A new alkali-thermostable azoreductase from Bacillus sp. strain SF

Heterogeneous biocatalytic system for effective decolorization of textile dye effluent

The current physicochemical methods for decolorizing toxic synthetic dyes are not sustainable to halt the environmental damage as they are expensive and often produce concentrated sludge, which may lead to secondary disposal problems. Biocatalysis (microbes and/or their enzymes) is a cost-effective, versatile, energy-saving and clean alternative. The most common enzymes involved in dye degradation are laccases, azoreductases and peroxidases. Toxic dyes could be converted into less harmful byproducts through the combined action of many enzymes or the utilization of whole cells. The action of whole cells to treat dye effluents is either by biosorption or degradation (aerobic or anaerobic). Using immobilized cells or enzymes will offer advantages such as superior stability, persistence against harsh environmental conditions, reusability and longer half-lives. This review envisages the recent strategies of immobilization and bioreactor considerations with the immobilized system as the effective treatment of textile dye effluents. Packed bed reactors are the most popular heterogeneous biocatalytic reactors for dye decolorization due to their efficiency and cost-effectiveness.

Eco-friendly degradation of reactive red 195, reactive blue 214, and reactive yellow 145 by Klebsiella pneumoniae MW815592 isolated from textile waste

The water is used in many textile manufacturing steps beyond cleaning. The quantity and the significant chemical load of the effluents generated constitute the primary challenge of the textile industry. In order to discover new sustainable methods to overcome this problem, the aim of this research was to study the potential for degradation of Reactive Blue 214, Reactive Red 195, and Reactive Yellow 145 using a dye degrading bacterium. Sequencing analysis reveals it to be Klebsiella pneumoniae MW815592. This strain completely decolorized artificial effluent (200 mg/L) after 42 h at pH 9 and 46 °C. The decolorization rate increased in the presence of glucose and yeast extract (2 g). In addition, our finding revealed that the decolorization is due to biodegradation rather than adsorption on the bacterial surface.

Biodegradation of Reactive Red 198 by textile effluent adapted microbial strains

A sustainable technology to eliminate the persistent reactive dyes from the textile effluents discharged indiscriminately in the environment is highly desirous given the explosive growth of textile industries. The present study investigated the potential of two different bacterial strains, Bacillus cereus SKB12 and Enterobacter hormaechei SKB16 isolated from the dye house effluent sludge in the biotransformation of Reactive Red 198 (RR 198). Process variables such as temperature, pH, shaking conditions and contact time were optimized for the successful decolourization of RR 198. Maximum decolourization of 80% and 85% of RR 198 was achieved at pH 6 and 7, and 40 °C in microaerophilic conditions on treatment with B. cereus and E. hormaechei, respectively. High-Performance Liquid Chromatography (HPLC), and Gas Chromatography-Mass Spectrometry (GC-MS) analyses conducted further affirmed that the decolourization of RR 198 was rather due to biodegradation than biosorption through shift in wavenumbers, retention time variations and the appearance of lesser molecular weight peaks. Degradative pathway for RR 198 predicted based on the enzyme assay data and dye degraded metabolite peaks acquired through GC-MS analysis highlighted the significance of azoreductase and laccase in the degradation of RR 198 into smaller non-toxic compounds. In addition, toxicity assessment through zootoxicological and phytotoxicological experiments using brine shrimp and Vigna radiata validated the detoxified status of the metabolites thus proving the promising potentials of the bacterial strains in the remediation of azo dyes.

The degradation of textile industry dyes using the effective bacterial consortium

The effluents from textile industries without proper treatment contains a remarkable amount of synthetic dyes which are harmful to the environment and a big challenge globally to degrade it with a eco-friendly way. Conventional methods are extremely energy-consuming, non-effective and generate a toxic sludge impacting the environment. Several microorganisms can be utilized to treat these effluents. The research deals with five bacteria isolated from textile effluent and their consortium for the biodegradation ability of Novacron dyes. The isolates were identified through the Biolog™ identification system and molecular technique. Biodegradation was confirmed by measuring optical density (OD) optimizing conditions (pH 7.0, temperature 37 °C, 10 % inoculums and 100 mg/L dye) under static condition. The isolates started decolourization at 24 h whereas, the consortium started decolourization at 18 h and exhibited a maximum after 72 h. The presence of low molecular weight protein as metabolite supported the biodegradation and non hazardous to environment. This study revealed that these bacteria might have degradation potentials, and research results will help to set up dye removal eco-friendly methods to expose the dye effulents to environment in future.

Extremophiles, a Nifty Tool to Face Environmental Pollution: From Exploitation of Metabolism to Genome Engineering

Extremophiles are microorganisms that populate habitats considered inhospitable from an anthropocentric point of view and are able to tolerate harsh conditions such as high temperatures, extreme pHs, high concentrations of salts, toxic organic substances, and/or heavy metals. These microorganisms have been broadly studied in the last 30 years and represent precious sources of biomolecules and bioprocesses for many biotechnological applications; in this context, scientific efforts have been focused on the employment of extremophilic microbes and their metabolic pathways to develop biomonitoring and bioremediation strategies to face environmental pollution, as well as to improve biorefineries for the conversion of biomasses into various chemical compounds. This review gives an overview on the peculiar metabolic features of certain extremophilic microorganisms, with a main focus on thermophiles, which make them attractive for biotechnological applications in the field of environmental remediation; moreover, it sheds light on updated genetic systems (also those based on the CRISPR-Cas tool), which expand the potentialities of these microorganisms to be genetically manipulated for various biotechnological purposes.

