Acidiphilium Cryptum Research Articles

Production of poly (3-hydroxybutyrate) and extracellular polymeric substances from glycerol by the acidophile Acidiphilium cryptum.

Acidiphilium cryptum is an acidophilic, heterotrophic, and metallotolerant bacteria able to use dissolved oxygen or Fe(III) as an electron sink. The ability of this extremophile to accumulate poly(3-hydroxybutyrate) (PHB) and secrete extracellular polymeric substances (EPS) has also been reported. Hence, the aim of this work is to characterize the production of PHB and EPS by the wild strain DSM2389 using glycerol in shaken flasks and bioreactor. Results showed that maximum PHB accumulation (37-42% w/w) was obtained using glycerol concentrations of 9 and 15gL-1, where maximum dry cell weight titers reached 3.6 and 3.9gL-1, respectively. The culture in the bioreactor showed that PHB accumulation takes place under oxygen limitation, while the redox potential of the culture medium could be used for online monitoring of the PHB production. Recovered EPS was analyzed by Fourier-transform infrared spectroscopy and subjected to gas chromatography-mass spectrometry after cleavage and derivatization steps. These analyses showed the presence of sugars which were identified as mannose, rhamnose and glucose, in a proportion near to 3.2:2.3:1, respectively. Since glycerol had not been used in previous works, these findings suggest the potential of A. cryptum to produce biopolymers from this compound at a large scale with a low risk of microbial contamination due to the low pH of the fermentation process.

A two-step bioleaching process enhanced the recovery of rare earth elements from phosphogypsum

This work proposes a novel bioleaching process for enhancing the recovery of rare earth elements from phosphogypsum by using a co-culture of Acidithiobacillus ferrooxidans CL and Acidiphilium cryptum LT. The maximum production of organic acids (13.8 mM) was obtained when Acidiphilium cryptum LT was cultured for 5 days in advance, and then co-cultured with Acidithiobacillus ferrooxidans CL. This two-step co-culture process exhibited a significant synergistic action through the decrease in pH caused by the protons released during the formation of jarosite and organic acids produced by Acidiphilium cryptum LT. This process exhibited the highest extraction of total REEs: 84.5% of La, 39.0% of Ce, 70.7% of Nd, and 83.9% of Y from phosphogypsum under the optimum conditions. This study provides new insights into the positive role of jarosite formation in the bioleaching of non-sulfides minerals and simultaneously substantiated that the two-step co-culture process enhanced the recovery of rare earth elements from phosphogypsum.

Removal of arsenic from acidic liquors using chemical and autotrophic and mixed heterotrophic bacteria-produced biogenic schwertmannites

Arsenic penetrates human society through a variety of geological and anthropogenic processes, posing significant health hazards. Acid mine drainage, which contains high concentrations of heavy metals and sulfate, is formed by the biological oxidation of pyrite and other metal-containing sulfidic minerals and is a significant environmental hazard. Adsorption is a simple and effective method for removing arsenic from water. In this study, co-precipitation and adsorption of arsenic with biogenic and chemically produced iron-containing settleable precipitates, i.e. schwertmannites were studied. Autotrophic Leptospirillum ferrooxidans and heterotrophic mixed culture of Alicyclobacillus tolerans and Acidiphilium cryptum oxidized iron at rates from 18 to 23 mg/(L.h) in the presence of 5 and 10 mg/L As3+, and both cultures tolerated up to 100 mg As3+/L although Fe2+ oxidation rates decreased to 3–4 mg/(L.h). At Fe/As ratios of ≥20, As removal efficiencies of ≥95% were obtained by co-precipitation with Fe3+ at pH 3.5–4.5. Because schwertmannite precipitates produced by the heterotrophic culture formed crystals, it was studied for adsorptive removals of As3+ and As5+ and compared with chemically synthesized schwertmannites. As3+ (100 mg/L) adsorption onto biogenic and chemical schwertmannite were 25 and 44%, respectively, at pH 4. At 100 mg As5+/L, adsorption capacity and efficiency onto biogenic schwertmannite were 47 mg/g and 50%, respectively. At 300 mg As5+/L, adsorption capacity and efficiency onto chemical schwertmannite were 169 mg/g and 56%, respectively. In summary, biogenic schwertmannite has potential for As removal via co-precipitation with Fe3+ at pH 3.5–4.5 and Fe/As ratios of ≥20 due to low production cost from acidic mine drainage. In contrast to the schwertmannite generation methods, which are usually performed with autotrophic acidophilic bacteria in the literature, this efficient and modular schwertmannite production process and its evaluation on arsenic adsorption is an important potential in acidic mine drainage treatment containing arsenic.

