Enhanced bioconversion of hemicellulosic biomass by microbial consortium for biobutanol production with bioaugmentation strategy

Comprehensive analysis of genomic variation, pan-genome and biosynthetic potential of Corynebacterium glutamicum strains

Corynebacterium glutamicum is a non-pathogenic species of the Corynebacteriaceae family. It has been broadly used in industrial biotechnology for the production of valuable products. Though it is widely accepted at the industrial level, knowledge about the genomic diversity of the strains is limited. Here, we investigated the comparative genomic features of the strains and pan-genomic characteristics. We also observed phylogenetic relationships among the strains based on average nucleotide identity (ANI). We found diversity between strains at the genomic and pan-genomic levels. Less than one-third of the C. glutamicum pan-genome consists of core genes and soft-core genes. Whereas, a large number of strain-specific genes covered about half of the total pan-genome. Besides, C. glutamicum pan-genome is open and expanding, which indicates the possible addition of new gene families to the pan-genome. We also investigated the distribution of biosynthetic gene clusters (BGCs) among the strains. We discovered slight variations of BGCs at the strain level. Several BGCs with the potential to express novel bioactive secondary metabolites have been identified. Therefore, by utilizing the characteristic advantages of C. glutamicum, different strains can be potential applicants for natural drug discovery.

Construction of Coculture System Containing Escherichia coli with Different Microbial Species for Biochemical Production

Microbial synthesis of target chemicals usually involves multienzymatic reactions in vivo, especially for compounds with a long metabolic pathway. However, when various genes are introduced into one single strain, it leads to a heavy metabolic burden. In contrast, the microbial coculture system can allocate metabolic pathways into different hosts, which will relieve the metabolic burdens. Escherichia coli is the most used chassis to synthesize biofuels and chemicals owing to its well-known genetics, high transformation efficiency, and easy cultivation. Accordingly, cocultures containing the cooperative E. coli with other microbial species have received great attention. In this review, the individual applications and boundedness of different combinations will be summarized. Additionally, the strategies for the self-regulation of population composition, which can help enhance the stability of a coculture system, will also be discussed. Finally, perspectives for the cocultures will be proposed.

Valorization of agricultural wastes for biofuel applications

Continuous environmental degradation, volatility in the oil market, and unimpressive functioning of fossil-based (FB) fuels in compression ignition engines have expanded the tempo of the search for alternative fuels. Due to the astronomical rise in global population, improved agricultural, commercial, and manufacturing activities, enhanced farming and other food production and utilization ventures, agricultural waste generation, renewable fuel consumption, and emission of toxic gases. The need for cost-effective, readily available, and environmentally benign agricultural waste to biofuels has never been more crucial. Biofuels are renewable, biodegradable, low-cost, and eco-friendly fuels that are produced by microorganisms from waste lignocellulosic biomass. Conversion of agricultural wastes to biofuel does not exacerbate food security, contributes to waste management, prevents environmental degradation, and ensures energy security. This study reviews the conversion of agricultural wastes into biofuels with special emphasis on bioethanol, biohydrogen, biobutanol, biomethane, biomethanol, and biodiesel for various applications. It is safe to conclude that wastes generated from agricultural activities and processes are useful and can be harnessed to meet the affordable and accessible global renewable energy target. The result of this investigation will improve the body of knowledge and provide novel strategies and pathways for the utilization of agricultural wastes. Going forward, more collaborative and interdisciplinary studies are required to evolve state-of-the-art, ecofriendly, and cost-effective conversion pathways for agricultural wastes to promote the utilization of the generated renewable fuels. More human, financial, and infrastructural investments are desirable to motivate the conversion of agricultural waste into biofuels to ensure environmental sanitation and sustainability, promote renewable fuel utilization, and avert the raging implosion of our planet.

