Regulation of ρ-coumaric acid tolerance in Clostridium beijerinckii by disturbing the intracellular electron transport chain

Application of fed-batch strategy to fully eliminate the negative effect of lignocellulose-derived inhibitors in ABE fermentation

BACKGROUND

Inhibitors that are released from lignocellulose biomass during its treatment represent one of the major bottlenecks hindering its massive utilization in the biotechnological production of chemicals. This study demonstrates that negative effect of inhibitors can be mitigated by proper feeding strategy. Both, crude undetoxified lignocellulose hydrolysate and complex medium supplemented with corresponding inhibitors were tested in acetone-butanol-ethanol (ABE) fermentation using Clostridium beijerinckii NRRL B-598 as the producer strain.

RESULTS

First, it was found that the sensitivity of C. beijerinckii to inhibitors varied with different growth stages, being the most significant during the early acidogenic phase and less pronounced during late acidogenesis and early solventogenesis. Thus, a fed-batch regime with three feeding schemes was tested for toxic hydrolysate (no growth in batch mode was observed). The best results were obtained when the feeding of an otherwise toxic hydrolysate was initiated close to the metabolic switch, resulting in stable and high ABE production. Complete utilization of glucose, and up to 88% of xylose, were obtained. The most abundant inhibitors present in the alkaline wheat straw hydrolysate were ferulic and coumaric acids; both phenolic acids were efficiently detoxified by the intrinsic metabolic activity of clostridia during the early stages of cultivation as well as during the feeding period, thus preventing their accumulation. Finally, the best feeding strategy was verified using a TYA culture medium supplemented with both inhibitors, resulting in 500% increase in butanol titer over control batch cultivation in which inhibitors were added prior to inoculation.

CONCLUSION

Properly timed sequential feeding effectively prevented acid-crash and enabled utilization of otherwise toxic substrate. This study unequivocally demonstrates that an appropriate biotechnological process control strategy can fully eliminate the negative effects of lignocellulose-derived inhibitors.

Biofuneling lignin-derived compounds into lipids using a newly isolated Citricoccus sp. P2

Lignin-derived compounds (LDCs) bioconversion into lipids is a promising yet challenging task. This study focuses on the isolation of the ligninolytic bacterium Citricoccus sp. P2 and investigates its mechanism for producing lipids from LDCs. Although strain P2 exhibits a relatively low lignin degradation rate of 44.63%, it efficiently degrades various concentrations of LDCs. The highest degradation rate is observed when incubated with 0.6 g/L vanillic acid, 0.6 g/L syringic acid, 0.8 g/L p-coumaric acid, and 0.4 g/L phenol, resulting in respective lipid yields of 0.16 g/L, 0.13 g/L, 0.24 g/L, and 0.13 g/L. The genome of strain P2 provides insights into LDCs bioconversion into lipids and stress tolerance. Moreover, Citricoccus sp. P2 has been successfully developed a non-sterilized lipid production using its native alkali-halophilic characteristics, which significantly enhances the lipid yield. This study presents a promising platform for lipids production from LDCs and has potential to promote valorization of lignin.

Current status, challenges and prospects for lignin valorization by using Rhodococcus sp

Lignin represents the most abundant renewable aromatics in nature, which has complicated and heterogeneous structure. The rapid development of biotransformation technology has brought new opportunities to achieve the complete lignin valorization. Especially, Rhodococcus sp. possesses excellent capabilities to metabolize aromatic hydrocarbons degraded from lignin. Furthermore, it can convert these toxic compounds into high value added bioproducts, such as microbial lipids, polyhydroxyalkanoate and carotenoid et al. Accordingly, this review will discuss the potentials of Rhodococcus sp. as a cell factory for lignin biotransformation, including phenol tolerance, lignin depolymerization and lignin-derived aromatic hydrocarbon metabolism. The detailed metabolic mechanism for lignin biotransformation and bioproducts spectrum of Rhodococcus sp. will be comprehensively discussed. The available molecular tools for the conversion of lignin by Rhodococcus sp. will be reviewed, and the possible direction for lignin biotransformation in the future will also be proposed.