Mixed azo dyes degradation by an intracellular azoreductase enzyme from alkaliphilic Bacillus subtilis: a molecular docking study

The rise of pollution due to the dye industries and textile wastes are evolving rapidly every day. The dyes are used in different trade names by the textile industries. The actual chemistry of dye is vague and difficult to understand even today though we are equipped technically. The toxic effects of the dyes and the reasons behind the acute toxicity are also an undiscovered mystery; therefore, no effective measures can be employed to degrade dyes. Deploying physical or chemical methods to pre-treat the azo dyes are expensive, extremely energy-consuming, and are not environment friendly. Hence, the use of microbes for textile dye degradation will be eco-friendly and is probably a cost-effective alternative to physicochemical methods. The present study was conducted to investigate the degradation of azo dyes isolated from textile effluent contaminated soil by employing the bacterial strains for degradation. The bacterial strains could degrade the optimum concentration of mixed azo dyes (200 mg/L) with an incubation up to 5 days. The decolourization of the dyes was expressed in terms of percentage of decolourization, and was found that about 87.35% of degradation by Bacillus subtilis strain. The enzyme responsible was analyzed as intracellular azoreductase involved in the degradation of mixed azo dyes. The enzymatic pathway and 1-phenyl-2-4(4-methyl phenyl)-diazene 1-oxide was observed as the major metabolite by GC-MS analysis. The in silico study determined the binding of mixed azo dye with azoreductase and hypothesized that their linking could be the main reason for the degradation of mixed azo dye.

Assessment of soil and groundwater contamination at a former Tannery district in Dhaka, Bangladesh

The uncontrolled and unplanned development of leather processing industries in Bangladesh has contaminated land and water, prompting concerns for public health. Hazaribagh, located in the southwestern part of Dhaka, has been the city's principal leather processing zone since the 1960s. In order to alleviate the environmental contamination and public health risks to citizens of Hazaribagh and downstream, a relocation project was launched to remove the tanning industry. However, soil and groundwater quality conditions of the former industrial sites must be assessed and/or remediated for commercial and residential use. Soil was collected from ten sites and tested for concentrations of potentially toxic metals (Pb, Cr, Zn, Cu, Ni and Cd), and groundwater was collected from six sites and analyzed for physiochemical parameters and potentially toxic metals. Concentrations of soil Cr, Zn and Cu exceeded the European Union maximum permissible concentrations. Deep groundwater Cr concentration in one location exceeded the Bangladesh DoE maximum limits; however, deep groundwater is overall of good-to-excellent quality. Spatial variations of soil and groundwater contamination in Hazaribagh indicate that contaminants have not spread laterally. Based on local conditions, current technologies, contamination level, time and cost, and ease of operation, it is suggested that soil flushing, electrokinetics and/or phytoremediation could be options for remediation of affected soil and groundwater in the Hazaribagh district.

Eco-friendly decolorization and degradation of reactive yellow 145 textile dye by Pseudomonas aeruginosa and Thiosphaera pantotropha

Dyes are toxic and inherently resistant to microbial degradation. In this study, decolorization and degradation of textile dye reactive yellow 145 (RY145) were evaluated using pure bacterial strains Pseudomonas aeruginosa (RS1) and Thiosphaera pantotropha ATCC 35512. In nutrient broth under static condition, complete decolorization of 50 mg L^-1 RY145 could be achieved within 96 h and 72 h, for Pseudomonas aeruginosa (RS1) and Thiosphaera pantotropha, respectively. In contrast, under shaking condition both the cultures could achieve only 50% decolorization in 96 h. Treatment under sequential static and shaking condition resulted in complete decolorization and 65% mineralization after 96 h. Higher dye concentration in excess of 100 mg L^-1 and 50 mg L^-1 decreased the extent of dye mineralization in Pseudomonas aeruginosa and Thiosphaera pantotropha, respectively. Even with the repetitive addition of the dye, both the strains were capable of decolorizing the dye. Acclimatized cultures showed 54% decolorization of RY145 in mineral media (MM) even in the absence of a readily degradable external carbon source. Amongst various individual carbon and nitrogen sources, maximum decolorization was observed in MM supplemented with peptone as carbon and nitrogen source at pH 7 under static condition.

Microbial use for azo dye degradation-a strategy for dye bioremediation

Azo dyes are aromatic compounds with one to many -N=N- groups as well as the leading class of synthetic dyes utilised in commercial solicitations. Azo dyes, released in the environment through textile effluents, have hazardous effects on the aquatic as well as human life. Their persistence and discharge into the environment are becoming a global concern; thus, the remediation of these contaminants has acquired great attention. The current review comprehensively discusses some of the main aspects of biodegradation of azo dyes. A variety of physicochemical approaches has already been utilised for treatment of textile effluents counting filtration, coagulation and chemical flocculation. Though these conventional techniques are effective, yet they are lavish and also comprise formation of concentrated sludge that makes a secondary disposal problem. In this regard, microbial usage is an effective, economical, bio-friendly and ecologically benign approach.