A Comparative Study on Bioleaching Properties of Various Sulfide Minerals Using Acidiphilium cryptum

Bioleaching has been regarded as a green alternative to chemical leaching in metal extraction processes. In this study, the bioleaching properties of indigenous acidophilic bacteria for various sulfide minerals were compared and evaluated in terms of pH reduction and metal leaching. The primary minerals in the samples were sphalerite (ZnS) (SP), galena (PbS) (GN1 and GN2), pyrite (FeS2) (PY), and chalcopyrite (CuFeS2) (CCP), and an indigenous acidophilic bacterium, Acidiphilium cryptum (99.56%), was applied for bioleaching experiments. The metal extraction in bioleaching differed according to the mineral content. The leached metal concentration of Zn was higher than that of Pb for the SP sample with a high ZnS content, whereas the concentration of Pb was higher than that of Zn for the GN1 and GN2 samples with a high PbS content. Meanwhile, the leaching rate of Zn was faster than that of Pb for all samples. Corrosion action was observed on the surface of bacterial residues in SP and GN1 samples. These results show that the bioleaching mechanism based on sulfide minerals proceeds through indirect biological oxidation, chemical oxidation, and direct bacterial oxidation. The results of this study can provide basic research data for process optimization when employing bioleaching to extract valuable metals.

Simulated acid mine drainage treatment in iron oxidizing ceramic membrane bioreactor with subsequent co-precipitation of iron and arsenic

Acid mine drainage (AMD), generated in the active and abandoned mine sites, is characterized by low pH and high metal concentrations. One AMD treatment possibility is biologically oxidizing Fe2+ followed by precipitation through pH control. As compared to autotrophic iron oxidizing microbial community, a microbial community enriched in the presence of organic nutrients was hypothesized to yield higher biomass during commissioning the bioreactor. In this study, the treatment of Fe, Cu, Co, Mn, Zn, Ni, and As containing simulated AMD was studied using an iron-oxidizing ceramic membrane bioreactor (CMBR) at varying hydraulic retention times (HRTs) (6–24 h) and two different feed Fe2+ concentrations (250 and 750 mg/L). The impact of tryptone soya broth (TSB) on the CMBR performance was also investigated. Almost complete Fe2+ oxidation and sustainable flux at around 5.0 L/(m2.h) were obtained in the CMBR with the Alicyclobacillus tolerans and Acidiphilium cryptum dominated enrichment culture. The Fe2+ oxidation rate, as assessed in batch operation cycles of CMBR, increased significantly with increasing Fe2+ loading to the bioreactor. The iron oxidation rate decreased by the elimination of organic matter from the feed. The increase of the CMBR permeate pH to 3.5–4.0 resulted in selective co-precipitation of As and Fe (over 99%) with the generation of biogenic schwertmannite.

METAGENOMICS (16S AMPLICON SEQUENCING) AND DGGE ANALYSIS OF BACTERIAL DIVERSITY OF ACID MINE DRAINAGE

Acid mine drainages (AMDs) are characterized as low pH water containing metal, which is the best-known habitat for the microorganisms described as acidophiles. Most of these members do not survive above pH 4.0; therefore they are acidophilic. Because of these metabolic properties of acidophilic microorganisms, AMD is accepted as a model system in biogeochemical studies on iron and sulfur cycles for the analysis of microbial ecology.
 In this study, microbial diversity of the acid mine drainage in Çan (Çanakkale) was investigated by using culture-independent techniques including Denaturing Gradient Gel Electrophoresis (DGGE) and high-throughput sequencing.
 A total of 105,593,810 and 115,689,952 bases read were performed. Following sequencing, raw data were proceeded in “Quantitative Insights Into Microbial Ecology” (QIIME 1.9.1). Acidiphilium, Acidocella and Acidosoma were genus obtained by bioinformatic analysis. The highest rate belonged to Acidiphilium for each water sample. Acidiphilium cryptum, Acidithiobacillus sp. and Acidibacter ferrireducens were determined according to DGGE analysis.