Production of butanol from lignocellulosic biomass: recent advances, challenges, and prospects

Due to energy and environmental concerns, biobutanol is gaining increasing attention as an alternative renewable fuel owing to its desirable fuel properties. Biobutanol production from lignocellulosic biomass through acetone-butanol-ethanol (ABE) fermentation has gained much interest globally due to its sustainable supply and non-competitiveness with food, but large-scale fermentative production suffers from low product titres and poor selectivity. This review presents recent developments in lignocellulosic butanol production, including pretreatment and hydrolysis of hemicellulose and cellulose during ABE fermentation. Challenges are discussed, including low concentrations of fermentation sugars, inhibitors, detoxification, and carbon catabolite repression. Some key process improvements are also summarised to guide further research and development towards more profitable and commercially viable butanol fermentation.

Unraveling the unique butyrate re-assimilation mechanism of Clostridium sp. strain WK and the application of butanol production from red seaweed Gelidium amansii through a distinct acidolytic pretreatment

Exploration of the algae-derived biobutanol synthesis has become one of the hotspots due to its highly cost-effective and environment-friendly features. In this study, a solventogenic strain Clostridium sp. strain WK produced 13.96 g/L butanol with a maximal yield of 0.41 g/g from glucose in the presence of 24 g/L butyrate. Transcriptional analysis indicated that the acid re-assimilation of this strain was predominantly regulated by genes buk-ptb rather than ctfAB, explaining its special phenotypes including high butyrate tolerance and the pH-independent fermentation. In addition, a butyric acid-mediated hydrolytic system was established for the first time to release a maximal yield of 0.35 g/g reducing sugars from the red algal biomass (Gelidium amansii). Moreover, 4.48 g/L of butanol was finally achieved with a significant enhancement by 29.9 folds. This work reveals an unconventional metabolic pathway for butanol synthesis in strain WK, and demonstrates the feasibility to develop renewable biofuels from marine resources.

Solid-state anaerobic digestion of sugarcane bagasse at different solid concentrations: Impact of bio augmented cellulolytic bacteria on methane yield and insights on microbial diversity

This study investigated the impact of the potential cellulose degrading bacteria that could be bioaugmented in the solid-state anaerobic digestion (SSAD) of bagasse to enhance the methane yield. The prospective anaerobic cellulose degrading bacteria was isolated from the soil. SSAD experiments were organized with & without bioaugmentation with a substrate total solid (TS) of 25%, 30%, 40% and 50% at an optimized feed to microorganism (F/M) ratio of 1:1. The maximum yield of 0.44 L CH₄/ (g VS added) was obtained from bioaugmented bagasse at a TS of 40% whereas it was 0.34 L CH₄/(g VS added) for non-bioaugmented bagasse. The isolated bacterial strain was identified that belongs species Pseudomonas of Gamma Proteobacteria which exhibited good cellulolytic activity. Metagenomic studies found 90% of archaeal microorganisms affiliated to Methanosaeta, a strict acetoclastic methanogen.

Emerging technologies for genetic modification of solventogenic clostridia: From tool to strategy development

Solventogenic clostridia has been considered as one of the most potential microbial cell factories for biofuel production in the biorefinery industry. However, the inherent shortcomings of clostridia strains such as low productivity, by-products formation and toxic tolerance still strongly restrict the large-scale application. Therefore, concerns regarding the genetic modification of solventogenic clostridia have spurred interests into the development of modern gene-editing tools. In this review, we summarize the latest advances of genetic tools involved in modifying solventogenic clostridia. Following a systematic comparison on their respective characteristics, we then review the corresponding strategies for overcoming the obstacles to the enhanced production. Discussing the progress of other microbial cell factories for solventogenesis, we finally describe the key challenges and trends with valuable recommendations for future large-scale biosolvent industrial application.