FRET-based fluorescent nanoprobe platform for sorting of active microorganisms by functional properties

Fluorescence-activated cell sorting (FACS) has rarely been applied to screening of microorganisms because of poor detection resolution, which is compromised by poor stability, toxicity, or interference from background fluorescence of the fluorescence sensors used. Here, a fluorescence-based rapid high-throughput cell sorting method was first developed using a fluorescence resonance energy transfer (FRET) fluorescent nanoprobe NP-RA, which was constructed by coating a silica nanoparticle with Rhodamine B and methyl-red (an azo dye). Rhodamine B (inner layer) is the FRET donor and methyl-red (outer layer) is the acceptor. This ready-to-use NP-RA is non-fluorescent, but fluoresces once the outer layer is degraded by microorganisms. In our experiment, NP-RA was ultrasensitive to model strain Shewanella decolorationis S12, showing a broad detection range from 8.0 cfu/mL to 8.7 × 10⁸ cfu/mL under confocal laser scanning microscopy, and from 1.1 × 10⁷ to 9.36 × 10⁸ cfu/mL under a fluorometer. In addition, NP-RA bioimaging can clearly identify other azo-respiring cells in the microbial community, including Bosea thiooxidans DSM 9653 and Lysinibacillus pakistanensis NCCP-54. Furthermore, the fluorescent probe NP-RA is compatible with downstream FACS so that azo-respiring cells can be rapidly sorted out directly from an artificial microbial community. To our knowledge, no fluorescent nanoprobe has yet been designed for tracking and sorting azo-respiration functional microorganisms.

Remarkable diversification of bacterial azoreductases: primary sequences, structures, substrates, physiological roles, and biotechnological applications

Azoreductases reductively cleave azo linkages by using NAD(P)H as an electron donor. The enzymes are widely found in bacteria and act on numerous azo dyes, which allow various unique applications. This review describes primary amino acid sequences, structures, substrates, physiological roles, and biotechnological applications of bacterial azoreductases to discuss their remarkable diversification. According to primary sequences, azoreductases were classified phylogenetically into four main clades. Most members of clades I-III are flavoproteins, whereas clade IV members include flavin-free azoreductases. Clades I and II prefer NADPH and NADH, respectively, as electron donors, whereas other members generally use both. Several enzymes formed no clades; moreover, some bacteria produce azoreductases with longer primary structures than those hitherto identified, which implies further diversification of bacterial azoreductases. The crystal structures commonly reveal the Rossmann folds; however, ternary structures are moderately varied with different quaternary conformation. Although physiological roles are obscure, several azoreductases have been shown to act on metabolites such as flavins, quinones, and metal ions more efficiently than on azo dyes. Considering that many homologs exclusively act on these metabolites, it is possible that azoreductases are actually side activities of versatile reductases that act on various substrates with different specificities. In parallel, this idea raises the possibility that homologous enzymes, even if these are already defined as other types of reductases, widely harbor azoreductase activities. Although azoreductases for which their genes have been identified are not abundant, it may be simple to identify azoreductases of biotechnological importance that have novel substrate specificities.

Living at the Frontiers of Life: Extremophiles in Chile and Their Potential for Bioremediation

Extremophiles are organisms capable of adjust, survive or thrive in hostile habitats that were previously thought to be adverse or lethal for life. Chile gathers a wide range of extreme environments: salars, geothermal springs, and geysers located at Altiplano and Atacama Desert, salars and cold mountains in Central Chile, and ice fields, cold lakes and fjords, and geothermal sites in Patagonia and Antarctica. The aims of this review are to describe extremophiles that inhabit main extreme biotopes in Chile, and their molecular and physiological capabilities that may be advantageous for bioremediation processes. After briefly describing the main ecological niches of extremophiles along Chilean territory, this review is focused on the microbial diversity and composition of these biotopes microbiomes. Extremophiles have been isolated in diverse zones in Chile that possess extreme conditions such as Altiplano, Atacama Desert, Central Chile, Patagonia, and Antarctica. Interesting extremophiles from Chile with potential biotechnological applications include thermophiles (e.g., Methanofollis tationis from Tatio Geyser), acidophiles (e.g., Acidithiobacillus ferrooxidans, Leptospirillum ferriphilum from Atacama Desert and Central Chile copper ores), halophiles (e.g., Shewanella sp. Asc-3 from Altiplano, Streptomyces sp. HKF-8 from Patagonia), alkaliphiles (Exiguobacterium sp. SH31 from Altiplano), xerotolerant bacteria (S. atacamensis from Atacama Desert), UV- and Gamma-resistant bacteria (Deinococcus peraridilitoris from Atacama Desert) and psychrophiles (e.g., Pseudomonas putida ATH-43 from Antarctica). The molecular and physiological properties of diverse extremophiles from Chile and their application in bioremediation or waste treatments are further discussed. Interestingly, the remarkable adaptative capabilities of extremophiles convert them into an attractive source of catalysts for bioremediation and industrial processes.

