Characterization of a xylitol dehydrogenase from Aspergillus flavus and its application in l-xylulose production.

An NAD+-dependent xylitol dehydrogenase from A. flavus (AfXDH) was cloned and successfully expressed in Escherichia coli. AfXDH gene sequence revealed an open reading frame of 1,110 bp, encoding a polypeptide of 369 amino acids with a calculated molecular mass of 38,893 Da. Among various polyols, sorbitol and xylitol were preferred substrates of AfXDH with Km values of 16.2 and 16.9 mM, respectively. AfXDH showed the highest activity in Tris-glycine-NaOH buffer (pH 9.5) at 50°C; it required Zn2+ or Mn2+ for enzyme activity. The half-life at 40°C and half denaturation temperature (T1/2) was 200 min and 45°C, respectively. Bioinformatic analyses along with biochemical properties confirmed that AfXDH belonged to the medium-chain dehydrogenase/reductase family. AfXDH exhibits higher thermostability and k cat values than those of other XDHs. The feasibility of using AfXDH in l-xylulose production was demonstrated. AfXDH, when coupled with Streptococcus pyogenes NADH oxidase, efficiently converted xylitol to l-xylulose with 97% yield, suggesting its usefulness for the industrial l-xylulose production from xylitol.

Mild Alkaline Pretreatment of Rice Straw as a Feedstock in Microbial Fuel Cells for Generation of Bioelectricity.

The dependency on non-renewable fossil fuels as an energy source has drastically increased global temperatures. Their continuous use poses a great threat to the existing energy reserves. Therefore, the energy sector has taken a turn toward developing eco-friendly, sustainable energy generation by using sustainable lignocellulosic wastes, such as rice straw (RS). For lignocellulosic waste to be utilized as an efficient energy source, it needs to be broken down into less complex forms by pretreatment processes, such as alkaline pretreatment using NaOH. Varied NaOH concentrations (0.5%,1.0%,1.5%,2%) for alkaline pretreatment of RS were used for the holocellulose generation. Amongst the four NaOH concentrations tested, RS-1.5% exhibited higher holocellulose generation of 80.1%, whereas 0.5%, 1 5 and 2% pointed 71.9%, 73.8%, and 78.5% holocellulose generation, respectively. Further, microbial fuel cells (MFCs) were tested for voltage generation by utilizing holocellulose generated from untreated (RS-0%) and mildly alkaline pretreated RS (RS-1.5%) as a feedstock. The MFC voltage and maximum power generation using RS-0% were 194 mV and 167 mW/m2, respectively. With RS-1.5%, the voltage and maximum power generation were 556 mV and 583 mW/m2, respectively. The power density of RS-1.5% was three-fold higher than that of RS-0%. The increase in MFC power generation suggests that alkaline pretreatment plays a crucial role in enhancing the overall performance.

Application of bioelectrochemical systems to regulate and accelerate the anaerobic digestion processes.

Anaerobic digestion (AD) serves as a potential bioconversion process to treat various organic wastes/wastewaters, including sewage sludge, and generate renewable green energy. Despite its efficiency, AD has several limitations that need to be overcome to achieve maximum energy recovery from organic materials while regulating inhibitory substances. Hence, bioelectrochemical systems (BESs) have been widely investigated to treat inhibitory compounds including ammonia in AD processes and improve the AD operational efficiency, stability, and economic viability with various integrations. The BES operations as a pretreatment process, inside AD or after the AD process aids in the upgradation of biogas (CO2 to methane) and residual volatile fatty acids (VFAs) to valuable chemicals and fuels (alcohols) and even directly to electricity generation. This review presents a comprehensive summary of BES technologies and operations for overcoming the limitations of AD in lab-scale applications and suggests upscaling and future opportunities for BES-AD systems.

Biotin and Zn2+ Increase Xylitol Production by Candida tropicalis.

