Effect of various factors on ethanol yields from lignocellulosic biomass by Thermoanaerobacterium AK₁₇

From waste management to circular economy: Leveraging thermophiles for sustainable growth and global resource optimization

Waste of any origin is one of the most serious global and man-made concerns of our day. It causes climate change, environmental degradation, and human health problems. Proper waste management practices, including waste reduction, safe handling, and appropriate treatment, are essential to mitigate these consequences. It is thus essential to implement effective waste management strategies that reduce waste at the source, promote recycling and reuse, and safely dispose of waste. Transitioning to a circular economy with policies involving governments, industries, and individuals is essential for sustainable growth and waste management. The review focuses on diverse kinds of environmental waste sources around the world, such as residential, industrial, commercial, municipal services, electronic wastes, wastewater sewerage, and agricultural wastes, and their challenges in efficiently valorizing them into useful products. It highlights the need for rational waste management, circularity, and sustainable growth, and the potential of a circular economy to address these challenges. The article has explored the role of thermophilic microbes in the bioremediation of waste. Thermophiles known for their thermostability and thermostable enzymes, have emerged to have diverse applications in biotechnology and various industrial processes. Several approaches have been explored to unlock the potential of thermophiles in achieving the objective of establishing a zero-carbon sustainable bio-economy and minimizing waste generation. Various thermophiles have demonstrated substantial potential in addressing different waste challenges. The review findings affirm that thermophilic microbes have emerged as pivotal and indispensable candidates for harnessing and valorizing a range of environmental wastes into valuable products, thereby fostering the bio-circular economy.

Uncovering the lignin-degrading potential of Serratia quinivorans AORB19: insights from genomic analyses and alkaline lignin degradation

BACKGROUND

Lignin is an intricate phenolic polymer found in plant cell walls that has tremendous potential for being converted into value-added products with the possibility of significantly increasing the economics of bio-refineries. Although lignin in nature is bio-degradable, its biocatalytic conversion is challenging due to its stable complex structure and recalcitrance. In this context, an understanding of strain's genomics, enzymes, and degradation pathways can provide a solution for breaking down lignin to unlock the full potential of lignin as a dominant valuable bioresource. A gammaproteobacterial strain AORB19 has been isolated previously from decomposed wood based on its high laccase production. This work then focused on the detailed genomic and functional characterization of this strain based on whole genome sequencing, the identification of lignin degradation products, and the strain's laccase production capabilities on various agro-industrial residues.

RESULTS

Lignin degrading bacterial strain AORB19 was identified as Serratia quinivorans based on whole genome sequencing and core genome phylogeny. The strain comprised a total of 123 annotated CAZyme genes, including ten cellulases, four hemicellulases, five predicted carbohydrate esterase genes, and eight lignin-degrading enzyme genes. Strain AORB19 was also found to possess genes associated with metabolic pathways such as the β-ketoadipate, gentisate, anthranilate, homogentisic, and phenylacetate CoA pathways. LC-UV analysis demonstrated the presence of p-hydroxybenzaldehyde and vanillin in the culture media which constitutes potent biosignatures indicating the strain's capability to degrade lignin. Finally, the study evaluated the laccase production of Serratia AORB19 grown with various industrial raw materials, with the highest activity detected on flax seed meal (257.71 U/L), followed by pea hull (230.11 U/L), canola meal (209.56 U/L), okara (187.67 U/L), and barley malt sprouts (169.27 U/L).

CONCLUSIONS

The whole genome analysis of Serratia quinivorans AORB19, elucidated a repertoire of genes, pathways and enzymes vital for lignin degradation that widens the understanding of ligninolytic metabolism among bacterial lignin degraders. The LC-UV analysis of the lignin degradation products coupled with the ability of S. quinivorans AORB19 to produce laccase on diverse agro-industrial residues underscores its versatility and its potential to contribute to the economic viability of bio-refineries.