Elevated levels of laccase synthesis by Pleurotus pulmonarius BPSM10 and its potential as a dye decolorizing agent

Laccases (EC 1.10.3.2) are a class of multi-copper oxidases that have industrial value. In the present study, forty-five isolates of wild mushrooms were screened for laccase production. Eight of the isolates exhibited exploitable levels of substrate oxidation capacity. Isolate BPSM10 exhibited the highest laccase activity of 103.50 U/ml. Internal Transcribed Spacer (ITS) rRNA gene sequencing was used to identify BPSM10 as Pleurotus pulmonarius. The response of BPSM10 to two nutritional media supplemented with various inducers was characterized and the results indicated that Malt Extract Broth (MEB) supplemented with Xylidine increased laccase production by 2.8× (349.5 U/ml) relative to the control (122 U/ml), while Potato Dextrose Broth (PDB) supplemented with xylidine increased laccase production by 1.9× (286 U/ml). BPSM10 has the ability to decolorize seven synthetic dyes in a liquid medium. Maximum decolorization was observed of malachite green (MG); exhibiting 68.6% decolorization at 100 mg/L. Fourier-transform infrared spectroscopy (FTIR) was employed to confirm the decolorization capacity. The present study indicates that P. pulmonarius BPSM10 has the potential to be used as a potent alternative biosorbent for the removal of synthetic dyes from aqueous solutions, especially in the detoxification of polluted water.

Screening of potential bioremediation enzymes from hot spring bacteria using conventional plate assays and liquid chromatography - Tandem mass spectrometry (Lc-Ms/Ms)

The search for an eco-friendly, non-toxic, economical and efficient means of cleaning water through bioremediation is not only more favourable but critical to maintaining water quality globally especially in water-scarce countries. Thermophilic bacteria including Bacillus species are an important source of novel enzymes for biotechnology applications. In this study, 56 bacterial isolates which were cultured from five hot springs in South Africa were identified predominantly as Bacillus sp. or Bacillus-related spp by 16S rDNA gene sequencing. These isolates were screened for potentially useful enzymes for water bioremediation. Using conventional agar plate assays, 56% (n = 43), 68% (n = 38) and 16% (n = 31) were positive for amylase, protease and bromothymol blue decolorisation respectively. In liquid starch culture, three amylase-positive isolates differentially degraded starch by 34% (isolate 20S) to 98% (isolate 9T). Phenol degradation revealed that five out of thirty reduced phenol up to 42% by colorimetric assay. A thermophilic strain of Anoxybacillus rupiensis 19S (optimal growth temperature of 50 °C), which degraded starch, protein and phenol, was selected for further analysis by tandem LC-MS/MS. This newer technique identified potential enzymes for water bioremediation relating to pollutants from the food industry (amylase, proteases), polyaromatic hydrocarbons and dye pollutants (catalase peroxidase, superoxide dismutase, azoreductase, quinone oxidoreductase), antibiotic residues (ribonucleases), solubilisation of phosphates (inorganic pyrophosphatase) and reduction of chromate and lead. In addition, potential enzymes for biomonitoring of environmental pollutants were also identified. Specifically, dehydrogenases were found to decrease as the level of inorganic heavy metals and petroleum increased in soil samples. This study concludes that bacteria found in South African hot springs are a potential source of novel enzymes with tandem LC-MS/MS revealing substantially more information compared with conventional assays, which can be used for various applications of water bioremediation.

Optimization and enhancement of textile reactive Remazol black B decolorization and detoxification by environmentally isolated pH tolerant Pseudomonas aeruginosa KY284155

Azo dyes are complex derivatives of diazene used in food and textile manufacture. They are highly recalcitrant compounds, and account for severe environmental and health problems. Different strains of Pseudomonas species were isolated from textile wastewater effluents. The bioconversion of Remazol black B (a commonly used water soluble dye) by Pseudomonas aeruginosa was observed in static conditions. The bio-decolorization process was optimized by a multi factorial Plackett–Burman experimental design. Decolorization of 200 mg L⁻¹ reached 100% in 32 h. Interestingly, the presence of yeast extract, magnesium and iron in the culture media, highly accelerated the rate of decolorization. Moreover, one of our isolates, P. aeruginosa KY284155, was kept high degradation rates at high pH (pH = 9), which represents the pH of most textile wastewater effluents, and was able to tolerate high concentration of dye up to 500 mg L⁻¹. In bacteria, azo-dye degradation is often initiated by reductive azo compound cleavage catalyzed by azo-reductases. Three genes encoding azo-reductases, paazoR1, paazoR2 and paazoR3, could be identified in the genome of the isolated P. aeruginosa stain (B1). Bioinformatics analyses of the paazoR1, paazoR2 and paazoR3 genes reveal their prevalence and conservation in other P. aeruginosa strains. Chemical oxygen demand dramatically decreased and phyto-detoxification of the azo dye was accomplished by photocatalytic post treatment of the biodegradation products. We suggest applying combined biological photocatalytic post treatment for azo dyes on large scale, for effective, cheap decolorization and detoxification of azo-dyes, rendering them safe enough to be discharged in the environment.