PROKARYOTIC DIVERSTY OF ACID MINE DRAINAGE PONDS IN ORE ENRICHMENT PLANT

The biodiversity of acidophilic prokaryotes was determined in three AMD ponds (pH 2.7-6.5) in Turkey (İzmir-Halıköy antimony ore enrichment plant) using 16S rRNA cloning and denaturing gradient gel electrophoresis methods. Water samples were taken two times in March 2014 and June 2015. The microbial diversity identified includes species such as Acidiphilium angustum, Acidocella sp., Ferroplasma acidiphilum, Acidithiobacillus ferriphilus, Acidithiobacillus ferrivorans, Acidiphilium rubrum, Thiomonas sp., Acidiphilium multivorum, Acidiphilium cryptum, Ferrovum myxofaciens, Acidocella aluminiidurans with the used techniques. In addition to, it has been determined that biodiversity is variable in the operating mine pools. Aciditihobacillus ferriphilus, Acidiphilium angustum, and Acidiphilium rubrum are new records for Turkey.

Iron oxidation in a ceramic membrane bioreactor using acidophilic bacteria isolated from an acid mine drainage

This study aims at investigating the iron oxidation and filtration performances of a ceramic membrane bioreactor (CMBR) inoculated with a mixed culture of iron-oxidizing acidophilic bacteria isolated from a real acid mine drainage (AMD). The CMBR was fed with a synthetic solution containing 1400 mg/L Fe2+ at pH 1.5−2.5. High iron oxidation performances (around 97%) at HRT range of 48−12 h were observed. Further decreasing HRT to 6 h led to significant decrease in the Fe2+ oxidation performances. CMBR was operated at varying fluxes. Significant fouling was not observed provided that flux was 5 LMH or lower. The mixed culture was composed of Acidiphilium cryptum and Alicyclobacillus cycloheptanicus. In addition to continuous flow experiments in CMBR, Fe2+ oxidation experiments were conducted under batch conditions. Oxidation kinetics well fitted Monod equation with a specific maximum iron oxidation rate and half-saturation constant of around 3.35 mg-Fe2+/(g-VSS.min) and 549 mg/L, respectively.

Boosting Escherichia coli\u2019s heterologous production rate of ectoines by exploiting the non-halophilic gene cluster from Acidiphilium cryptum

The compatible solutes ectoine and hydroxyectoine are synthesized by many microorganisms as potent osmostress and desiccation protectants. Besides their successful implementation into various skincare products, they are of increasing biotechnological interest due to new applications in the healthcare sector. To meet this growing demand, efficient heterologous overproduction solutions for ectoines need to be found. This study is the first report on the utilization of the non-halophilic biosynthesis enzymes from Acidiphilium cryptum DSM 2389T for efficient heterologous production of ectoines in Escherichia coli. When grown at low salt conditions (≤ 0.5% NaCl) and utilizing the cheap carbon source glycerol, the production was characterized by the highest specific production of ectoine [2.9 g/g dry cell weight (dcw)] and hydroxyectoine (2.2 g/g dcw) reported so far and occurred at rapid specific production rates of up to 345 mg/(g dcw × h). This efficiency in production was related to an unprecedented carbon source conversion rate of approx. 60% of the theoretical maximum. These findings confirm the unique potential of the here implemented non-halophilic enzymes for ectoine production processes in E. coli and demonstrate the first efficient heterologous solution for hydroxyectoine production, as well as an extraordinary efficient low-salt ectoine production.

Electron shuttles stimulate the reductive dissolution of jarosite by Acidiphilium cryptum

Jarosite is stable under oxidizing conditions at low pH, but Acidiphilium cryptum can promote its reductive dissolution to Fe(II). Since electron shuttles enhance the bioreduction of iron oxides at circumneutral pH, nine compounds were selected to study their effect on jarosite dissolution at low pH. Gallocyanine, methylene blue and thionine stimulated the bioreduction of jarosite by factors of four, six and ten, respectively. Cyclic voltammetry test at pH 2.5 showed that these three compounds have a half-wave potential in the range 276–375 mV vs SHE. Similar results have been reported for neutrophiles, although the range obtained in this work was shifted to positive values. This behavior would be related to the ability of A. cryptum to reduce electron shuttles and thermodynamic data for reduction of ferric compounds.

