacteria in this case. An increase in the level of rusticyanin together with the enhancement of the ferrous iron oxidation rate were observed in both sulfur-adsorbed and unadsorbed cells. On the other hand, sulfur oxidation by the adsorbed bacteria was not affected by the presence of ferrous iron in the medium. When bacteria that were adsorbed to sulfur prills were grown at a higher pH (ca. 2.5) in the presence of ferrous iron, they rapidly lost both ferrous iron and sulfur oxidation capacities and became inactive, apparently because of the deposition of a jarosite-like precipitate onto the surface to which they were attached.     

Characterization of arsenic resistant and arsenopyrite oxidizing Acidithiobacillus ferrooxidans from Hutti gold leachate and effluents

Four arsenic resistant ferrous oxidizers were isolated from Hutti Gold Mine Ltd. (HGML) samples. Characterization of these isolates was done using conventional microbiological, biochemical and molecular methods. The ferrous oxidation rates with these isolates were 16, 48, 34 and 34 mg L(-1)h(-1) and 15, 47, 34 and 32 mg L(-1)h(-1) in absence and presence of 20 mM of arsenite (As3+) respectively. Except isolate HGM 8, other three isolates showed 2.9-6.3% inhibition due to the presence of 20 mM arsenite. Isolate HGM 8 was able to grow in presence of 14.7 g L(-1) of arsenite, with 25.77 mg L(-1)h(-1) ferrous oxidation rate. All the four isolates were able to oxidize iron and arsenopyrite from 20 g L(-1) and 40 g L(-1) refractory gold ore and 20 g L(-1) refractory gold concentrate. Once the growth was established pH adjustment was not needed inspite of ferrous oxidation, which could be due to concurrent oxidation of pyrite. Isolate HGM 8 showed the final cell count of as high as 1.12 x 10(8) cells mL(-1) in 40 g L(-1) refractory gold ore. The isolates were grouped into one haplotypes by amplified ribosomal DNA restriction analysis (ARDRA). The phylogenetic position of HGM 8 was determined by 16S rDNA sequencing. It was identified as Acidithiobacillus ferrooxidans and strain name was given as SRHGM 1.     

海藻酸钙包埋氧化亚铁硫杆菌的固定化研究

采用非稳态法测定FeSO4在包埋和未包埋氧化亚铁硫杆菌的凝胶中的有效扩散系数。结果表明,FeSO4在凝胶中的有效扩散系数De随着海藻酸钠浓度的升高而降低,当海藻酸钠浓度为2%时最优;凝胶剂CaCl2的浓度对扩散系数的影响较小。包埋的氧化亚铁硫杆菌在10h达到增殖平衡,而FeSO4在包埋细菌的凝胶内扩散系数明显减少。     

Anaerobic and aerobic oxidation of ferrous iron at neutral pH by chemoheterotrophic nitrate-reducing bacteria

Nine out of ten anaerobic enrichment cultures inoculated with sediment samples from various freshwater, brackish-water, and marine sediments exhibited ferrous iron oxidation in mineral media with nitrate and an organic cosubstrate at pH 7.2 and 30° C. Anaerobic nitrate-dependent ferrous iron oxidation was a biological process. One strain isolated from brackish-water sediment (strain HidR2, a motile, nonsporeforming, gram-negative rod) was chosen for further investigation of ferrous iron oxidation in the presence of acetate as cosubstrate. Strain HidR2 oxidized between 0.7 and 4.9 mM ferrous iron aerobically and anaerobically at pH 7.2 and 30° C in the presence of small amounts of acetate (between 0.2 and 1.1 mM). The strain gained energy for growth from anaerobic ferrous iron oxidation with nitrate, and the ratio of iron oxidized to acetate provided was constant at limiting acetate supply. The ability to oxidize ferrous iron anaerobically with nitrate at approximately pH 7 appears to be a widespread capacity among mesophilic denitrifying bacteria. Since nitrate-dependent iron oxidation closes the iron cycle within the anoxic zone of sediments and aerobic iron oxidation enhances the reoxidation of ferrous to ferric iron in the oxic zone, both processes increase the importance of iron as a transient electron carrier in the turnover of organic matter in natural sediments. Received: 24 April 1997 / Accepted: 22 September 1997     

Enhancement of photo-hydrogen production in a biofilm photobioreactor using optical fiber with additional rough surface

In this study, a biofilm photobioreactor with optical fibers that have additional rough surface (OFBP-R) was developed and it was shown that additional rough surface greatly enhanced the biofilm formation and thus increased the cell concentration, leading to an improvement in the hydrogen production performance. The effects of operational conditions, including the influent substrate concentration, flow rate, temperature and influent medium pH, on the performance of OFBP-R were also investigated. The experimental results showed that the optimum operational conditions for hydrogen production were: the influent substrate concentration 60 mM, flow rate 30 mL/h, temperature 30 °C and influent medium pH 7. Under the optimal operation conditions discovered in this work, the OFBP-R yielded fairly good and stable long-term performance with hydrogen production rate of 1.75 mmol/L/h, light conversion efficiency of 9.3% and substrate degradation efficiency of 75%.     