Spatial variation of electrode position in bioelectrochemical treatment system: Design consideration for azo dye remediation

In the present study, three bio-electrochemical treatment systems (BET) were designed with variations in cathode electrode placement [air exposed (BET1), partially submerged (BET2) and fully submerged (BET3)] to evaluate azo-dye based wastewater treatment at three dye loading concentrations (50, 250 and 500 mg L^-1). Highest dye decolorization (94.5 ± 0.4%) and COD removal (62.2 ± 0.8%) efficiencies were observed in BET3 (fully submerged electrodes) followed by BET1 and BET2, while bioelectrogenic activity was highest in BET1 followed by BET2 and BET3. It was observed that competition among electron acceptors (electrode, dye molecules and intermediates) critically regulated the fate of bio-electrogenesis to be higher in BET1 and dye removal higher in BET3. Maximum half-cell potentials in BET3 depict higher electron acceptance by electrodes utilized for dye degradation. Study infers that spatial positioning of electrodes in BET3 is more suitable towards dye remediation, which can be considered for scaling-up/designing a treatment plant for large-scale industrial applications.

Efficient degradation of Azo dyes by a newly isolated fungus Trichoderma tomentosum under non-sterile conditions

A fast-growing fungus with remarkable ability to degrade several azo dyes under non-sterile conditions was isolated and identified. This fungus was identified as Trichoderma tomentosum. Textile effluent of ten-fold dilution could be decolorized by 94.9% within 72h before optimization. Acid Red 3R model wastewater with a concentration of 85.5mgL^-1 could be decolorized by 99.2% within the same time after optimization. High-level of manganese peroxidase and low-level of lignin peroxidase activities were detected during the process of decolorization from the culture supernatant, indicating the possible involvement of two enzymes in azo dye decolorization. No aromatic amine products were detected from the degradation products of Acid Red 3R by gas chromatography-mass spectrometry (GC/MS) analysis, indicating the possible involvement of a special symmetrical oxidative degradation pathway. Phytotoxicity assay confirmed the lower toxicity toward the test plant seeds of the degradation products when compared to the original dye.

Biodegradation and detoxification of Direct Black G textile dye by a newly isolated thermophilic microflora

The biodegradation and detoxification of azo dye - Direct Black G (DBG) with a newly isolated thermophilic microflora was investigated in the present study. It was found this microflora can decolorize DBG at a wide range of pH from 5 to 10, and grow well under high concentration of dye (600 mg·L^-1) and salinity (50 g·L^-1). Its decolorization ratio could reach 97% with 8 h of incubation at optimal conditions. The induction of laccase, manganese peroxidase, lignin peroxidase and azoreductase suggests their synergetic involvements in the degradation process of DBG. In addition, the phytotoxicity analysis indicated the thermophilic microflora converted toxic dye DBG into low toxicity metabolites. PCR-DGGE analysis revealed that there are nine different bacteria presented in this microflora. Furthermore, a new degradation pathway of DBG degradation by this microflora was proposed based on the intermediates identified by LC-ESI-MS/MS.

Electrochemical decolorization of methyl red by RuO2-IrO2-TiO2 electrode and biodegradation with Pseudomonas stutzeri MN1 and Acinetobacter baumannii MN3: An integrated approach

Textile effluent consists of enormous quantities of toxic dyes, which are being discharged into natural aqueous system and thus contaminate the water quality. Hence it is important to develop an eco-friendly and cost effective technology to treat the dyes contaminated wastewater. In this research, an integrated approach of electrochemical oxidation (EO) and biodegradation process (BP) was studied of methyl red (MR) dye. In EO, RuO₂-IrO₂-TiO₂ is used as anode and titanium mesh electrode as cathode. This was followed by BP of the treated EO effluent. Various parameters viz., pH (5-10), sodium chloride concentrations (NaCl) (1-5 g L^-1) and current density (10-30 mA cm²) were optimized. The results of the EO showed 99.96% of MR decolorization within 10 min at pH of 5, NaCl of 2 g L^-1 and current density of 30 mA cm². The EO treated MR was further treated by BP Pseudomonas stutzeri MN1, Acinetobacter baumannii MN3 and mixed consortia of MN1 and MN3. The out of three treatments, the results of mixed consortium BP showed 90% removal of COD at the end of 24 h. The phytotoxic evaluation using Vigna radiata seeds confirmed the toxicity of untreated MR solution, whereas, 100% germination was observed in treated (biodegraded) MR solution. Overall these results evidenced that MR dye was completely decolorized and mineralized by EO and BP within 10 min and 24 h respectively. Hence, this integrated approach can be used as an effective degradation method to treat dyes in the textile industry.