In silico genome analysis of an acid mine drainage species, Acidiphilium multivorum, for potential commercial acetic acid production and biomining

The genome of Acidiphilium multivorum strain AIU 301, acidophilic, aerobic Gram-negative bacteria, was investigated for potential metabolic pathways associated with organic acid production and metal uptake. The genome was compared to other acidic mine drainage isolates, Acidiphilium cryptum JF-5 and Acidithiobacillus ferrooxidans ATCC 23270, as well as Acetobacter pasteurianus 386B, which ferments cocoa beans. Plasmids between two Acidiphilium spp. were compared, and only two of the sixteen plasmids were identified as potentially similar. Comparisons of the genome size to the number of protein coding sequences indicated that A. multivorum and A. cryptum follow the line of best fit unlike A. pasteurianus 386B, which suggests that it was improperly annotated in the database. Pathways between these four species were analyzed bioinformatically and are discussed here. A. multivorum AIU 301, shares pathways with A. pasteurianus 386B including aldehyde and alcohol dehydrogenase pathways, which are used in the generation of vinegar. Mercury reductase, arsenate reductase and sulfur utilization proteins were identified and discussed at length. The absence of sulfur utilization proteins from A. multivorum AIU 301 suggests that this species uses previously undefined pathways for sulfur acquisition. Bioinformatic examination revealed novel pathways that may benefit commercial fields including acetic acid production and biomining.

The significance of pH in dictating the relative toxicities of chloride and copper to acidophilic bacteria

The ability of acidophilic bacteria to grow in the presence of elevated concentrations of cationic transition metals, though varying between species, has long been recognized to be far greater than that of most neutrophiles. Conversely, their sensitivity to both inorganic and organic anions, with the notable exception of sulfate, has generally been considered to be far more pronounced. We have compared the tolerance of different species of mineral-oxidizing Acidithiobacillus and Sulfobacillus, and the heterotrophic iron-reducer Acidiphilium cryptum, to copper and chloride when grown on ferrous iron, hydrogen or glucose as electron donors at pH values between 2.0 and 3.0. While tolerance of copper varied greatly between species, these were invariably far greater at pH 2.0 than at pH 3.0, while their tolerance of chloride showed the opposite pattern. The combination of copper and chloride in liquid media appeared to be far more toxic than when these elements were present alone, which was thought to be due to the formation of copper–chloride complexes. The results of this study bring new insights into the understanding of the physiological behaviour of metal-mobilising acidophilic bacteria, and have generic significance for the prospects of bioleaching copper ores and concentrates in saline and brackish waters.

The contribution of Acidiphilium cryptum to the dissolution of low-grade manganese ores

Microorganisms contribute to metal mobilization by catalyzing the dissolution of diverse compounds in both acidic and circumneutral environments. Since Mn(II) is more soluble than Mn(IV), acidophilic iron-reducing microorganisms, such as Acidiphilium cryptum, may increase the manganese contained in acid mine drainage from both active and abandoned mines. Two low-grade manganese ores (7% and 25% Mn) were used to perform dissolution tests with A. cryptum JF-5 in medium amended with oxalic acid, dissolved iron and thionine. The statistical analysis performed with a 99% confidence level demonstrated that the presence of Acidiphilium cryptum was the most important factor for manganese dissolution, increasing the process rate up to sevenfold (from 23.2 to 175.8nmol/L·d). Pareto charts showed that the oxalate alone and the interaction of bacteria-Fe(II)-oxalate were also relevant in the dissolution process. The sulfuric acid used for pH control was proportional to the extracted amounts of manganese and iron, obtaining a consumption of 1.3mol of H2SO4 per mol of metal. In the presence of A. cryptum, oxalic acid led to the selective dissolution of manganese, while thionine increased the dissolution of iron and decreased the process selectivity. Deoxygenation of the culture medium limited the bacteria proliferation and dissolution processes. Although the mechanism used by A. cryptum to dissolve both ores was not identified, the synthesis of bacterial appendages, or an iron- (or iron-chelate) mediated process may be essential when performing manganese dissolution.

Palladium bionanoparticles production from acidic Pd(II) solutions and spent catalyst leachate using acidophilic Fe(III)-reducing bacteria.

