A review on variation in crude glycerol composition, bio-valorization of crude and purified glycerol as carbon source for lipid production

Strategies for the efficient biosynthesis of β-carotene through microbial fermentation

β-Carotene is an orange fat-soluble compound, which has been widely used in fields such as food, medicine and cosmetics owing to its anticancer, antioxidant and cardiovascular disease prevention properties. Currently, natural β-carotene is mainly extracted from plants and algae, which cannot meet the growing market demand, while chemical synthesis of β-carotene cannot satisfy the pursuit for natural products of consumers. The β-carotene production through microbial fermentation has become a promising alternative owing to its high efficiency and environmental friendliness. With the rapid development of synthetic biology and in-depth study on the synthesis pathway of β-carotene, microbial fermentation has shown promising applications in the β-carotene synthesis. Accordingly, this review aims to summarize the research progress and strategies of natural carotenoid producing strain and metabolic engineering strategies in the heterologous synthesis of β-carotene by engineered microorganisms. Moreover, it also summarizes the adoption of inexpensive carbon sources to synthesize β-carotene as well as proposes new strategies that can further improve the β-carotene production.

Environmental impacts of valorisation of crude glycerol from biodiesel production - A life cycle perspective

Biodiesel production produces significant quantities of impure crude glycerol as a by-product. Recent increases in the global biodiesel production have led to a surplus of crude glycerol, rendering it a waste. As a result, different methods for its valorisation are currently being investigated. This paper assesses the life cycle environmental impacts of an emerging technology for purification of crude glycerol - a multi-step physico-chemical treatment - in comparison to incineration with energy recovery commonly used for its disposal. For the former, three different acids (H3PO4, H2SO4 and HCl) are considered for the acidification step in the purification process. The results suggest that the H2SO4-based treatment is the best option with 17 net-negative impacts out of the 18 categories considered; this is due to system credits for the production of purified glycerol, heat and potassium salts. In comparison to incineration with energy recovery, the H2SO4-based process has lower savings for the climate change impact (-311 versus -504 kg CO2 eq./t crude glycerol) but it performs better in ten other categories. Sensitivity analyses suggest that that the impacts of the physico-chemical treatment are highly dependent on crude glycerol composition, allocation of burdens to crude glycerol and credits for glycerol production. For example, treating crude glycerol with lower glycerol content would increase all impacts except climate change and fossil depletion due to the higher consumption of chemicals and lower production of purified glycerol. Considering crude glycerol as a useful product rather than waste and allocating to it burdens from biodiesel production would increase most impacts significantly, including climate change (22-40 %), while fossil depletion, freshwater and marine eutrophication would become net-positive. The findings of this research will be of interest to the biodiesel industry and other industrial sectors that generate crude glycerol as a by-product.

A systematic review on utilization of biodiesel-derived crude glycerol in sustainable polymers preparation

With the rapid development of biodiesel, biodiesel-derived glycerol has become a promising renewable bioresource. The key to utilizing this bioresource lies in the value-added conversion of crude glycerol. While purifying crude glycerol into a pure form allows for diverse applications, the intricate nature of this process renders it costly and environmentally stressful. Consequently, technology facilitating the direct utilization of unpurified crude glycerol holds significant importance. It has been reported that crude glycerol can be bio-transformed or chemically converted into high-value polymers. These technologies provide cost-effective alternatives for polymer production while contributing to a more sustainable biodiesel industry. This review article describes the global production and quality characteristics of biodiesel-derived glycerol and investigates the influencing factors and treatment of the composition of crude glycerol including water, methanol, soap, matter organic non-glycerol, and ash. Additionally, this review also focused on the advantages and challenges of various technologies for converting crude glycerol into polymers, considering factors such as the compatibility of crude glycerol and the control of unfavorable factors. Lastly, the application prospect and value of crude glycerol conversion were discussed from the aspects of economy and environmental protection. The development of new technologies for the increased use of crude glycerol as a renewable feedstock for polymer production will be facilitated by the findings of this review, while promoting mass market applications.