Simultaneous Decolorization and Biohydrogen Production from Xylose by Klebsiella oxytoca GS-4-08 in the Presence of Azo Dyes with Sulfonate and Carboxyl Groups

Biohydrogen production from the pulp and paper effluent containing rich lignocellulosic material could be achieved by the fermentation process. Xylose, an important hemicellulose hydrolysis product, is used less efficiently as a substrate for biohydrogen production. Moreover, azo dyes are usually added to fabricate anticounterfeiting paper, which further increases the complexity of wastewater. This study reports that xylose could serve as the sole carbon source for a pure culture of Klebsiella oxytoca GS-4-08 to achieve simultaneous decolorization and biohydrogen production. With 2 g liter^-1 of xylose as the substrate, a maximum xylose utilization rate (UR_xyl) and a hydrogen molar yield (HMY) of 93.99% and 0.259 mol of H₂ mol of xylose^-1, respectively, were obtained. Biohydrogen kinetics and electron equivalent (e^- equiv) balance calculations indicated that methyl red (MR) penetrates and intracellularly inhibits both the pentose phosphate pathway and pyruvate fermentation pathway, while methyl orange (MO) acted independently of the glycolysis and biohydrogen pathway. The data demonstrate that biohydrogen pathways in the presence of azo dyes with sulfonate and carboxyl groups were different, but the azo dyes could be completely reduced during the biohydrogen production period in the presence of MO or MR. The feasibility of hydrogen production from industrial pulp and paper effluent by the strain if the xylose is sufficient was also proved and was not affected by toxic substances which usually exist in such wastewater, except for chlorophenol. This study offers a promising energy-recycling strategy for treating pulp and paper wastewaters, especially for those containing azo dyes.IMPORTANCE The pulp and paper industry is a major industry in many developing countries, and the global market of pulp and paper wastewater treatment is expected to increase by 60% between 2012 and 2020. Such wastewater contains large amounts of refractory contaminants, such as lignin, whose reclamation is considered economically crucial and environmentally friendly. Furthermore, azo dyes are usually added in order to fabricate anticounterfeiting paper, which further increases the complexity of the pulp and paper wastewater. This work may offer a better understanding of biohydrogen production from xylose in the presence of azo dyes and provide a promising energy-recycling method for treating pulp and paper wastewater, especially for those containing azo dyes.

Tannery Effluent Treatment by Yeast Species Isolates from Watermelon

The quest for an effective alternative means for effluent treatment is a major concern of the modern-day scientist. Fungi have been attracting a growing interest for the biological treatment of industrial wastewater. In this study, Saccharomycescerevisiae and Torulasporadelbrueckii were isolated from spoiled watermelon and inoculated into different concentrations of effluent. The inoculants were incubated for 21-days to monitor the performance of the isolates by measurement of biochemical oxygen demand (BOD), chemical oxygen demand (COD), nitrates, conductivity, phosphates, sulphates and turbidity. The results showed that Saccharomycescerevisiae had the highest percentage decrease of 98.1%, 83.0%, 60.7%, 60.5%, and 54.2% for turbidity, sulphates, BOD, phosphates and COD, respectively, of the tannery effluent. Torulasporadelbrueckii showed the highest percentage decrease of 92.9%, 90.6%, and 61.9% for sulphates, COD, and phosphates, respectively, while the syndicate showed the highest percentage reduction of 87.4% and 70.2% for nitrate and total dissolve solid (TDS), respectively. The least percentage decrease was displayed by syndicate organisms at 51.2%, 48.1% and 40.3% for BOD, COD and conductivity, respectively. The study revealed that Saccharomycescerevisiae and Torulasporadelbrueckii could be used in the biological treatment of tannery-effluent. Hence, it was concluded that the use of these organisms could contribute to minimizing the adverse environmental risks and health-hazards associated with the disposal of untreated tannery-effluents.

Aerobic and anaerobic microbial degradation of crude (4-methylcyclohexyl)methanol in river sediments

Cyclohexane and some of its derivatives have been a major concern because of their significant adverse human health effects and widespread occurrence in the environment. The 2014 West Virginia chemical spill has raised public attention to (4-methylcyclohexyl)methanol (4-MCHM), one cyclohexane derivative, which is widely used in coal processing but largely ignored. In particular, the environmental fate of its primary components, cis- and trans-4-MCHM, remains largely unexplored. This study aimed to investigate the degradation kinetics and mineralization of cis- and trans-4-MCHM by sediment microorganisms under aerobic and anaerobic conditions. We found the removal of cis- and trans-4-MCHM was mainly attributed to biodegradation with little contribution from sorption. A nearly complete aerobic degradation of 4-MCHM occurred within 14 days, whereas the anaerobic degradation was reluctant with residual percentages of 62.6% of cis-4-MCHM and 85.0% of trans-4-MCHM after 16-day incubation. The cis-4-MCHM was degraded faster than the trans under both aerobic and anaerobic conditions, indicating an isomer-specific degradation could occur during the 4-MCHM degradation. Nitrate addition enhanced 4-MCHM mineralization by about 50% under both aerobic and anaerobic conditions. Both cis- and trans-4-MCHM fit well with the first-order kinetic model with respective degradation rates of 0.46-0.52 and 0.19-0.31 day(-)(1) under aerobic condition. Respective degradation rates of 0.041-0.095 and 0.013-0.052 day(-)(1) occurred under anaerobic condition. One bacterial strain capable of effectively degrading 4-MCHM isomers was isolated from river sediments and identified as Bacillus pumilus at the species level based on 16S rRNA gene sequence and 97% identity. Our findings will provide critical information for improving the prediction of the environmental fate of 4-MCHM and other cyclohexane derivatives with similar structure as well as enhancing the development of feasible treatment technologies to mitigate these compounds.

