Cellulosic ethanol production by consortia of Scheffersomyces stipitis and engineered Zymomonas mobilis

Abstract

Background As one of the clean and sustainable energies, lignocellulosic ethanol has achieved much attention around the world. The production of lignocellulosic ethanol does not compete with people for food, while the consumption of ethanol could contribute to the carbon dioxide emission reduction. However, the simultaneous transformation of glucose and xylose to ethanol is one of the key technologies for attaining cost-efficient lignocellulosic ethanol production at an industrial scale. Genetic modification of strains and constructing consortia were two approaches to resolve this issue. Compared with strain improvement, the synergistic interaction of consortia in metabolic pathways should be more useful than using each one separately. Results In this study, the consortia consisting of suspended Scheffersomyces stipitis CICC1960 and Zymomonas mobilis 8b were cultivated to successfully depress carbon catabolite repression (CCR) in artificially simulated 80G40XRM. With this strategy, a 5.52% more xylose consumption and a 6.52% higher ethanol titer were achieved by the consortium, in which the inoculation ratio between S. stipitis and Z. mobilis was 1:3, compared with the Z. mobilis 8b mono-fermentation. Subsequently, one copy of the xylose metabolic genes was inserted into the Z. mobilis 8b genome to construct Z. mobilis FR2, leading to the xylose final-consumption amount and ethanol titer improvement by 15.36% and 6.81%, respectively. Finally, various corn stover hydrolysates with different sugar concentrations (glucose and xylose 60, 90, 120 g/L), were used to evaluate the fermentation performance of the consortium consisting of S. stipitis CICC1960 and Z. mobilis FR2. Fermentation results showed that a 1.56–4.59% higher ethanol titer was achieved by the consortium compared with the Z. mobilis FR2 mono-fermentation, and a 46.12–102.14% higher ethanol titer was observed in the consortium fermentation when compared with the S. stipitis CICC1960 mono-fermentation. Furthermore, qRT-PCR analysis of xylose/glucose transporter and other genes responsible for CCR explained the reason why the initial ratio inoculation of 1:3 in artificially simulated 80G40XRM had the best fermentation performance in the consortium. Conclusions The fermentation strategy used in this study, i.e., using a genetically modified consortium, had a superior performance in ethanol production, as compared with the S. stipitis CICC1960 mono-fermentation and the Z. mobilis FR2 mono-fermentation alone. This result showed that this strategy has potential for future lignocellulosic ethanol production.

Development and characterization of efficient xylose utilization strains of Zymomonas mobilis

“…Despite that the ethanol yield of 8b-S38 in 100 g/L xylose media was lower than that of AD50 strain (Additional file 2 : Table S2), its yield in mixed-sugar media was comparable with AD50 (96%) [ 30 ] in mixed sugars, with 97.58% in G2X10 and 92.85% in G2X15 media (Additional file 2 : Table S3). Strain FR2 was another newly engineered strain for efficient xylose fermentation with another copy of xylAB and talB - tktA inserted in the genome of parental strain 8b and its ethanol yield in the mixed-sugar media was 95.5% [ 41 ]. Interestingly, another engineered strain FR1 was also constructed in that work with only talB - tktA inserted in 8b, but showed no difference to the parental strain.…”

Synergy of Cellulase Systems between Acetivibrio thermocellus and Thermoclostridium stercorarium in Consolidated-Bioprocessing for Cellulosic Ethanol

“…For example, a co-culture of cellulolytic At and ethanolgenic Clostridium thermolacticum achieved enhanced ethanol yield, which is 80% of the theoretical maximum during cellulose fermentation [ 51 ]. Another study showed that a co-culture of A. thermocellum and Thermoanaerobacter pseudethanolicus is effective for improved ethanol production during batch fermentation [ 52 , 53 ]. Here, we proposed a CBP strategy by employing a co-culture of At and Ts ( Figure 4 ), in which lignocellulose biomass can be synergetically hydrolyzed by both cellulase systems of At and Ts, and cellobiose and cellodextrins are produced from cellulose and can be utilized directly by both At and Ts while hemicellulose-derived pentoses can be only utilized by Ts.…”

Research Progress on Lignocellulosic Biomass Degradation Catalyzed by Enzymatic Nanomaterials

“…Among them, enzyme‐catalyzed degradation has been widely used because of its low cost and environmental friendliness, which can overcome the shortcomings of other degradation methods (e. g., chemical and physical methods), including corrosiveness to the reactor, harsh reaction conditions, formation of by‐products, and so on [16] . Even lignocellulose can be hydrolyzed directly by microorganisms, but all involve enzyme systems [17] . Moreover, the premise of efficient use of enzymes in LCB degradation is to solve the challenges like high cost of enzyme degradation, and long reaction cycle.…”

“…[16] Even lignocellulose can be hydrolyzed directly by microorganisms, but all involve enzyme systems. [17] Moreover, the premise of efficient use of enzymes in LCB degradation is to solve the challenges like high cost of enzyme degradation, and long reaction cycle.…”