The acidophilic, Fe(III)-reducing heterotrophic bacteria Acidocella aromatica PFBCT and Acidiphilium cryptum SJH were utilized to produce palladium (Pd) bionanoparticles via a simple 1-step microbiological reaction. Monosaccharide (or intracellular NADH)-dependent reactions lead to visualization of intra/extra-cellular enzymatic Pd(0) nucleation. Formic acid-dependent reactions proceeded via the first slow Pd(0) nucleation phase and the following autocatalytic Pd(II) reduction phase regardless of the presence or viability of the cells. However, use of active cells (with full enzymatic and membrane protein activities) at low formic acid concentration (5mM) was critical to allow sufficient time for Pd(II) biosorption and the following enzymatic Pd(0) nucleation, which consequently enabled production of fine, dense and well-dispersed Pd(0) bionanoparticles. Differences of the resultant Pd(0) nanoparticles in size, density and localization between the two bacteria under each condition tested suggested different activity and location of enzymes and membrane "Pd(II) trafficking" proteins responsible for Pd(0) nucleation. Despite the inhibitory effect of leaching lixiviant and dissolved metal ions, Pd(0) bionanoparticles were effectively formed by active Ac. aromatica cells from both acidic synthetic Pd(II) solutions and from the actual spent catalyst leachates at equivalent 18-19nm median size with comparable catalytic activity.

Characterization of acidophilic bacteria related to Acidiphilium cryptum from a coal-mining-impacted river of South Brazil

Three acidophilic bacteria were isolated from water and sediment samples collected at a coal mining-impacted river, Rio Sangão, located in the city of Criciúma, Santa Catarina state, in south Brazil. These microorganisms were isolated in acid media and were phylogenetic related to the species Acidiphilium cryptum by its 16s rRNA gene sequences, although they differ from this species in the assimilation of some carbon sources. The optimum and the maximum pH for growth of all strains were nearly 3.0 and 5.0-6.0, respectively. Two of the strains were slightly more acidophilic, with the minimum pH for growth of 2.0. All strains also tolerate the four tested metals (Ni, Zn, Cu and Se) at variable concentrations, with LAMA 1486 being the most metal-resistant strain. These bacteria may belong to different ecotypes of A. cryptum, or even represent new species in the genus. Besides they bear characteristics that make them useful in the development of bioremediation process, for the treatment of sites contaminated with multiple toxic metals, including coal mining drainage

Indirect Redox Transformations of Iron, Copper, and Chromium Catalyzed by Extremely Acidophilic Bacteria.

Experiments were carried out to examine redox transformations of copper and chromium by acidophilic bacteria (Acidithiobacillus, Leptospirillum, and Acidiphilium), and also of iron (III) reduction by Acidithiobacillus spp. under aerobic conditions. Reduction of iron (III) was found with all five species of Acidithiobacillus tested, grown aerobically on elemental sulfur. Cultures maintained at pH 1.0 for protracted periods displayed increasing propensity for aerobic iron (III) reduction, which was observed with cell-free culture liquors as well as those containing bacteria. At. caldus grown on hydrogen also reduced iron (III) under aerobic conditions, confirming that the unknown metabolite(s) responsible for iron (III) reduction were not (exclusively) sulfur intermediates. Reduction of copper (II) by aerobic cultures of sulfur-grown Acidithiobacillus spp. showed similar trends to iron (III) reduction in being more pronounced as culture pH declined, and occurring in both the presence and absence of cells. Cultures of Acidithiobacillus grown anaerobically on hydrogen only reduced copper (II) when iron (III) (which was also reduced) was also included; identical results were found with Acidiphilium cryptum grown micro-aerobically on glucose. Harvested biomass of hydrogen-grown At. ferridurans oxidized iron (II) but not copper (I), and copper (I) was only oxidized by growing cultures of Acidithiobacillus spp. when iron (II) was also included. The data confirmed that oxidation and reduction of copper were both mediated by acidophilic bacteria indirectly, via iron (II) and iron (III). No oxidation of chromium (III) by acidophilic bacteria was observed even when, in the case of Leptospirillum spp., the redox potential of oxidized cultures exceeded +900 mV. Cultures of At. ferridurans and A. cryptum reduced chromium (VI), though only when iron (III) was also present, confirming an indirect mechanism and contradicting an earlier report of direct chromium reduction by A. cryptum. Measurements of redox potentials of iron, copper and chromium couples in acidic, sulfate-containing liquors showed that these differed from situations where metals are not complexed by inorganic ligands, and supported the current observations of indirect copper oxido-reduction and chromium reduction mediated by acidophilic bacteria. The implications of these results for both industrial applications of acidophiles and for exobiology are discussed.