Combination of alkaline biodiesel-derived crude glycerol pretreated corn stover with dilute acid pretreated water hyacinth for highly-efficient single cell oil production by oleaginous yeast Cutaneotrichosporon oleaginosum

Single cell oil (SCO) prepared from biodiesel-derived crude glycerol (BCG) and lignocellulosic biomass (LCB) via oleaginous yeasts is an intriguing alternative precursor of biodiesel. Here, a novel strategy combining alkaline BCG pretreated corn stover and dilute acid pretreated water hyacinth for SCO overproduction was developed. The mixed pretreatment liquors (MPLs) were naturally neutralized and adjusted to a proper carbon-to-nitrogen ratio beneficial for SCO overproduction by Cutaneotrichosporon oleaginosum. The toxicity of inhibitors was relieved by dilution detoxification. The enzymatic hydrolysate of solid fractions was suitable for SCO production either separately or simultaneously with MPLs. Fed-batch fermentation of the MPLs resulted in high cell mass, SCO content, and SCO titer of 80.7 g/L, 75.7 %, and 61.1 g/L, respectively. The fatty acid profiles of SCOs implied high-quality biodiesel characteristics. This study offers a novel BCG&LCB-to-SCO route integrating BCG-based pretreatment and BCG/LCB hydrolysates co-utilization, which provides a cost-effective technical route for micro-biodiesel production.

Bacteria for Bioplastics: Progress, Applications, and Challenges

Bioplastics are one of the answers that can point society toward a sustainable future. Under this premise, the synthesis of polymers with competitive properties using low-cost starting materials is a highly desired factor in the industry. Also, tackling environmental issues such as nonbiodegradable waste generation, high carbon footprint, and consumption of nonrenewable resources are some of the current concerns worldwide. The scientific community has been placing efforts into the biosynthesis of polymers using bacteria and other microbes. These microorganisms can be convenient reactors to consume food and agricultural wastes and convert them into biopolymers with inherently attractive properties such as biodegradability, biocompatibility, and appreciable mechanical and chemical properties. Such biopolymers can be applied to several fields such as packing, cosmetics, pharmaceutical, medical, biomedical, and agricultural. Thus, intending to elucidate the science of microbes to produce polymers, this review starts with a brief introduction to bioplastics by describing their importance and the methods for their production. The second section dives into the importance of bacteria regarding the biochemical routes for the synthesis of polymers along with their advantages and disadvantages. The third section covers some of the main parameters that influence biopolymers' production. Some of the main applications of biopolymers along with a comparison between the polymers obtained from microorganisms and the petrochemical-based ones are presented. Finally, some discussion about the future aspects and main challenges in this field is provided to elucidate the main issues that should be tackled for the wide application of microorganisms for the preparation of bioplastics.

Recovery of potassium salt by acidification of crude glycerol derived from biodiesel production

Crude glycerol (CG) is a major byproduct of biodiesel production. Most of it cannot be utilized due to major impurities. The CG generally contains alkalis, which generate the residual salts in a series of its purification stages. This study aims to obtain the optimum process conditions and acid molar ratio to produce a higher potassium salt yield while improving the purity of glycerol by a simple acidification procedure. The CG was obtained from the transesterification of palm oil using a catalyst based on potassium carbonate. A phosphoric acid (85%) is utilized at various molar ratios and the process temperature is 60-80 °C. The strong acid was slowly added to the CG and heated for 30 minutes with a mixing speed of 250 rpm. The optimum acidification process occurred at a temperature of 70 °C with a molar crude glycerol ratio to phosphoric acid of 1 : 0.5. The glycerol purity was increased from 43.3% to 67.63% (w/w). It effectively obtains a potassium phosphate salt with a yield of 6.78%. The functional group infrared (IR) and X-ray fluorescence (XRF) spectra identified the salt residue as potassium dihydrogen phosphate (KH2PO4). This is composed predominantly of potassium oxide (K2O) and phosphorus pentoxide (P2O5), 50% and 47.9%, respectively.