Metabolic engineering of Thermoanaerobacterium AK17 for increased ethanol production in seaweed hydrolysate

Sustainably produced renewable biomass has the potential to replace fossil-based feedstocks, for generation of biobased fuels and chemicals of industrial interest, in biorefineries. In this context, seaweeds contain a large fraction of carbohydrates that are a promising source for enzymatic and/or microbial biorefinery conversions. The thermoanaerobe Thermoanaerobacterium AK17 is a versatile fermentative bacterium producing ethanol, acetate and lactate from various sugars. In this study, strain AK17 was engineered for more efficient production of ethanol by knocking out the lactate and acetate side-product pathways. This was successfully achieved, but the strain reverted to acetate production by recruiting enzymes from the butyrate pathway. Subsequently this pathway was knocked out and the resultant strain AK17_M6 could produce ethanol close to the maximum theoretical yield (90%), leading to a 1.5-fold increase in production compared to the wild-type strain. Strain AK17 was also shown to successfully ferment brown seaweed hydrolysate from Laminaria digitata to ethanol in a comparatively high yield of 0.45 g/g substrate, with the primary carbon sources for the fermentations being mannitol, laminarin-derived glucose and short laminari-oligosaccharides. As strain AK17 was successfully engineered and has a wide carbohydrate utilization range that includes mannitol from brown seaweed, as well as hexoses and pentoses found in both seaweeds and lignocellulose, the new strain AK17_M6 obtained in this study is an interesting candidate for production of ethanol from both second and third generations biomass.

Biochemical hydrogen potential assay for predicting the patterns of the kinetics of semi-continuous dark fermentation

The performance and kinetics response of thermophilic semi-continuous dark fermentation (DF) of simulated complex carbohydrate-rich waste was investigated at various hydraulic retention times (HRT) (2, 2.5, and 3 d) and compared with data obtained from biochemical hydrogen potential assay (BHP). A culture of Thermoanaerobacterium thermosaccharolyticum was used as the inoculum and dominated both in BHP and semi-continuous reactor. Both the modified Gompertz and first-order models described the DF kinetics well (R² = 0.97-1.00). HRT of 2.5 d was found to be optimal in terms of maximum hydrogen production rate and hydrogen potential, which were 3.97 and 1.26 times higher, respectively, than in BHP. The hydrolysis constant was highest at HRT of 3 d and was closest to the value obtained in the BHP. Overall, BHP has been shown to be a useful tool for predicting H₂ potential and the hydrolysis constant, while the maximum H₂ production rate is greatly underestimated.

Evaluation of Laminaria Digitata Hydrolysate for the Production of Bioethanol and Butanol by Fermentation

Seaweeds (macroalgae) are gaining attention as potential sustainable feedstock for the production of fuels and chemicals. This comparative study focuses on the characterization of the microbial production of alcohols from fermentable carbohydrates in the hydrolysate of the macroalgae Laminaria digitata as raw material. The potential of a hydrolysate as a carbon source for the production of selected alcohols was tested, using three physiologically different fermentative microbes, in two main types of processes. For the production of ethanol, Saccharomyces cerevisiae was used as a benchmark microorganism and compared with the strictly anaerobic thermophile Thermoanaerobacterium strain AK17. For mixed production of acetone/isopropanol, butanol, and ethanol (A/IBE), three strictly anaerobic Clostridium strains were compared. All strains grew well on the hydrolysate, and toxicity constraints were not observed, but fermentation performance and product profiles were shown to be both condition- and strain-specific. S. cerevisiae utilized only glucose for ethanol formation, while strain AK17 utilized glucose, mannitol, and parts of the glucan oligosaccharides. The clostridia strains tested showed different nutrient requirements, and were able to utilize glucan, mannitol, and organic acids in the hydrolysate. The novelty of this study embodies the application of different inoculates for fermenting a common brown seaweed found in the northern Atlantic Ocean. It provides important information on the fermentation properties of different microorganisms and pinpoints the value of carbon source utilization when selecting microbes for efficient bioconversion into biofuel and chemical products of interest.