In this study, the medium requirements to increase the production of xylitol by using Candida tropicalis (CT) have been investigated. The technique of single addition or omission of medium components was applied to determine the nutritional requirements. The addition of amino acids such as Asp, Glu, Gln, Asn, Thr, and Gly had no significant effect on CT growth. However, in the absence of other metal ions, there was a higher concentration of cell growth and xylitol production when only Zn2+ was present in the medium. The analysis of various vitamins unveiled that biotin and thiamine were the only vitamins required for the growth of CT. Surprisingly, when only biotin was present in the medium as a vitamin, there was less growth of CT than when the medium was complete, but the amount of xylitol released was significantly higher. Overall, this study will increase the xylitol production using the single omission or addtion technique.

Correction to: Biomethanol Production from Methane by Immobilized Co-cultures of Methanotrophs.

[This corrects the article DOI: 10.1007/s12088-020-00883-6.].

Site-directed lysine modification of xylanase for oriented immobilization onto silicon dioxide nanoparticles.

Enhanced covalent immobilization of xylanase from Chaetomium globosum (XylCg) onto SiO2 nanoparticles was achieved by the modification of surface residues. The mutation of surface residues to lysine by site-directed mutagenesis increased the immobilization efficiency (IE) and immobilization yield (IY). The immobilized mutant XylCg (N172K-H173K-S176K-K133A-K148A) exhibited an IY of 99.5% and IE of 135%, which were 1.8- and 4.3-fold higher than immobilized wildtype (WT). Regarding the catalytic properties, the kcat and kcat/Km values were 1850 s-1 and 2030 mL mg-1 s-1 for the immobilized mutant, and 331 s-1 and 404 mL mg-1 s-1 for the immobilized WT, respectively. Additionally, the immobilized mutant exhibited four times higher thermal stability than the immobilized WT at 60 °C. These results suggest that surface-mutated lysine residues confer good stability and orientation on the support matrix, thus improving the overall performance of xylanase.

Biomethanol Production from Methane by Immobilized Co-cultures of Methanotrophs.

Methanol production by co-culture of methanotrophs Methylocystis bryophila and Methyloferula stellata was examined from methane, a greenhouse gas. Co-culture exhibited higher methanol yield of 4.72 mM at optimum ratio of M. bryophila and M. stellata (3:2) compared to individual cultures. The immobilized co-culture within polyvinyl alcohol (PVA) showed relative efficiency of 90.1% for methanol production at polymer concentration of 10% (v/v). The immobilized co-culture cells within PVA resulted in higher bioprocess stability over free cells at different pH, and temperatures. Free and encapsulated co-cultures showed maximum methanol production of 4.81 and 5.37 mM under optimum conditions, respectively. After five cycles of reusage under batch conditions, free and encapsulated co-cultures retained methanol production efficiency of 23.8 and 61.9%, respectively. The present investigation successfully revealed the useful influence of co-culture on the methanol production over pure culture. Further, encapsulation within the polymeric matrix proved to be a better approach for the enhanced stability of the bioprocess.

Conversion of biogas to methanol by methanotrophs immobilized on chemically modified chitosan.

In this study, chitosan modified with glutaraldehyde (GLA), 3-aminopropyltriethoxysilane (APTES), polyethyleneimine, and APTES followed by GLA (APTES-GLA) as a support material was used to improve methanol production from biogas. Among these support materials, chitosan-APTES-GLA showed the highest increase in immobilization yield and relative efficiency of Methylomicrobium album up to 56.4% and 97.7%, respectively. Maximum cell loading of 236 mg dry cell mass per g-support was observed for M. album., which is 7.7-fold higher than that of chitosan. The immobilized M. album maintained a 23.9-fold higher methanol production compared to free cells after 8 cycles of reuse; it also produced 6.92 mmol·L-1 methanol from biogas that originated from anaerobic digestion of rice straw, thereby validating its industrial application. This is the first report on the immobilization of methanotrophs on chemically modified chitosans to improve cell loading and relative efficiency, and its potential applications in the conversion of greenhouse gases to methanol.

Characterization of Cellobiohydrolases from Schizophyllum commune KMJ820.