Microbial debromination of hexabromocyclododecanes

Hexabromocyclododecanes (HBCDs), a new sort of brominated flame retardants (BFRs), are globally prevalent and recalcitrant toxic environmental pollutants. HBCDs have been found in many environmental media and even in the human body, leading to serious health concerns. HBCDs are biodegradable in the environment. By now, dozens of bacteria have been discovered with the ability to transform HBCDs. Microbial debromination of HBCDs is via HBr-elimination, HBr-dihaloelimination, and hydrolytic debromination. Biotic transformation of HBCDs yields many hydroxylated and lower brominated compounds which lack assessment of ecological toxicity. Bioremediation of HBCD pollution has only been applied in the laboratory. Here, we review the current knowledge about microbial debromination of HBCDs, aiming to promote the bioremediation applied in HBCD contaminated sites. KEY POINTS: • Microbial debromination of HBCDs is via hydrolytic debromination, HBr-elimination, and HBr-dihaloelimination. • Newly occurred halogenated contaminants such as HBCDs hitch the degradation pathway tamed by previously discharged anthropogenic organohalides. • Strategy that combines bioaugmentation with phytoremediation for bioremediation of HBCD pollution is promising.

Recent Progress on Chemical Production From Non-food Renewable Feedstocks Using Corynebacterium glutamicum

Due to the non-renewable nature of fossil fuels, microbial fermentation is considered a sustainable approach for chemical production using glucose, xylose, menthol, and other complex carbon sources represented by lignocellulosic biomass. Among these, xylose, methanol, arabinose, glycerol, and other alternative feedstocks have been identified as superior non-food sustainable carbon substrates that can be effectively developed for microbe-based bioproduction. Corynebacterium glutamicum is a model gram-positive bacterium that has been extensively engineered to produce amino acids and other chemicals. Recently, in order to reduce production costs and avoid competition for human food, C. glutamicum has also been engineered to broaden its substrate spectrum. Strengthening endogenous metabolic pathways or assembling heterologous ones enables C. glutamicum to rapidly catabolize a multitude of carbon sources. This review summarizes recent progress in metabolic engineering of C. glutamicum toward a broad substrate spectrum and diverse chemical production. In particularly, utilization of lignocellulosic biomass-derived complex hybrid carbon source represents the futural direction for non-food renewable feedstocks was discussed.

Acidolysis as a biorefinery approach to producing advanced bioenergy from macroalgal biomass: A state-of-the-art review

Facing fossil fuels consumption and its accompanying environmental pollution, macroalgae, as a major part of the third-generation (3G) biomass, has great potential for bioenergy development due to its species-abundant, renewable and carbohydrate-rich properties. Diluted acid treatment is one of the most effective approaches to releasing fermentable sugars from macroalgal biomass in a short period, but the optimal conditions need to be explored to maximize the hydrolytic yield for the subsequent microbial conversion. Therefore, this review aims to summarize the latest advances in various acids and other auxiliary methods adopted to increase the hydrolytic efficiency of macroalgae. Following an overview of the strategies of different algal types, methods involved in the bioconversion of biofuels and microbial fuel cells (MFC) from algal hydrolysates are also described. For the 3G biorefinery development, the review further discusses key challenges and trends for future utilizing marine biomass to achieve the large-scale industrial production.

Integrated Evaluation of the Impediments to the Adoption of Coconut Palm Wood as a Sustainable Material for Building Construction

Coconut palm wood is commonly regarded as a sustainable building material. Nonetheless, its adoption as a green building material by the construction industry is limited, particularly in West Africa. This paper analyses the impediments to the effective adoption of coconut palm wood in residential building construction. The research data was collected from literature reviews, expert-based surveys, and semi-structured interviews with specialists from the construction sector, African Timber Organization, governmental institutions related to forestry and construction, university professors, and researchers in the areas of sustainable building materials and construction. Thirteen crucial impediments were identified and an integrated evaluation of the impediments was conducted using the Interpretive Structural Modelling (ISM) technique to examine the hierarchical structure of the relations between the impediments. A further technique, Cross-impact Matrix Multiplication Applied to Classification (MICMAC), was used to categorize the impediments from a driving to driven perspective. This categorization provides a unique profile for the impediments, which is different from that of conventional evaluation techniques for evaluating impediments. The findings of this paper offer useful guide to practitioners and policy makers in formulating effective policies, regulations, and standards that will promote the development and wide adoption of coconut palm wood in the construction industry.