Peculiar Response in the Co-Culture Fermentation of Leuconostoc mesenteroides and Lactobacillus plantarum for the Production of ABE Solvents

Two bacterial strains (CL11A and CL11D) that are capable of ABE fermentation, identified as Leuconostoc mesenteroides and Weissella cibari, were isolated from the soil surrounding the roots of bean plants. Another strain (ZM 3A), identified as Lactobacillus plantarum, which is capable of purely ethanolic fermentation was isolated from sugarcane. Glucose was used as a standard substrate to investigate the performance of these strains in mono—and co-culture fermentation for ABE production. The performance parameters employed in this study were substrate degradation rates, product and metabolite yields, pH changes and microbial growth rates. Both ABE isolates were capable of producing the three solvents but Leuconostoc mesenteroides had a higher specificity for ethanol than Weissella cibari. The co-culturing of Leuconostoc mesenteroides and Lactobacillus plantarum enhanced ethanol production at the expense of both acetone and butanol, and also influenced the final substrate consumption rate and product yield. The experiments indicated the potential of these niche environments for the isolation of ABE-producing microorganisms. This study contributes to the formulation of ideal microbial co-culture and consortia fermentation, which seeks to maximize the yield and production rates of favored products.

Microbial application of thermophilic Thermoanaerobacterium species in lignocellulosic biorefinery

Recently, thermophilic Thermoanaerobacterium species have attracted increasing attentions in consolidated bioprocessing (CBP), which can directly utilize lignocellulosic materials for biofuels production. Compared to the mesophilic process, thermophilic process shows greater prospects in CBP due to its relatively highly efficiency of lignocellulose degradation. In addition, thermophilic conditions can avoid microbial contamination, reduce the cooling costs, and further facilitate the downstream product recovery. However, only few reviews specifically focused on the microbial applications of thermophilic Thermoanaerobacterium species in lignocellulosic biorefinery. Accordingly, this review will comprehensively summarize the recent advances of Thermoanaerobacterium species in lignocellulosic biorefinery, including their secreted xylanases and bioenergy production. Furthermore, the co-culture can significantly reduce the metabolic burden and achieve the more complex work, which will be discussed as the further perspectives. KEY POINTS: • Thermoanaerobacterium species, promising chassis for lignocellulosic biorefinery. • Potential capability of hemicellulose degradation for Thermoanaerobacterium species. • Efficient bioenergy production by Thermoanaerobacterium species through metabolic engineering.

Draft Genome Sequence of Thermophilic Bacillus sp. TYF-LIM-B05 Directly Producing Ethanol from Various Carbon Sources Including Lignocellulose

Bacillus sp. TYF-LIM-B05, which is isolated from spoilage vinegar, is resistant to high temperature, high concentrated alcohol, acid, and salt, and can produce ethanol from mono-, di-, polysaccharide, and complex biomass as the sole carbon source. Thus, this strain is a potential candidate for consolidated bioprocessing (CBP) of fermenting lignocellulose to ethanol in a single step. To provide insight into the key enzymes involved in lignocellulose degradation and ethanol production, a draft genome of TYF-LIM-B05 was developed in this study. The results indicated that 348 genes are related to carbohydrate transport and metabolism according to the clusters of orthologous groups of proteins and annotated 187 CAZy domains from a total of 61 different families. The presence of genes encoding laccases, quinone oxidoreductases/reductases, and aryl-alcohol dehydrogenases further implies that TYF-LIM-B05 has the potential to degrade lignin. Remarkably, this strain has the ability to catalyze the oxidative decarboxylation of pyruvate to acetyl-CoA by pyruvate dehydrogenase complexes. The genomic information provided in this study will help researchers to better understand the mechanisms of the lignocellulose degradation and ethanol production pathway in thermophiles.