Transcriptomic studies of solventogenic clostridia, Clostridium acetobutylicum and Clostridium beijerinckii

Solventogenic clostridia are not a strictly defined group within the genus Clostridium but its representatives share some common features, i.e. they are anaerobic, non-pathogenic, non-toxinogenic and endospore forming bacteria. Their main metabolite is typically 1-butanol but depending on species and culture conditions, they can form other metabolites such as acetone, isopropanol, ethanol, butyric, lactic and acetic acids, and hydrogen. Although these organisms were previously used for the industrial production of solvents, they later fell into disuse, being replaced by more efficient chemical production. A return to a more biological production of solvents therefore requires a thorough understanding of clostridial metabolism. Transcriptome analysis, which reflects the involvement of individual genes in all cellular processes within a population, at any given (sampling) moment, is a valuable tool for gaining a deeper insight into clostridial life. In this review, we describe techniques to study transcription, summarize the evolution of these techniques and compare methods for data processing and visualization of solventogenic clostridia, particularly the species Clostridium acetobutylicum and Clostridium beijerinckii. Individual approaches for evaluating transcriptomic data are compared and their contributions to advancements in the field are assessed. Moreover, utilization of transcriptomic data for reconstruction of computational clostridial metabolic models is considered and particular models are described. Transcriptional changes in glucose transport, central carbon metabolism, the sporulation cycle, butanol and butyrate stress responses, the influence of lignocellulose-derived inhibitors on growth and solvent production, and other respective topics, are addressed and common trends are highlighted.

Triton X-100 supplementation regulates growth and secondary metabolite biosynthesis during in-depth extractive fermentation of Monascus purpureus

Extractive fermentation has been proven to be efficient in enhancing the secretion and production of secondary metabolites in submerged fermentation by Monascus spp., owing to increased cell membrane permeability and resolved product inhibition. In this study, we investigated the regulation effect of Triton X-100 on cell growth and secondary metabolite biosynthesis in submerged fermentation of M. purpureus DK. The results show that the maximum monascus pigments (MPs), citrinin (CIT) production, and specific growth rate are 136.86 U/mL, 4.57 mg/L, and 0.04 h^-1, respectively, when 3 g/L of Triton X-100 is supplemented after fermentation for 10 d, and the extracellular MPs and CIT increased by 127.48% and 288.57%, respectively. RT-qPCR shows that the expression levels of MPs and CIT biosynthesis gene clusters are significantly upregulated, whereas those of glycolysis, tricarboxylic acid cycle, respiratory chains, and ATP synthase are downregulated. This study provides a vital strategy for extractive fermentation under extreme environmental conditions for further enhancing MP production.

Delignification of Cistus ladanifer Biomass by Organosolv and Alkali Processes

Residues of Cistus ladanifer obtained after commercial steam distillation for essential oil production were evaluated to produce cellulose enriched solids and added-value lignin-derived compounds. The delignification of extracted (CLRext) and extracted and hydrothermally pretreated biomass (CLRtreat) was studied using two organosolv processes, ethanol/water mixtures (EO), and alkali-catalyzed glycerol (AGO), and by an alkali (sodium hydroxide) process (ASP) under different reaction conditions. The phenolic composition of soluble lignin was determined by capillary zone electrophoresis and by Py-GC/MS, which was also used to establish the monomeric composition of both the delignified solids and isolated lignin. The enzymatic saccharification of the delignified solids was also evaluated. The ASP (4% NaOH, 2 h) lead to both the highest delignification and enzymatic saccharification (87% and 79%, respectively). A delignification of 76% and enzymatic hydrolysis yields of 72% were obtained for AGO (4% NaOH) while EO processes led to lower delignification (maximum lignin removal 29%). The residual lignin in the delignified solids were enriched in G- and H-units, with S-units being preferentially removed. The main phenolics present in the ASP and AGO liquors were vanillic acid and epicatechin, while gallic acid was the main phenolic in the EO liquors. The results showed that C. ladanifer residues can be a biomass source for the production of lignin-derivatives and glucan-rich solids to be further used in bioconversion processes.