Glycerol carbonate synthesis via transesterification of enriched glycerol and dimethyl carbonate using a Li-incorporated MCM-41 framework

Waste crude glycerol was successfully enriched and utilized as an inexpensive source for producing value-added chemicals, such as glycerol carbonate (GC) - a valuable compound with extensive industrial applications. The Li/MCM-41 heterogeneous catalyst was synthesized and used for the transesterification of enriched glycerol and dimethyl carbonate (DMC) to produce GC. The catalyst's physicochemical properties were characterized using thermogravimetric, Hammett indicator, inductively coupled plasma-optical emission spectroscopy, nitrogen adsorption-desorption, X-ray diffractometry, scanning electron microscopy, and Fourier-transform infrared spectroscopy analyses. Reaction conditions were optimized using response surface methodology and analysis of variance, yielding an accurate quadratic model to predict the GC yield under different transesterification variables. The results revealed that 5%Li/MCM-41 served as the optimal catalyst, achieving the highest TOF of 4.72 h-1. The DMC: enriched glycerol molar ratio had the greatest impact on the GC yield, with an R2 = 0.9743 and adjusted R2 = 0.9502. The optimal GC yield (58.77%) with a final purity of 78% was attained at a 5.15 wt% catalyst loading relative to the initial amount of enriched glycerol, DMC: enriched glycerol molar ratio of 4.24 : 1, and a reaction temperature of 86 °C for 165 min. The 5%Li/MCM-41 heterogeneous catalyst could be reused for four cycles with a decreased GC yield from 58.77% to 45.72%. Thus, the Li/MCM-41 catalyst demonstrated a remarkable efficiency and potential as a heterogeneous catalyst for synthesizing GC. This method not only contributes to environmental sustainability by making use of a byproduct from biodiesel production but also aligns with the principles of a circular economy.

Application of adaptive laboratory evolution to improve the tolerance of Rhodotorula strain to methanol in crude glycerol and development of an effective method for cell lysis

Rhodotorula toruloides can utilize crude glycerol as the low-cost carbon source for lipid production, but its growth is subjected to inhibition by methanol in crude glycerol. Here, transcriptome profiling demonstrated that 1004 genes were significantly regulated in the strain R. toruloides TO2 under methanol stress. Methanol impaired the function of membrane transport and subsequently weakened the utilization of glycerol, activities of the primary metabolism and functions of nucleus and ribosome. Afterwards the tolerance of TO2 to methanol was improved by using two-round adaptive laboratory evolution (ALE). The final strain M2-ale had tolerance up to 3.5% of methanol. 1 H NMR-based metabolome analysis indicated that ALE not only improved the tolerance of M2-ale to methanol but also tuned the carbon flux towards the biosynthesis of glycerolipid-related metabolites. The biomass and lipid titer of M2-ale reached 14.63 ± 0.45 g L-1 and 7.06 ± 0.44 g L-1 at 96 h in the crude glycerol medium, which increased up to 17.69% and 31.39%, respectively, comparing with TO2. Afterwards, an effective method for cell lysis was developed by combining sonication and enzymatic hydrolysis (So-EnH). The lytic effect of So-EnH was validated by using confocal imaging and flow cytometry. At last, lipid recovery rate reached 95.4 ± 2.7% at the optimized condition.

The effect of slime accumulated in a long-term operating UASB using crude glycerol to treat S-rich wastewater

An up-flow anaerobic sludge blanket (UASB) reactor targeting sulfate reduction was operated under a constant TOC/S-SO42- ratio of 1.5 ± 0.3 g C/g S for 639 days using crude glycerol as carbon source. A filamentous and fluffy flocculant material, namely slime-like substances (SLS), was gradually accumulated in the bioreactor after the cease of methanogenic activity. The accumulation of SLS was followed by a decrease in the removal efficiencies and a deterioration in the performance. Selected characteristics of SLS were investigated to explore the causes of its formation and the effect of SLS on the UASB performance. Results showed that glycerol fermentation and sulfate reduction processes taking place in the reactor were mainly accomplished in the bottom part of the UASB reactor, as the sludge concentration in the bottom was higher. The accumulation of SLS in the UASB reactor caused sludge flotation that further led to biomass washout, which decreased the sulfate and glycerol removal efficiencies. Batch activity tests performed with granular sludge (GS), slime-covered granular sludge (SCGS) and SLS showed that there was no difference between GS and SLS in the mechanism of glycerol fermentation and sulfate reduction. However, the specific sulfate reduction rate of GS was higher than that of SLS, while SLS showed a higher glycerol fermentation rate than that of GS. The different rates in GS and SLS were attributed to the higher relative abundances of fermentative microorganisms found in SLS and higher relative abundances of sulfate reducing bacteria (SRB) found in GS.