Utilization of waste cake for fermentative ethanol production

In this study, utilization of waste cake for bioethanol production via a two-step of enzymatic hydrolysis and ethanol fermentation was developed. At the first step, namely waste cake hydrolysis, effects of α-amylase volumes (C1: 0.1 mL/L, C2: 0.4 mL/L, C3, 0.8 mL/L) on the performance of hydrolysis were investigated. Both chemical oxygen demand (COD) and reducing sugar (RS) could reach the highest values after 80 min for all the conditions. The maximum COD of 86.3 g/L, RS of 44 g/L and waste reduction of 85.2% were achieved at C3. At the second step, namely ethanol fermentation, the produced waste cake hydrolysate was used as the sole feedstock for fermentative ethanol production, and the highest ethanol production of 46.6 g/L and ethanol yield of 1.13 g/g RS were obtained (C3), respectively. This corresponds to an overall ethanol yield of 1.12 g ethanol/g initial dry cake which is the highest ethanol yield compared to using other food wastes reported to date. These values are higher than using the glucose as feedstock since the waste cake hydrolysate could provide the carbon and nitrogen sources for ethanol fermentation.

Recent trends in bioethanol production from food processing byproducts

The widespread use of corn starch and sugarcane as sources of sugar for the production of ethanol via fermentation may negatively impact the use of farmland for production of food. Thus, alternative sources of fermentable sugars, particularly from lignocellulosic sources, have been extensively investigated. Another source of fermentable sugars with substantial potential for ethanol production is the waste from the food growing and processing industry. Reviewed here is the use of waste from potato processing, molasses from processing of sugar beets into sugar, whey from cheese production, byproducts of rice and coffee bean processing, and other food processing wastes as sugar sources for fermentation to ethanol. Specific topics discussed include the organisms used for fermentation, strategies, such as co-culturing and cell immobilization, used to improve the fermentation process, and the use of genetic engineering to improve the performance of ethanol producing fermenters.

Furfural tolerance and detoxification mechanism in Candida tropicalis

BACKGROUND

Current biomass pretreatment by hydrothermal treatment (including acid hydrolysis, steam explosion, and high-temperature steaming) and ionic liquids generally generate inhibitors to the following fermentation process. Furfural is one of the typical inhibitors generated in hydrothermal treatment of biomass. Furfural could inhibit cell growth rate and decrease biofuel productivity of microbes. Candida tropicalis is a promising microbe for the production of biofuels and value-added chemicals using hemicellulose hydrolysate as carbon source. In this study, C. tropicalis showed a comparable ability of furfural tolerance during fermentation. We investigated the mechanism of C. tropicalis's robust tolerance to furfural and relevant metabolic responses to obtain more information for metabolic engineering of microbes for efficient lignocellulose fermentation.

RESULTS

Candida tropicalis showed comparable intrinsic tolerance to furfural and a fast rate of furfural detoxification. C. tropicalis's half maximal inhibitory concentration for furfural with xylose as the sole carbon source was 3.69 g/L, which was higher than that of most wild-type microbes reported in the literature to our knowledge. Even though furfural prolonged the lag phase of C. tropicalis, the final biomass in the groups treated with 1 g/L furfural was slightly greater than that in the control groups. By real-time PCR analysis, we found that the expression of ADH1 in C. tropicalis (ctADH1) was induced by furfural and repressed by ethanol after furfural depletion. The expression of ctADH1 could be regulated by both furfural and ethanol. After the disruption of gene ctADH1, we found that C. tropicalis's furfural tolerance was weakened. To further confirm the function of ctADH1 and enhance Escherichia coli's furfural tolerance, ctADH1 was overexpressed in E. coli BL21 (DE3). The rate of furfural degradation in E. coli BL21 (DE3) with pET-ADH1 (high-copy plasmid) and pCS-ADH1 (medium-copy plasmid) was increased by 1.59-fold and 1.28-fold, respectively.

