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Bioleaching of tannery sludge is an efficient and environmentally friendly way for chromium (Cr) removal, which supports the sustainable development of the leather industry. Acidithiobacillus thiooxidans has been reported effective in Cr bioleaching of tannery sludge. However, little is known about whether the presence of other benefiting species could further improve the Cr leaching efficiency of A. thiooxidans . Here, we studied the enhancing roles of four species namely Acidiphilium cryptum , Sulfobacillus acidophilus , Alicyclobacillus cycloheptanicus , and Rhodotorula mucilaginosa in chromium bioleaching of tannery sludge with A. thiooxidans by batch bioleaching experiments. We found that each of the four species facilitated the quick dominance of A. thiooxidans in the bioleaching process and significantly improved the bioleaching performance including bioleaching rate and efficiency. The bioleaching efficiency of Cr in the tannery sludge could reach 100% on the sixth day by co-inoculating A. thiooxidans and four auxiliary species. The achievements shed a light on the role of the community-level interactions on bioleaching and may also serve as guidance for managing bioleaching consortiums for better outcomes.

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 15 g L−1, where maximum dry cell weight titers reached 3.6 and 3.9 g L−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.

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.

For the first time, a microbial fuel cell has been developed using an acidophile, Acidiphilium cryptum, as the anode biocatalyst. Electricity production using its natural electron acceptor, iron, as the electron mediating agent at pH values <=4.0 was demonstrated. Accumulation of Fe(III) at the electrode, however, restricted current output. The combination of nitrilotriacetic acid and Phenosafranin as electron mediators increased the power output to 12.7 mW/m² in a two-chamber air-sparged fuel cell. Direct electron transfer from the microorganisms to the anode was also investigated but was not detected under the conditions studied.

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.

Acidiphilium cryptum is an acidophilic, heterotrophic α-Proteobacterium which thrives in acidic, metal-rich environments (e.g. acid mine drainage). Recently, an ectABCDask gene cluster for biosynthesis of the compatible solutes ectoine and hydroxyectoine was detected in the genome sequence of A. cryptum JF-5. We were able to demonstrate that the type strain A. cryptum DSM 2389 T is capable of synthesizing the compatible solute hydroxyectoine in response to moderate osmotic stress caused by sodium chloride and aluminium sulphate, respectively. Furthermore, we used the A. cryptum JF-5 sequence to amplify the ectABCDask gene cluster from strain DSM 2389 T and achieved heterologous expression of the gene cluster in Escherichia coli . Hence, we could for the first time prove metabolic functionality of the genes responsible for hydroxyectoine biosynthesis in the acidophile A. cryptum . In addition, we present information on specific enzyme activity of A. cryptum DSM 2389 T ectoine synthase (EctC) in vitro. In contrast to EctCs from halophilic microorganisms, the A. cryptum enzyme exhibits a higher isoelectric point, thus a lower acidity, and has maximum specific activity in the absence of sodium chloride.

The effect of glucose and elemental sulfur on the growth and PHB accumulation of Acidiphilium cryptum DX1-1 was investigated. Meanwhile, the differential expressions of 19 genes related with PHB accumulation, sulfur metabolism and carbon fixed in heterotrophy, phytotrophy and mixotrophy were studied by RT-qPCR. The results showed that strain DX1-1 could accumulate PHB with sulfur as the energy substance and atmospheric CO₂ as carbon resource. Glucose could improve the growth of strain DX1-1 cultured in medium with sulfur as the energy substance, and almost all the key enzyme-encoding genes related with PHB, sulfur metabolism and carbon fixed were basically up-regulated. PHB polymerase (Arcy_3030), ribulose-bisphosphate carboxylase (Acry_0825), ribulose-phosphate-epimerase (Acry_0022), and cysteine synthase A (Acry_2560) played important role in PHB accumulation, the modified expression of which could influence the PHB yield. With CO₂ as carbon resource, the main initial substance of PHB accumulation for strain DX1-1 was acetyl-CoA, instead of acetate with the glucose as the carbon resource. Because of accumulating PHB by fixed atmospheric CO₂ while independent of light, A. cryptum DX1-1 may have specifically potential in production of PHB.

A microbial fuel cell using an acidophilic microorganism, Acidiphilium cryptum, as the anode biocatalyst was investigated. The mode of electron transfer by this organism to the electrode was studied. Electricity production in the presence of a mediator was demonstrated using its natural electron acceptor, iron, as well as phenosafranin as the electron mediating agent. Production of Fe(II), as a result of iron reduction, at a pH of 4.0 or below was found to support electricity production. Accumulation of the oxidized iron, Fe(III) as a result of electron donation to the electrode, however, restricted higher current output. Addition of nitrilotriacetic acid helped resolve the problem by redissolution of deposited Fe(III). Further, use of phenosafranin as a secondary mediator resulted in improvement in power output. At a cell loading equivalent to OD600 of 1.0, a power output of 12.7 mW/m2 was obtained in a two-chamber air-sparged fuel cell. Potential for direct electron transfer was also investigated but not detected under the conditions studied.

We report the ¹H, ¹³C, and ¹⁵N chemical shift assignments of both oxidized and reduced forms of an abundant periplasmic c-type cytochrome, designated ApcA, isolated from the acidophilic gram-negative facultatively anaerobic metal-reducing alphaproteobacterium Acidiphilium cryptum. These resonance assignments prove that ApcA is a monoheme cytochrome c ₂ and the product of the Acry_2099 gene. An absence of resonance peaks in the NMR spectra for the 21N-terminal residues suggests that a predicted N-terminal signal sequence is cleaved. We also describe the preparation and purification of the protein in labeled form from laboratory cultures of A. cryptum growing on ¹³C- and ¹⁵N- labeled substrates.

The growth of acidophilic iron respiring bacteria at pH > 4.5 may be a key to the transition from acidic to circumneutral conditions that would occur during restoration of acid mine drainage sites. Flasks containing Acidiphilium cryptum ATCC 33463 were incubated initially under aerobic conditions in liquid medium containing Fe 2(SO 4) 3 and glucose at an initial pH of 5. Significant iron respiration was observed after flasks were sealed to prevent oxygenation; at the same time, medium pH increased from 4.5 to 6. No soluble Fe(III) was detected throughout the experiments, consistent with pH conditions, indicating that bacteria were able to respire using precipitated ferric iron species. In addition, the concentration of soluble Fe 2+ reached a plateau, even though iron respiration appeared to continue, possibly due to precipitation of mixed Fe (II)/Fe(III)-oxide as magnetite. Results suggest that A. cryptum has a wide range of pH tolerance, which may enable it to play a role in controlling acid generation by means of establishing growth conditions favorable to neutrophilic bacteria such as sulfate reduction.