Novel Strategy for Tracking the Microbial Degradation of Azo Dyes with Different Polarities in Living Cells

Direct visualization evidence is important for understanding the microbial degradation mechanisms. To track the microbial degradation pathways of azo dyes with different polar characterizations, sensors based on the fluorescence resonance energy transfer (FRET) from 1,8-naphthalimide to azo dyes were synthesized, in which the quenched fluorescence will recover when the azo bond was cleaved. In living cells, the sensor-tracking experiment showed that the low polarity and hydrophobic azo dye can be taken up into the cells and reduced inside the cells, whereas the high polarity and hydrophilic azo dye can be reduced only outside the cells because of the selective permeability of the cell membranes. These results indicated that there were two different bacterial degradation pathways available for different polarity azo dyes. To our knowledge, no fluorescent sensor has yet been designed for illuminating the microbial degradation mechanisms of organic pollutants with different characteristics.

Decolorization characteristics of a newly isolated salt-tolerant Bacillus sp. strain and its application for azo dye-containing wastewater in immobilized form

Strain CICC 23870 capable of decolorization of various azo dyes under high saline conditions was isolated from saline-alkali soil. The oxygen-insensitive azoreductase in crude extracts exhibited a wide substrate adaptively in the presence of NADH as a cofactor. The decolorization process by free cells followed first-order kinetics, with a high Methyl Orange (MO) tolerance concentration up to 100 mg l(-1) estimated by Haldane model. The average decolorization rate of free cell system was 26.30 mg g(-1) h(-1) at initial MO concentration of 32.7 mg l(-1). However, the values for the systems of immobilized cells (4 mm) in alginate, alginate and nano-TiO2, and alginate and powered activated carbon (PAC) were 6.83, 4.64, and 11.34 mg g(-1) h(-1), respectively. The effective diffusion factors in the tree different matrices were calculated by diffusion-based mathematic model. The diffusion step controls the overall decolorization rate, and the effective diffusion coefficients varied with internal structure of the bead matrices. The diffusion coefficients were increased from 4.98 × 10(-9) to 2.25 × 10(-8) cm(2) s(-1) when PAC was added, but decreased to 6.62 × 10(-10) cm(2) s(-1) when nano-TiO2 was added. The immobilized matrices could be reused for at least three cycles but with a decreased decolorization rate, possibly due to the breakage of beads at the end of each cycle, which led to the loss of immobilized bacteria.

Non-classical azoreductase secretion in Clostridium perfringens in response to sulfonated azo dye exposure

Clostridium perfringens, a strictly anaerobic microorganism and inhabitant of the human intestine, has been shown to produce an azoreductase enzyme (AzoC), an NADH-dependent flavin oxidoreductase. This enzyme reduces azo dyes into aromatic amines, which can be carcinogenic. A significant amount of work has been completed on the activity of AzoC. Despite this, much is still unknown, including whether azoreduction of these dyes occurs intracellularly or extracellulary. A physiological study of C. perfringens involving the effect of azo dye exposure was completed to answer this question. Through exposure studies, azo dyes were found to cause cytoplasmic protein release, including AzoC, from C. perfringens in dividing and non-dividing cells. Sulfonation (negative charge) of azo dyes proved to be the key to facilitating protein release of AzoC and was found to be azo-dye-concentration-dependent. Additionally, AzoC was found to localize to the Gram-positive periplasmic region. Using a ΔazoC knockout mutant, the presence of additional azoreductases in C. perfringens was suggested. These results support the notion that the azoreduction of these dyes may occur extracellularly for the commensal C. perfringens in the intestine.

Effect of quinoid redox mediators on the aerobic decolorization of azo dyes by cells and cell extracts from Escherichia coli

It is widely accepted that the addition of redox mediators increases the decolorization rates of azo dyes by bacterial strains under anaerobic conditions. However, little information exists about whether quinoid redox mediators can enhance the performance of aerobic azo dye decolorization. In the present study, quinone-mediated decolorization of different azo dyes by whole cells and cell extracts from the Escherichia coli strain CD-2 under aerobic conditions were investigated. The results demonstrated that reduction rates of different azo dyes were greatly increased when quinone compounds were used as redox mediators. Compared with menadione, 2-hydroxy-1,4-naphthoquinone (lawsone) was more effective at aiding azo dye degradation and the optimum concentration for lawsone is 0.1 mM. Strain CD-2 and the anthraquinone were co-immobilized by entrapment in different polymeric matrices. The co-immobilized beads exhibited good catalytic activity for azo dye degradation and kept stable during successive repeated experiments. The mechanism of the quinone-mediated reduction showed that although whole cells incubated with quinones could significantly increase the rate of decolorization of azo dyes, the quinone compounds did not directly promote azoreductase activity. According to the survey, this is the first report to confirm that the addition of quinoid redox mediators to bacteria increased decolorization under aerobic conditions.

Bio-electro degradation of azo-dye in a combined anaerobic–aerobic process along with energy recovery

Complete removal of reactive orange 16 in a microbial fuel cell coupled aerobic post-treatment process along with energy recovery.