Recent Development of Extremophilic Bacteria and Their Application in Biorefinery

The biorefining technology for biofuels and chemicals from lignocellulosic biomass has made great progress in the world. However, mobilization of laboratory research toward industrial setup needs to meet a series of criteria, including the selection of appropriate pretreatment technology, breakthrough in enzyme screening, pathway optimization, and production technology, etc. Extremophiles play an important role in biorefinery by providing novel metabolic pathways and catalytically stable/robust enzymes that are able to act as biocatalysts under harsh industrial conditions on their own. This review summarizes the potential application of thermophilic, psychrophilic alkaliphilic, acidophilic, and halophilic bacteria and extremozymes in the pretreatment, saccharification, fermentation, and lignin valorization process. Besides, the latest studies on the engineering bacteria of extremophiles using metabolic engineering and synthetic biology technologies for high-efficiency biofuel production are also introduced. Furthermore, this review explores the comprehensive application potential of extremophiles and extremozymes in biorefinery, which is partly due to their specificity and efficiency, and points out the necessity of accelerating the commercialization of extremozymes.

Deciphering the d-/l-lactate-producing microbiota and manipulating their accumulation during solid-state fermentation of cereal vinegar

Symphony orchestra of multi-microorganisms characterizes the solid-state acetic acid fermentation process of Chinese cereal vinegars. Lactate is the predominant non-volatile acid and plays indispensable roles in flavor formation. This study investigated the microbial consortia driving the metabolism of D-/l-lactate during fermentation. Sequencing analysis based on D-/l-lactate dehydrogenase genes demonstrated that Lactobacillus (relative abundance: > 95%) dominated the production of both d-lactate and l-lactate, showing species-specific features between the two types. Lactobacillus helveticus (>65%) and L. reuteri (~80%) respectively dominated l- and d-lactate-producing communities. D-/l-lactate production and utilization capabilities of eight predominant Lactobacillus strains were determined by culture-dependent approach. Subsequently, D-/l-lactate producer L. plantarum M10-1 (d:l ≈ 1:1), l-lactate producer L. casei 21M3-1 (D:L ≈ 0.2:9.8) and D-/l-lactate utilizer Acetobacter pasteurianus G3-2 were selected to modulate the metabolic flux of D-/l-lactate of microbial consortia. The production ratio of D-/l-lactate was correspondingly shifted coupling with microbial consortia changes. Bioaugmentation with L.casei 21M3-1 merely enhanced l-lactate production, displaying ~4-fold elevation at the end of fermentation. Addition of L.plantarum M10-1 twice increased both D- and l-lactate production, while A. pasteurianus G3-2 decreased the content of D-/l-isomer. Our results provided an alternative strategy to specifically manipulate the metabolic flux within microbial consortia of certain ecological niches.

Performance analysis and optimization of ammonium removal in a new biological folded non-aerated filter reactor

A new type of biological folded non-aerated filter (BFNAF) was found to be superior and feasible for the treatment of NH₄⁺-N wastewater. It was constructed with the folded structure suitable for the nylon biomass carrier. The advantages of the BFNAF included low energy consumption, long reaction path, large biofilm surface area and non-clogging compared to the traditional biological aerated filter. In this study, the effects of hydraulic retention time (HRT), and the influent NH₄⁺-N concentration on the performance of BFNAF were investigated and optimized by the response surface methodology. Under the optimal operating condition (HRT, 10 h; NH₄⁺-N concentration, 52 mg/L), the removal efficiency and removal rate were 94.62 ± 0.63% and 0.106 kg-NH₄⁺ m^-3 day^-1, respectively. The results showed that the BFNAF reactor could remove NH₄⁺-N from wastewater and realized the nitrification process effectively under natural ventilation conditions.