A novel cellobiohydrolase (CBH)-generating fungi have been isolated and categorized as Schizophyllum commune KMJ820 based on morphology and rDNA gene sequence. Cellulose powder was used as carbon source, the total enzyme activity was 11.51 U/ml is noted; which is among the highest amounts of CBH-generating microbes studied. CBH have been purified to homogenize, with pursual of serial chromatography using S. commune supernatants and two different CBHs were found; CBH 1 and 2. The filtered CBHs showed greater activity (V max = 51.4 and 20.8 U/mg) in contrast to CBHs from earlier studies. The MW (molecular weights) of S. commune CBH 1 and 2 were verified to be approximately 50 kDa and 150 kDa, respectively, by size exclusion chromatography. Even though CBHs have been evaluated from other sources, but S. commune CBH is prominent in comparison to other CBHs by its high enzyme activity.

Bioelectrochemical Detoxification of Phenolic Compounds during Enzymatic Pre-Treatment of Rice Straw.

The use of lignocellulosic biomass such as rice straw can help subsidize the cost of producing value-added chemicals. However, inhibitory compounds, such as phenolics, produced during the pre-treatment of biomass, hamper the saccharification process. Laccase and electrochemical stimuli are both well known to reduce phenolic compounds. Therefore, in this study, we implemented a bioelectrochemical detoxification system (BEDS), a consolidated electrochemical and enzymatic process involving laccase, to enhance the detoxification of phenolics, and thus achieve a higher saccharification efficiency. Saccharification of pretreated rice straw using BEDS at 1.5 V showed 90% phenolic reduction (Phr), thereby resulting in a maximum saccharification yield of 85%. In addition, the specific power consumption when using BEDS (2.2 W/Kg Phr) was noted to be 24% lower than by the electrochemical process alone (2.89 W/kg Phr). To the best of our knowledge, this is the first study to implement BEDS for reduction of phenolic compounds in pretreated biomass.

Exploitation of Citrus Peel Extract as a Feedstock for Power Generation in Microbial Fuel Cell (MFC).

Microbial fuel cells (MFCs) are envisioned as an evolving cost-effective process for treating organic wastes to simultaneously generate bioelectricity. Therefore, in present study a single chambered mediator- less air cathode MFC was operated for bioelectricity generation using citrus waste (CW) as a feedstock. The MFC was operated at four organic loading conditions (OLs; 3, 6, 9 and 12 kg/m3). The voltage generation and organic content reduction demonstrated the possibility of utilizing CW as a substrate in MFC. The polarization analysis revealed a high-power generation of 71.1 mW/m2 with low OL of 3 kg/m3. The decrease in pH and high volatile fatty acids (VFAs) generation was noted at high OL. Our current findings suggest better performance of MFC, in terms of energy generation and organic reduction at high OL.

Continuous autotrophic denitrification process for treating ammonium-rich leachate wastewater in bioelectrochemical denitrification system (BEDS).

The reduction of nitrogen compounds in aqueous solution is extremely important for sustainable management of ecosystem and human health. An autotrophic bioelectrochemical denitrification (BED) process was evaluated at various conditions for enhanced treatment of synthetic wastewater (SW) and ammonium-rich leachate. With SW, a decrease in hydraulic retention time (HRT: 41.6 to 8.3 h) resulted in a 370% increase in denitrification rate from 0.026 to 0.096 kg NO3-N/m3. D. An increase in applied voltage (0.7 to 2 V) enhanced nitrate removal (81 to 97% removal), but coulombic efficiency decreased from 74% to 19%. With doubled cathode electrodes, the nitrate removal rate was doubled from 0.056 to 0.114 kg NO3-N/m3. D. Moreover, leachate wastewater was successfully denitrified with the maximum removal rate of 0.121 kg NO3-N/m3. D. These results point towards the practical potential for the combination of nitrification systems with BEDS for reduction of nitrogen for discharge purposes.

Utilization of residual organics of Labaneh whey for renewable energy generation through bioelectrochemical processes: Strategies for enhanced substrate conversion and energy generation.