Bacterial chromate reductase, a potential enzyme for bioremediation of hexavalent chromium: a review

Hexavalent chromium is mobile, highly toxic and considered as a priority environmental pollutant. Chromate reductases, found in chromium resistant bacteria are known to catalyse the reduction of Cr(VI) to Cr(III) and have recently received particular attention for their potential use in bioremediation process. Different chromate reductases such as ChrR, YieF, NemA and LpDH, have been identified from bacterial sources which are located either in soluble fractions (cytoplasm) or bound to the membrane of the bacterial cell. The reducing conditions under which these enzymes are functional can either be aerobic or anaerobic or sometimes both. Enzymatic reduction of Cr(VI) to Cr(III) involves transfer of electrons from electron donors like NAD(P)H to Cr(VI) and simultaneous generation of reactive oxygen species (ROS). Based on the steps involved in electron transfer to Cr(VI) and the subsequent amount of ROS generated, two reaction mechanisms, namely, Class I "tight" and Class II "semi tight" have been proposed. The present review discusses on the types of chromate reductases found in different bacteria, their mode of action and potential applications in bioremediation of hexavalent chromium both under free and immobilize conditions. Besides, techniques used in characterization of the Cr (VI) reduced products were also discussed.

Intracellular azo decolorization is coupled with aerobic respiration by a Klebsiella oxytoca strain

Reduction of azo dye methyl red coupled with aerobic respiration by growing cultures of Klebsiella oxytoca GS-4-08 was investigated. In liquid media containing dye and 0.6 % glucose in a mineral salts base, 100 mg l(-1) of the dye are completely removed in 3 h under shaking conditions. The dye cannot be aerobically decolorized by strain GS-4-08 without extra carbon sources, indicating a co-metabolism process. Higher initial dye concentration prolonged the lag phase of the cell growth, but final cell concentrations of each batches reached a same level with range from 6.3 to 7.6 mg l(-1) after the dye adaption period. This strain showed stronger dye tolerance and decolorization ability than many reported strains. Furthermore, a new intracellular oxygen-insensitive azoreductase was isolated from this strain, and the specific activity of enzyme was 0.846 and 0.633 U mg(-1) protein in the presence of NADH and NADPH, respectively. N,N dimethyl-p-phenylenediamine and anthranilic acid were stoichiometrically released from MR dye, indicating the breakage of azo bonds accounts for the intracellular decolorization. Combining the characteristics of azoreductase, the stoichiometry of EMP, and TCA cycle, the electron transfer chain theory of aerobic respiration, and the possible mechanism of aerobic respiration coupled with azo reduction by K. oxytoca GS-4-08 are proposed. This study is expected to provide a sound theoretical basis for the development of the K. oxytoca strain in aerobic process for azo dye containing wastewaters.

Properties of NAD (P) H azoreductase from alkaliphilic red bacteria Aquiflexum sp. DL6

Azoreductase plays a key role in bioremediation and biotransformation of azo dyes. It initializes the reduction of azo bond in azo dye metabolism under aerobic or anaerobic conditions. In the present study, we isolated an alkaliphilic red-colored Aquiflexum sp. DL6 bacterial strain and identified by 16S rRNA method. We report nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate-dependent azoreductase purified from Aquiflexum sp. DL6 by a combination of ammonium sulfate precipitation and chromatography methods. The azoreductase was purified up to 30-fold with 37 % recovery. The molecular weight was found to be 80 kDa. The optimum activity was observed at pH 7.4 and temperature 60 °C with amaranth azo dye as a substrate. The thermal stability of azoreductase was up to 80 °C. The azoreductase has shown a wide range of substrate specificity, including azo dyes and nitro aromatic compounds. Metal ions have no significant inhibitory action on azoreductase activity. The apparent K m and V max values for amaranth azo dye were 1.11 mM and 30.77 U/mg protein respectively. This NAD (P) H azoreductase represents the first azoreductase to be characterized from alkaliphilic bacteria.

Improving kinetic or thermodynamic stability of an azoreductase by directed evolution

Protein stability arises from a combination of factors which are often difficult to rationalise. Therefore its improvement is better addressed through directed evolution than by rational design approaches. In this study, five rounds of mutagenesis/recombination followed by high-throughput screening (≈10,000 clones) yielded the hit 1B6 showing a 300-fold higher half life at 50°C than that exhibited by the homodimeric wild type PpAzoR azoreductase from Pseudomonas putida MET94. The characterization using fluorescence, calorimetry and light scattering shows that 1B6 has a folded state slightly less stable than the wild type (with lower melting and optimal temperatures) but in contrast is more resistant to irreversible denaturation. The superior kinetic stability of 1B6 variant was therefore related to an increased resistance of the unfolded monomers to aggregation through the introduction of mutations that disturbed hydrophobic patches and increased the surface net charge of the protein. Variants 2A1 and 2A1-Y179H with increased thermodynamic stability (10 to 20°C higher melting temperature than wild type) were also examined showing the distinctive nature of mutations that lead to improved structural robustness: these occur in residues that are mostly involved in strengthening the solvent-exposed loops or the inter-dimer interactions of the folded state.

Feasibility study on anaerobic biodegradation of azo dye reactive orange 16

Higher colour removal (>90%) shows feasibility of azo dye degradation by anaerobic digestion. Hydrogenotrophic methanogens are the key methane producers. Long retention time is useful for degrading aromatic amines under anaerobic condition.