Regulation of secondary metabolite biosynthesis in Monascus purpureus via cofactor metabolic engineering strategies

This study investigated the effects of cofactor metabolism on secondary metabolite production in M. purpureus through the application of different cofactor engineering strategies. Total pigment production dramatically increased by 39.08% and 40.89%, and yellow pigment production increased by 74.62% and 114.06% after the addition of 1.0 mg/L of the exogenous cofactor reagents methyl viologen and rotenone, respectively, in submerged batch-fermentation. The extracellular red pigment tone changed to yellow with the application of electrolytic stimulation at 800 mV/cm², but almost no citrinin production was detected. In addition, the total pigment, yellow pigment and citrinin production increased by 35.46%, 54.89% and 6.27% after disruption of the nuoⅠ gene that encodes NADH-quinone oxidoreductase, respectively. Thus, cofactor metabolic engineering strategies could be extended to the industrial production of Monascus pigment or high yellow pigment with free citrinin production.

Reassessment of the role of CaCO3 in n-butanol production from pretreated lignocellulosic biomass by Clostridium acetobutylicum

In this study, the role of CaCO₃ in n-butanol production was further investigated using corn straw hydrolysate (CSH) media by Clostridium acetobutylicum CICC 8016. CaCO₃ addition stimulated sugars utilization and butanol production. Further study showed that calcium salts addition to CSH media led to the increase in Ca²⁺ concentration both intracellularly and extracellularly. Interestingly, without calcium salts addition, intracellular Ca²⁺ concentration in the synthetic P2 medium was much higher than that in the CSH medium despite the lower extracellular Ca²⁺ concentrations in the P2 medium. These results indicated that without additional calcium salts, Ca²⁺ uptake by C. acetobutylicum CICC 8016 in the CSH medium may be inhibited by non-sugar biomass degradation compounds, such as furans, phenolics and organic acids. Comparative proteomics analysis results showed that most enzymes involved in glycolysis, redox balance and amino acids metabolism were up-regulated with CaCO₃ addition. This study provides further insights into the role of CaCO₃ in n-butanol production using real biomass hydrolysate.

Co-production of solvents and organic acids in butanol fermentation by Clostridium acetobutylicum in the presence of lignin-derived phenolics

Co-production of solvents (butanol, acetone, and ethanol) and organic acids (butyrate and acetate) by Clostridium acetobutylicum using lignocellulosic biomass as a substrate could further enlarge the application scope of butanol fermentation. This is mainly because solvents and organic acids could be used for production of fine chemicals such as butyl butyrate, butyl oleate, etc. However, many phenolic fermentation inhibitors are formed during the pretreatment process because of lignin degradation. The present study investigated the effects of five typical lignin-derived phenolics on the biosynthesis of solvents and organic acids in C. acetobutylicum ATCC 824. Results obtained in 100 mL anaerobic bottles indicated that butanol concentration was enhanced from 10.29 g L⁻¹ to 11.36 g L⁻¹ by the addition of 0.1 g L⁻¹ vanillin. Subsequently, a pH-control strategy was proposed in a 5 L anaerobic fermenter to alleviate the “acid crash” phenomenon and improve butanol fermentation performance, simultaneously. Notably, organic acid concentration was enhanced from 6.38 g L⁻¹ (control) to a high level of 9.21–12.57 g L⁻¹ with vanillin or/and vanillic acid addition (0.2 g L⁻¹) under the pH-control strategy. Furthermore, the butyrate/butanol ratio reached the highest level of 0.80 g g⁻¹ with vanillin/vanillic acid co-addition, and solvent concentration reached 13.85 g L⁻¹, a comparable level to the control (13.69 g L⁻¹). The effectiveness and robustness of the strategy for solvent and organic acid co-production was also verified under five typical phenolic environments. In conclusion, these results suggest that the proposed process strategy would potentially promote butanol fermentative products from renewable biomass.

Lignin-derived phenolics enhance solvent and organic acid biosynthesis in butanol fermentation by Clostridium acetobutylicum ATCC 824.