CONCLUSIONS

Candida tropicalis was a robust strain with intrinsic tolerance to inhibitor furfural. The mechanism of furfural detoxification and metabolic responses were identified by multiple analyses. Alcohol dehydrogenase 1 was confirmed to be responsible for furfural detoxification. C. tropicalis showed a complex regulation system during furfural detoxification to minimize adverse effects caused by furfural. Furthermore, the mechanism we uncovered in this work was successfully applied to enhance E. coli's furfural tolerance by heterologous expression of ctADH1. The study provides deeper insights into strain modification for biofuel production by efficient lignocellulose fermentation.

Determining sucrose and glucose levels in dual-purpose sorghum stalks by Fourier transform near infrared (FT-NIR) spectroscopy

BACKGROUND

Sorghum is an advanced biomass feedstock from which grain, sugar and stover can be used for biofuel production. Determinations of specific sugar contents in sorghum stalks help to make strategic decisions during plant breeding, processing, storage and optimization of fermentation conditions. In this study, Fourier transform near infrared (FT-NIR) spectroscopy was used as a relatively fast, low-cost, high-throughput assay to predict sucrose and glucose levels in stalks of 40 dwarf grain sorghum inbreds.

RESULTS

The diffuse reflection spectra were pretreated with multiplicative scatter correction (MSC) and first-derivative Savitzy-Golay (SG-1). Calibrated models were developed by partial least squares regression (PLSR) analysis. Martens' uncertainty test was used to determine the most effective spectral region. The PLSR model for stalk sucrose content was built on 380 significant wavenumbers in the 4000-6999 cm(-1) range. The model was based on four factors and had RPD = 2.40, RMSEP = 1.77 and R(2) = 0.81. Similarly, the model for stalk glucose was built using 4000-9000 cm(-1) and six factors, with RPD = 2.45, RMSEP = 0.73 and R(2) = 0.81.

CONCLUSION

PLSR models were developed based on FT-NIR spectra coupled with multivariate data analysis to provide a quick and low-cost estimate of specific sugar contents in grain sorghum stalks. This sugar information helps decision making for sorghum-based biomass processing and storage strategies.

A comparative multidimensional LC-MS proteomic analysis reveals mechanisms for furan aldehyde detoxification in Thermoanaerobacter pseudethanolicus 39E

BACKGROUND

Chemical and physical pretreatment of lignocellulosic biomass improves substrate reactivity for increased microbial biofuel production, but also restricts growth via the release of furan aldehydes, such as furfural and 5-hydroxymethylfurfural (5-HMF). The physiological effects of these inhibitors on thermophilic, fermentative bacteria are important to understand; especially as cellulolytic strains are being developed for consolidated bioprocessing (CBP) of lignocellulosic feedstocks. Identifying mechanisms for detoxification of aldehydes in naturally resistant strains, such as Thermoanaerobacter spp., may also enable improvements in candidate CBP microorganisms.

RESULTS

Thermoanaerobacter pseudethanolicus 39E, an anaerobic, saccharolytic thermophile, was found to grow readily in the presence of 30 mM furfural and 20 mM 5-HMF and reduce these aldehydes to their respective alcohols in situ. The proteomes of T. pseudethanolicus 39E grown in the presence or absence of 15 mM furfural were compared to identify upregulated enzymes potentially responsible for the observed reduction. A total of 225 proteins were differentially regulated in response to the 15 mM furfural treatment with 152 upregulated versus 73 downregulated. Only 87 proteins exhibited a twofold or greater change in abundance in either direction. Of these, 54 were upregulated in the presence of furfural and 33 were downregulated. Two oxidoreductases were upregulated at least twofold by furfural and were targeted for further investigation. Teth39_1597 encodes a predicted butanol dehydrogenase (BdhA) and Teth39_1598, a predicted aldo/keto reductase (AKR). Both genes were cloned from T. pseudethanolicus 39E, with the respective enzymes overexpressed in E. coli and specific activities determined against a variety of aldehydes. Overexpressed BdhA showed significant activity with all aldehydes tested, including furfural and 5-HMF, using NADPH as the cofactor. Cell extracts with AKR also showed activity with NADPH, but only with four-carbon butyraldehyde and isobutyraldehyde.