Labaneh whey (LW) that is rich in residual organics was evaluated for bioelectricity generation using microbial fuel cell (MFC) in two different configurations namely single chamber (MFC-SC) and dual chamber (MFC-DC) MFCs. The whole study was executed in three stages: The first stage evidenced promising amount of bioelectricity generation (DC, 643 mV; SC, 545 mV) along with chemical oxygen demand removal (CODr: DC, 60.63%; SC, CODr: 55.25%). In the second phase, activity of anodic electrogenic microbes was improved with short time poising at potentials of 400, 600 and 800 mV, among which 800 mV evidenced 2.24 (DC) and 1.60 (SC) fold enhancement in power generation along with significant improvement in substrate degradation. The third phase was solely focused on bioelectrochemical treatment of LW through applied potentials for extended period. This phase achieved 89 and 94% chemical oxygen demand (COD) degradation using SC and DC configurations, respectively at 800 mV.

Bioelectrogenesis from Raw Algal Biomass Through Microbial Fuel Cells: Effect of Acetate as Co-substrate.

Algae are autotrophic organisms that are widespread in water bodies. Increased pollution in water bodies leads to eutrophication. However, algae growing in lakes undergoing eutrophication could be utilized towards the generation of added-value bio-electricity using microbial fuel cells (MFCs). In the present study, two methods of electricity generation using raw algae (RA) and RA + acetate (AC) as co-substrate were analyzed in single chamber air cathode MFCs. MFCs supplemented with RA and RA + AC clearly showed higher power density, greater current generation, and improved COD (chemical oxygen demand) removal, which demonstrated the feasibility of using AC as substrate for MFC. The MFC-RA + AC (0.48 mA) generated 28% higher current relative to that generated by MFC with RA alone. Notably, the maximum power densities generated by MFC-RA and MFC-RA + AC were 230 and 410 mW/m2, respectively. MFC-RA and MFC-RA + AC exhibited TCOD (total chemical oxygen reduction) removal values of 77% and 86.6%, respectively. Despite the high influent TCOD (758 mg/l) concentration, the MFC-RA + AC exhibited an 8.5% higher COD removal relative to that of MFC-RA (525 mg/l). Our current findings demonstrated effective energy generation using algae biomass with a co-substrate.

Methane as a Substrate for Energy Generation Using Microbial Fuel Cells.

Methane (CH4) is a well-known and abundant feedstock for natural gas, and is readily available from various sources. In thermal plants, the CH4 generated from anthropogenic sources is converted into electrical energy via combustion. Microbial fuel cell (MFC) technology has proven to be an efficient strategy for the biological conversion of a many substrates, including biogas (CH4), to electricity. MFC technology uses gaseous substrate along with an enriched and selective microbial consortium. Predominantly, methanotrophs and electrochemically active Geobacter were utilized in a syntrophic association on the anode of an MFC. This review focuses on the exploitation of CH4 as a substrate for bioelectrogenesis via MFCs.

Enhanced treatment of petroleum refinery wastewater by short-term applied voltage in single chamber microbial fuel cell.

Electrochemically active anodic biofilm that has adapted under mild applied potentials in the range 100-500 mV was evaluated for its improved bioelectrogenesis and bioelectrochemical treatment of petroleum refinery wastewater (PRW) in a single chamber air cathode microbial fuel cell (MFC). MFC operation with 500 mV as supplemental voltage has exhibited a maximum power density of 132 mW/m2, which was three times higher than control MFC (45 mW/m2). Similarly, highest substrate removal efficiency (48%) was also obtained with the MFC of 500 mV, followed by 300 mV (37%), 100 mV (32%) and control (27%). Adaptation under applied potential conditions also exhibited enhanced degradation efficiency of diesel range organics (DROs)/straight chain-alkanes. The strategy efficiently reduced DROs with the maximum efficiency of 89% (500 mV), which is almost 50% higher than that of the control system (59%), demonstrating the effectiveness of using supplemented voltage in treating PRW.

The performance and long-term stability of low-cost separators in single-chamber bottle-type microbial fuel cells.

