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Application of Extremophiles in the Area of Bioenergy
Published in Pratibha Dheeran, Sachin Kumar, Extremophiles, 2022
Thermophilic clostridia are capable of ethanol fermentation and can grow at temperature range between 60°C and 65°C (Lamed and Zeikus 1980). Additionally, this strain is able to hydrolyse lignocellulosic substrate by secreting different types of cellulases and hemicellulases such as endo-β-glucanases, exoglucanases, and β-glucosidases (Demain 2005). It is reported that Clostridium thermocellum is capable of converting cellulose to cellobiose and cellodextrins and hemicellulose to xylose, xylobiose and other pentose sugars and simultaneously to ferment them into ethanol, acetate, lactate, H2 and CO2 as well (Demain 2005). However, C. thermocellum is not absolutely efficient to ferment hemicelluloses derived pentose sugars to ethanol (Taylor 2005). Mixed culture therefore can solve the problem of fermenting all the sugars derived from lignocellulosic feedstock. Several themophillic bacteria such as Thermoanaerobacterium thermosaccharolyticum (Mori 1990), Thermoanaerobacter thermohydrosulphuricum (Mori 1990), Thermoanaerobacter ethanolicus (Demain 2005), and Geobacillus stearothermophilus (Sharma 1991) are available which can be cultured with C. thermocellum to ferment pentose sugars for bioethanol production. Thermoanaerobacterium has also been reported as hemicellulolytic thermophilic anaerobe which can convert xylose to ethanol in an efficient manner. It is capable of utilizing pentose sugars such as xylose to produce ethanol (Lacis and Lawford 1991). Thermoanaerobacter ethanolicus isoriginated from Clostridium species which can ferment both D-glucose and D-xylose to ethanol, and it has low ethanol tolerance capacity (Burdette et al. 2002, Fong et al. 2006).
Metabolic Engineering of Yeast, Zymomonas mobilis, and Clostridium thermocellum to Increase Yield of Bioethanol
Published in Ayerim Y. Hernández Almanza, Nagamani Balagurusamy, Héctor Ruiz Leza, Cristóbal N. Aguilar, Bioethanol, 2023
S. Sánchez-Muñoz, M. J. Castro-Alonso, F. G. Barbosa, E. Mier-Alba, T. R. Balbino, D. Rubio-Ribeaux, I. O. Hernández-De Lira, J. C. Santos, C. N. Aguilar, S. S. Da Silva
Among genetic tools, gene overexpression has provided substantial information about the complexity of the clostridial metabolism and the possible ways for increasing bioethanol production [212]. Similarly, heterologous expression of bioethanol production pathways has been performed in native strains as a potential solution to the commercialization of this biofuel [213, 214]. In addition, the carbon flux and metabolic pathways affected in C. thermocellum by the inactivation or complete removal of specific genes have been another alternative selected by researchers [215]. Since bioethanol production demands the reduction of metabolic power for increasing its yield, this has been a key point to achieve large-scale processes. Thus, to enhance the NADPH pool, the most frequently engineered metabolic route is PPP [216, 217]. In this sense, due to the abundance of pentose sugars in LCB, and the lack of xylose consumption by C. thermocellum, studies have been developed to enhance the assimilation of this economic substrate [218, 219]. For instance, Verbeke et al. [218] reported that the deletion of the ATP-dependent transporter (CbpD) in C. thermocellum partially alleviated xylose inhibition. Moreover, the authors observed a decrease in the total and molar yields (mol xylitol: mol cellobiose consumed) of ~41% and ~46% respectively, when deleted a putative XD, encoded by Clo1313_0076. Banerjee et al. [122] also analyzed the heterologous expression in C. thermocellum of the genes xylA (xylose isomerase) and xylB (xylulokinase) from a thermophilic anaerobic bacterium Thermoanaerobacter ethanolicus. The results from this study suggested that the combined activity of these enzymes converted xylose to xylulose-5-phosphate, which was then converted to ethanol through the incorporation to PPP, glycolysis, and malate shunt pathway. Searching for new metabolic reactions through enzyme engineering is one of the expanding areas to improve actual scenario in the industrial sector [198].
An overview of simultaneous saccharification and fermentation of starchy and lignocellulosic biomass for bio-ethanol production
Published in Biofuels, 2019
To improve the existing production of bio-ethanol by the SHF process, a concept for simultaneous saccharification and fermentation was developed. Initially the concept of the process was configured with enzymatic hydrolysis of cellulose and simultaneous fermentation. SSF was first introduced by Gulf Oil Company, USA, and the University of Arkansas [64,65]. The sequence of the process for SSF is virtually the same as the SHF process except that the saccharification and fermentation steps are combined in one vessel. The disadvantages associated with such process are the presence of yeast or bacteria along with enzymes which minimizes the sugar accumulation in the vessel and utilization of enzymes makes the process expensive. The presence of ethanol in the process makes the mixture less susceptible to contamination by unwanted microorganisms. After liquefaction by α-amylase, enzyme glucoamylase is added to the slurry and consequently yeasts are also added to the slury. In the microbe based advanced SSF process, both saccharification and fermentation are achieved simultaneously in a single vessel at optimized enzyme activity with least accumulation of sugars [66,67]. To make the process less time consuming, two organisms with a synergistic relationship are co-cultured together in the same vessel. The process has been diagrammatically represented in Figure 7. Reports on bio-ethanol production suggest that SSF is superior in terms of ethanol yield and productivity than bio-ethanol produced by SHF [43]. SSCF is an alternate process to SSF which facilitates pentose fermentation. Microorganisms used for fermentation in SSCF should have similar operating pH and temperature. Successful co-culturing of C. shehatae and S.cerevisiae in the SSCF process was successfully reported [37]. Consolidated bioprocessing (CBP) is a similar approach which facilitates direct microbial conversion (DMC) and it integrates maximum biotransformation of biomass into ethanol in a single reactor by a single microorganism community [68]. It shows how one microbial community can be utilized to give maximum production of cellulases and fermentation, i.e. cellulase production, cellulose hydrolysis, and fermentation are carried out in one step. In CBP, thermophilic cellulolytic anaerobic bacteria like Thermoanaerobacter ethanolicus, Clostridium thermohydrosulfuricum, Thermoanaerobacter mathranii, Thermoanaerobium brockii, and Clostridium thermosaccharolyticum strains have been explored for bio-ethanol production. These anaerobic bacteria are superior than conventionally used yeasts for bio-ethanol production for their ability to direct conversion of various cheaper biomass feedstocks for bio-ethanol production at extremely high temperature. Different substrates like sludge-containing mash, starch of rice flour, overnight soaked sweet potato, broken rice, and corn cobs are successfully implemented for microbial SSF process [69–73] .