Influence of biomass production and detachment forces on biofilm structures in a biofilm airlift suspension reactor

The influence of process conditions (substrate loading rate and detachment force) on the structure of biofilms grown on basalt particles in a Biofilm Airlift Suspension (BAS) reactor was studied. The structure of the biofilms (density, surface shape, and thickness) and microbial characteristics (biomass yield) were investigated at substrate loading rates of 5, 10, 15, and 20 kg COD/m3. day with basalt concentrations of 60 g/L, 150 g/L, and 250 g/L. The basalt concentration determines the number of biofilm particles in steady state, which is the main determining factor for the biofilm detachment in these systems. In total, 12 experimental runs were performed. A high biofilm density (up to 67 g/L) and a high biomass concentration was observed at high detachment forces. The higher biomass content is associated with a lower biomass substrate loading rate and therefore with a lower biomass yield (from 0.4 down to 0.12 gbiomass/gacetate). Contrary to general beliefs, the observed biomass detachment decreased with increasing detachment force. In addition, smoother (fewer protuberances), denser and thinner compact biofilms were obtained when the biomass surface production rate decreased and/or the detachment force increased. These observations confirmed a hypothesis, postulated earlier by Van Loosdrecht et al. (1995b), that the balance between biofilm substrate surface loading (proportional to biomass surface production rate, when biomass yield is constant) and detachment force determines the biofilm structure. When detachment forces are relatively high only a patchy biofilm will develop, whereas at low detachment forces, the biofilm becomes highly heterogeneous with many pores and protuberances. With the right balance, smooth, dense and stable biofilms can be obtained. Copyright 1998 John Wiley & Sons, Inc.     

Benefits associated with the stalk of Gallionella ferruginea,evaluated by comparison of a stalk-forming and a non-stalk-forming strain and biofilm studies in situ

Factors that regulate and induce stalk formation by the iron-oxidizing and stalk-forming bacterium Gallionella ferruginea were studied in laboratory cultures and in situ. A stalk-forming strain, Sta⁺, and a non-stalk-forming strain, Sta^-, were used for comparative studies of the benefits associated with the stalk. Two different growth media were used: a ferrous sulfide medium (FS-medium), with slow oxidation of iron giving high concentrations of toxic oxygen radicals and a ferrous carbonate medium (FC-medium), with fast iron oxidation giving low concentration of the toxic oxygen radicals. It was found that Sta⁺ cells grown in the FS-medium survived 3 weeks longer than Sta^- cells grown in the FS-medium. When each strain was grown in the FC-medium, the Sta^- cells had an advantage and survived 8 weeks longer than the Sta⁺ cells. No difference in survival was found for Sta⁺ cells grown in FS-medium compared to growth in FC-medium. In laboratory cultures, the average stalk length per cell values were 7–2.5 times higher (92 h and 150–300 h growth, respectively) in a medium with 620 m iron than in a medium with 290 m iron. Gallionella ferruginea Sta⁺ outcompeted Sta^- cells when inoculated as mixed populations in FC-medium. It has previously been suggested that stalk formation in vitro is induced by oxygen. To confirm this observation, biofilm development in natural waters was studied in two wells, one with trace amounts of oxygen (LH) and one without (TH). A dense biofilm developed on surfaces exposed to flowing well LH water, but no biofilm developed in well TH. Stalks were formed in water samples from both wells when allowed to make contact with air. This work demonstrates for the first time that the stalk has a protecting function against the toxic oxygen radicals formed during the chemical iron oxidation. It also shows that it is the oxidation rate of the ferrous iron and not its concentration that is harmful to the cells. The stalk gives G. ferruginea a unique possibility to colonize and survive in habitats with high contents of iron, inaccessible for bacteria without a defense system against the oxidation of iron. Correspondence to: L. Hallbeck     

Kinetics of ferrous iron oxidation by batch and continuous cultures of thermoacidophilic Archaea at extremely low pH of 1.1–1.3

The extreme acid conditions required for scorodite (FeAsO₄·2H₂O) biomineralization (pH below 1.3) are suboptimal for growth of most thermoacidophilic Archaea. With the objective to develop a continuous process suitable for biomineral production, this research focuses on growth kinetics of thermoacidophilic Archaea at low pH conditions. Ferrous iron oxidation rates were determined in batch-cultures at pH 1.3 and a temperature of 75°C for Acidianus sulfidivorans, Metallosphaera prunea and a mixed Sulfolobus culture. Ferrous iron and CO₂ in air were added as sole energy and carbon source. The highest growth rate (0.066 h⁻¹) was found with the mixed Sulfolobus culture. Therefore, this culture was selected for further experiments. Growth was not stimulated by increase of the CO₂ concentration or by addition of sulphur as an additional energy source. In a CSTR operated at the suboptimal pH of 1.1, the maximum specific growth rate of the mixed culture was 0.022 h⁻¹, with ferrous iron oxidation rates of 1.5 g L⁻¹ d⁻¹. Compared to pH 1.3, growth rates were strongly reduced but the ferrous iron oxidation rate remained unaffected. Influent ferrous iron concentrations above 6 g L⁻¹ caused instability of Fe²⁺ oxidation, probably due to product (Fe³⁺) inhibition. Ferric-containing, nano-sized precipitates of K-jarosite were found on the cell surface. Continuous cultivation stimulated the formation of an exopolysaccharide-like substance. This indicates that biofilm formation may provide a means of biomass retention. Our findings showed that stable continuous cultivation of a mixed iron-oxidizing culture is feasible at the extreme conditions required for continuous biomineral formation.