Reductive Dissolution of Jarosite by Acidiphilium cryptum in Presence of Chelating Agents and Dissolved Iron

ABSTRACTThe mutual effects of Acidiphilium cryptum, nine ligands and dissolved iron on jarosite dissolution were studied in a 23 full factorial experiment. Ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA) and oxalic acid were able to dissolve jarosite chemically, but bacterial action enhanced this dissolution; from 88 ± 1, 46 ± 0 and 36 ± 0 µmol/L d to 129 ± 3, 177 ± 5 and 106 ± 14 µmol/L d, respectively. Oxalate and NTA exhibited a synergistic effect with dissolved iron and bacteria that may be explained by a model in which iron complexes behave as redox mediators in the microbial respiration in which jarosite, a solid substrate, serves as terminal electron acceptor.

Generation of acid mine drainage around the Karaerik copper mine (Espiye, Giresun, NE Turkey): implications from the bacterial population in the Acısu effluent.

The Karaerik Cu mine is a worked-out deposit with large volumes of tailings and slags which were left around the mine site without any protection. Natural feeding of these material and run-off water from the mineralised zones into the Acısu effluent causes a serious environmental degradation and creation of acid mine drainage (AMD) along its entire length. This research aims at modelling the formation of AMD with a specific attempt on the characterisation of the bacterial population in association with AMD and their role on its occurrence. Based on 16SrRNA analyses of the clones obtained from a composite water sample, the bacterial community was determined to consist of Acidithiobacillus ferrivorans, Ferrovum myxofaciens, Leptospirillum ferrooxidans and Acidithiobacillus ferrooxidans as iron-oxidising bacteria, Acidocella facilis, Acidocella aluminiidurans, Acidiphilium cryptum and Acidiphilium multivorum as iron-reducing bacteria, and Acidithiobacillus ferrivorans, Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Acidiphilium cryptum as sulphur-oxidising bacteria. This association of bacteria with varying roles was interpreted as evidence of a concomitant occurrence of sulphur and iron cycles during the generation of AMD along the Acısu effluent draining the Karaerik mine.

Psychrophile spoilers dominate the bacterial microbiome in musculature samples of slaughter pigs

The aim of this study was to disentangle the microbial diversity on porcine musculature. The hypervariable V1–V2 region of the 16S rRNA gene was amplified from DNA samples of clinically healthy slaughter pigs (n=8). Pyrosequencing yielded 37,000 quality-controlled reads and a diverse microbiome with 54–159 OTUs per sample was detected. Interestingly, 6 out of 8 samples were strongly dominated by 1–2 highly abundant OTUs (best hits of highly abundant OTUs: Serratia proteamaculans, Pseudomonas syringae, Aeromonas allosaccharophila, Brochothrix thermosphacta, Acidiphilium cryptum and Escherichia coli). In 1g musculature scraping, 3.20E+06 16S rRNA gene copies and 4.45E+01 Enterobacteriaceae rRNA gene copies were detected with qPCR. We conclude that i.) next-generation sequencing technologies help encompass the full content of complex, bacterial contamination, ii.) psychrophile spoilers dominated the microbiota and iii.) E. coli is an effective marker species for pork contamination, as it was one of very few abundant species being present in all samples.

Genomic Analysis Unravels Reduced Inorganic Sulfur Compound Oxidation of Heterotrophic Acidophilic Acidicaldus sp. Strain DX-1.

Although reduced inorganic sulfur compound (RISC) oxidation in many chemolithoautotrophic sulfur oxidizers has been investigated in recent years, there is little information about RISC oxidation in heterotrophic acidophiles. In this study, Acidicaldus sp. strain DX-1, a heterotrophic sulfur-oxidizing acidophile, was isolated. Its genome was sequenced and then used for comparative genomics. Furthermore, real-time quantitative PCR was performed to identify the expression of genes involved in the RISC oxidation. Gene encoding thiosulfate: quinone oxidoreductase was present in Acidicaldus sp. strain DX-1, while no candidate genes with significant similarity to tetrathionate hydrolase were found. Additionally, there were genes encoding heterodisulfide reductase complex, which was proposed to play a crucial role in oxidizing cytoplasmic sulfur. Like many heterotrophic sulfur oxidizers, Acidicaldus sp. strain DX-1 had no genes encoding enzymes essential for the direct oxidation of sulfite. An indirect oxidation of sulfite via adenosine-5′-phosphosulfate was proposed in Acidicaldus strain DX-1. However, compared to other closely related bacteria Acidiphilium cryptum and Acidiphilium multivorum, which harbored the genes encoding Sox system, almost all of these genes were not detected in Acidicaldus sp. strain DX-1. This study might provide some references for the future study of RISC oxidation in heterotrophic sulfur-oxidizing acidophiles.