Nitrogen removal from wastewater via simultaneous nitrification and denitrification using a biological folded non-aerated filter

A conventional biological filter has been shown to be a viable method for removing nitrogenous compounds from wastewater, but it still has many disadvantages. In this study, a biological folded non-aerated filter (BFNAF) was designed, and its feasibility for nitrogen-loaded wastewater treatment has been confirmed. Effects of the HRT and the COD/N ratio on the performance of BFNAF were investigated. Through response surface method, when the COD/N ratio and the HRT were 5.39 and 10.83 h, removal efficiencies of NH₄⁺, COD and TN reached maximum values of 88.62 ± 0.81%, 76.12 ± 0.57%, and 50.48 ± 1.02%, respectively. In addition, it was found that several denitrifying bacteria, such as Azoarcus, Arcobacter, Flavobacterium, along with many ammonia-oxidizing bacteria and nitrite-oxidizing bacteria, co-existed in the community of the biofilm. All the results showed that the BFNAF could realize the simultaneous nitrification and denitrification (SND) process effectively.

High butanol production from glycerol by using Clostridium sp. strain CT7 integrated with membrane assisted pervaporation

In this study, a unique butanol-ethanol fermentation process from glycerol integrated with pervaporation by using Clostridium sp. strain CT7 was investigated. 20.4 g/L of butanol and 0.3 g/L of ethanol were produced from 51.3 g/L of glycerol in the PV coupled batch fermentation process with butanol productivity of 0.15 g/L/h and yield of 0.40 g/g due to the reduced butanol inhibition by butanol removal. Subsequently, 41.9 g/L of butanol and 0.4 g/L of ethanol were obtained from 103.3 g/L of glycerol, with butanol productivity of 0.21 g/L/h and yield of 0.41 g/g in the PV coupled fed-batch fermentation process. The high butanol production could be attributed to the thin PDMS layer and negligible transportation resistance of the support. These results indicated the PV coupled fermentation process from glycerol using PDMS/ceramic composite membrane by Clostridium sp. strain CT7 might show a great potential for sustainable biobutanol production from low-cost carbon sources.

Sustainable Performance Measurements for Public–Private Partnership Projects: Empirical Evidence from China

The world is witnessing a global momentum of public–private partnership (PPP) development, along with the inherent complexities associated with the long-term construction, operation, and maintenance periods of numerous PPP projects. Performance measurements for PPP projects are critical for ensuring stakeholders’ interests in a sustainable way, without scarifying future generations’ interests in terms of economic, environmental, and social sustainability. A system of five-dimension sustainable performance measurements for PPP projects is proposed in this study and a questionnaire survey was conducted to solicit professional opinions on its effectiveness based on current PPP practice in China. A total of 79 professionals with PPP working experiences in China participated in the survey, among which five were chosen for interviews to validate and deepen the understandings of the findings. It reveals that the five dimensions have essential impacts on current PPP practice in China, though with varying extent of importance. The private sector has developed a sense of achieving a long-term financial return, which might have indeterminate results on the benefits of end users. The increasing trend of sustainability concerns in the PPP project performance measurement is confirmed, and it is partially due to the central government’s “High-Quality Development” initiatives. This study contributes to the theoretical discussions of establishing sustainable performance measurements for PPP projects, and it has practical implications for the public sector to improve PPP project evaluation methods and incentive mechanisms, so as to promote project sustainability.

The Benefits and Barriers for Promoting Bamboo as a Green Building Material in China—An Integrative Analysis

Bamboo is commonly considered as a green, environmentally friendly material. However, it appears that bamboo finds limited application in the form of green building materials in the Chinese construction sector. In order to explain this phenomenon and promote the material’s application, this study summarizes the benefits of applying bamboo materials and presents an analysis on barriers affecting the effective application of bamboo materials in this specific building sector. Research data are collected from both literature surveys and semi-structured interviews with a group of carefully selected experts from the Chinese building sector. Fifteen characteristic barriers are identified, such as ineffective action by government departments. An integrative analysis is conducted, including investigation on the hierarchy structure among characteristic barriers using the interpretive structural modeling (ISM) method and the classification of barriers from a driving-driven perspective using the Cross-impact Matrix Multiplication Applied to Classification (MICMAC) technique. This classification provides a different profile for the characteristic barriers from that of traditional barrier analysis methods. The findings provide valuable references for helping policy makers and practitioners adopt effective policies and measures to promote the application of bamboo for green materials in building sector.