This study evaluates long-term stability of low-cost separators in single-chamber bottle-type microbial fuel cells with domestic wastewater. Low-cost separators tested in this study were nonwoven fabrics (NWF) of polypropylene (PP80, PP100), textile fabrics of polyphenylene sulfide (PPS), sulfonated polyphenylene sulfide (SPPS), and cellulose esters. NWF PP80 separator generated the highest power density of 280 mW/m2, which was higher than with ion-exchange membranes (cation exchange membrane; CEM = 271 mW/m2, cation exchange membrane; CMI = 196 mW/m2, Nafion = 260 mW/m2). MFC operations with other size-selective separators such as SPPS, PPS, and cellulose esters exhibited power densities of 261, 231, and 250 mW/m2, respectively. During a 280-day operation, initial power density of PP80 (278 mW/m2) was decreased to 257 mW/m2, but this decrease was smaller than with others (Nafion: 265-230 mW/m2; PP100: 220-126 mW/m2). The anode potential of around -430 mV did not change much with all separators in the long-term operation, but the initial cathode potential gradually decreased. Fouling analysis suggested that the presence of carbonaceous substance on Nafion and PP80 after 280 days of operation and Nafion was subject to be more biofouling.

Bioelectrochemical methane (CH4) production in anaerobic digestion at different supplemental voltages.

Microbial electrolysis cells (MECs) at various cell voltages (0.5, 0.7 1.0 and 1.5V) were operated in anaerobic fermentation. During the start-up period, the cathode potential decreased from -0.63 to -1.01V, and CH4 generation increased from 168 to 199ml. At an applied voltage of 1.0V, the highest methane yields of 408.3ml CH4/g COD glucose was obtained, which was 30.3% higher than in the control tests (313.4ml CH4/g COD glucose). The average current of 5.1mA was generated at 1.0V at which the maximum methane yield was obtained. The other average currents were 1.42, 3.02, 0.53mA at 0.5, 0.7, and 1.5V, respectively. Cyclic voltammetry and EIS analysis revealed that enhanced reduction currents were present at all cell voltages with biocatalyzed cathode electrodes (no reduction without biofilm), and the highest value was obtained with 1V external voltage.

Optimum spacing between electrodes in an air-cathode single chamber microbial fuel cell with a low-cost polypropylene separator.

The performance of a single chamber microbial fuel cell (MFC) with a low-cost polypropylene separator was investigated at various electrode interspaces in a separator electrode assembly (SEA). The lag period was shortened (3.74-0.17 days) and voltage generation was enhanced (0.2-0.5 V) as electrode spacing was increased from 0 to 9 mm. Power density was increased from 220 to 370 mW/m2 with increased spacing. The highest power density of 488 mW/m2 was obtained in polarization analysis with 6 mm. The oxygen mass transfer coefficients with 0 mm (K o = 3.69 × 10-5 cm/s) electrode spacing were 3.8 times higher than with 9 mm (K o = 0.96 × 10-5 cm/s) spacing. Columbic efficiency (CE) was increased from 5 to 32% due to less oxygen diffusion with increase in electrode spacing, but on contrary the ohmic resistance (R oh) was increased from 2 to 4 Ω. In a long-term operation (200 days), a gradual decrease in cathode potentials was observed in all electrode spacing as the main limiting factor of stable MFC performance.

Bioelectrochemical reduction of volatile fatty acids in anaerobic digestion effluent for the production of biofuels.

This study proves for the first time the feasibility of biofuel production from anaerobic digestion effluent via bioelectrochemical cell operation at various applied cell voltages (1.0, 1.5 and 2.0 V). An increase in cell voltage from 1 to 2 V resulted in more reduction current generation (-0.48 to -0.78 mA) at a lowered cathode potential (-0.45 to -0.84 mV vs Ag/AgCl). Various alcohols were produced depending on applied cell voltages, and the main products were butanol, ethanol, and propanol. Hydrogen and methane production were also observed in the headspace of the cell. A large amount of lactic acid was unexpectedly formed at all conditions, which might be the primary cause of the limited biofuel production. The addition of neutral red (NR) to the system could increase the cathodic reduction current, and thus more biofuels were produced with an enhanced alcohol formation compared to without a mediator.