CONCLUSIONS

T. pseudethanolicus 39E displays intrinsic tolerance to the common pretreatment inhibitors furfural and 5-HMF. Multidimensional proteomic analysis was used as an effective tool to identify putative mechanisms for detoxification of furfural and 5-HMF. T. pseudethanolicus was found to upregulate an NADPH-dependent alcohol dehydrogenase 6.8-fold in response to furfural. In vitro enzyme assays confirmed the reduction of furfural and 5-HMF to their respective alcohols.

Simultaneous saccharification and fermentation of fungal pretreated cornstalk for hydrogen production using Thermoanaerobacterium thermosaccharolyticum W16

In this research, environmentally friendly fungal pretreatment was first adopted for deconstruction of cornstalk. Then the fungal-pretreated cornstalk was employed to produce hydrogen in simultaneous saccharification and fermentation (SSF) using crude enzyme from Trichoderma viride and Thermoanaerobacterium thermosaccharolyticum W16. The influence of various factors including substrate concentration, initial pH, and enzyme loading on hydrogen production were evaluated. The highest hydrogen yield of 89.3 ml/g-cornstalk was obtained with an initial pH 6.5, 0.75% substrate concentration, and 34 FPU/g cellulose. Compared the result with SSF of physical or chemical pretreated lignocellulosic materials, this research suggested an economic and efficient way for hydrogen production from lignocellulosic biomass.

Efficient ethanol production from corncob residues by repeated fermentation of an adapted yeast

For economically feasible lignocellulosic ethanol production, it is crucial to obtain a robust strain and develop an efficient fermentation process. An earlier-screened yeast strain Pichia guilliermondii was adapted to corncob residues (CCR) hydrolysate and used for high titer ethanol production without any detoxification or external nutrient supplementation. With an optimized fed-batch strategy, the maximum ethanol titer and productivity reached 56.3 g/l and 0.47 g l(-1) h(-1), respectively. To further increase the ethanol productivity, the fed-batch process was repeated three times with cell reuse, and the maximum ethanol titer and productivity reached 51.2 g/l and 1.11 g l(-1) h(-1), respectively. The results demonstrated that the combination of fed-batch with repeated fermentation was effective in improving the fermentation efficiency and achieving high ethanol productivity from CCR. The reported system is considered promising for commercial production of bioethanol from biomass hydrolysate in the future.

Production of ethanol from sugars and lignocellulosic biomass by Thermoanaerobacter J1 isolated from a hot spring in Iceland

Thermophilic bacteria have gained increased attention as candidates for bioethanol production from lignocellulosic biomass. This study investigated ethanol production by Thermoanaerobacter strain J1 from hydrolysates made from lignocellulosic biomass in batch cultures. The effect of increased initial glucose concentration and the partial pressure of hydrogen on end product formation were examined. The strain showed a broad substrate spectrum, and high ethanol yields were observed on glucose (1.70 mol/mol) and xylose (1.25 mol/mol). Ethanol yields were, however, dramatically lowered by adding thiosulfate or by cocultivating strain J1 with a hydrogenotrophic methanogen with acetate becoming the major end product. Ethanol production from 4.5 g/L of lignocellulosic biomass hydrolysates (grass, hemp stem, wheat straw, newspaper, and cellulose) pretreated with acid or alkali and the enzymes Celluclast and Novozymes 188 was investigated. The highest ethanol yields were obtained on cellulose (7.5 mM·g⁻¹) but the lowest on straw (0.8 mM·g⁻¹). Chemical pretreatment increased ethanol yields substantially from lignocellulosic biomass but not from cellulose. The largest increase was on straw hydrolysates where ethanol production increased from 0.8 mM·g⁻¹ to 3.3 mM·g⁻¹ using alkali-pretreated biomass. The highest ethanol yields on lignocellulosic hydrolysates were observed with hemp hydrolysates pretreated with acid, 4.2 mM·g⁻¹.