Biochemical and molecular characterization of a novel glycerol dehydratase from Klebsiella pneumoniae 2e with high tolerance against crude glycerol impurities

BACKGROUND

The direct bioconversion of crude glycerol, a byproduct of biodiesel production, into 1,3-propanediol by microbial fermentation constitutes a remarkably promising value-added applications. However, the low activity of glycerol dehydratase, which is the key and rate-limiting enzyme in the 1,3-propanediol synthetic pathway, caused by crude glycerol impurities is one of the main factors affecting the 1,3-propanediol yield. Hence, the exploration of glycerol dehydratase resources suitable for crude glycerol bioconversion is required for the development of 1,3-propanediol-producing engineered strains.

RESULTS

In this study, the novel glycerol dehydratase 2eGDHt, which has a tolerance against crude glycerol impurities from Klebsiella pneumoniae 2e, was characterized. The 2eGDHt exhibited the highest activity toward glycerol, with Km and Vm values of 3.42 mM and 58.15 nkat mg-1, respectively. The optimum pH and temperature for 2eGDHt were 7.0 and 37 °C, respectively. 2eGDHt displayed broader pH stability than other reported glycerol dehydratases. Its enzymatic activity was increased by Fe2+ and Tween-20, with 294% and 290% relative activities, respectively. The presence of various concentrations of the crude glycerol impurities, including NaCl, methanol, oleic acid, and linoleic acid, showed limited impact on the 2eGDHt activity. In addition, the enzyme activity was almost unaffected by the presence of an impurity mixture that mimicked the crude glycerol environment. Structural analyses revealed that 2eGDHt possesses more coil structures than reported glycerol dehydratases. Moreover, molecular dynamics simulations and site-directed mutagenesis analyses implied that the existence of unique Val744 from one of the increased coil regions played a key role in the tolerance characteristic by increasing the protein flexibility.

CONCLUSIONS

This study provides insight into the mechanism for enzymatic action and the tolerance against crude glycerol impurities, of a novel glycerol dehydratase 2eGDHt, which is a promising glycerol dehydratase candidate for biotechnological conversion of crude glycerol into 1,3-PDO.

Catalytic dehydration of glycerol over Cu-Fe-Al-based oxides: understanding changes in active sites throughout the reaction

The glycerol conversion into acetol using Fe, Al and Cu-based oxides was investigated. XRD results indicate the formation of nanosized particles with high phase dispersion, however, Raman, Mössbauer, 27Al NMR and XPS spectroscopies suggest the presence of iron(iii) oxide, Al2O3 and CuO phases. The FTIR with pyridine adsorption revealed high Lewis acidity. The TPR profile showed the reduction temperature range for the Fe3+ and Cu2+ sites, indicating the suitable condition for pretreatment. The N2 adsorption-desorption isotherms indicated the presence of micro-mesopores with interesting textural properties and specific area varying between 71 and 220 m2 g-1, while the porous morphology was observed by SEM and TEM images. The optimized catalytic tests showed glycerol conversion of 60% and acetol selectivity of 92% with 17% of coke according to TG profile. The recycling tests confirmed the efficiency of the solid, reaching 28% conversion and 91% acetol selectivity after four reuses and, after reactivation in an oxidizing atmosphere, the catalytic performance obtained results close to the second reuse. The interaction between the different Lewis acid sites involved in the mechanisms for the acetol and coke formation on the catalyst surface is discussed. The charge distribution represented by colors which indicates the acid-base surface was evaluated by a simple theoretical-computational study based on the DFT approach. The synergy between the active sites indicates that the presence of Cu0/Cu+ drastically increases the acetol selectivity which is a more important characteristic than the high Lewis acidity of Fen+ and Al3+.

Economical Production of Phenazine-1-carboxylic Acid from Glycerol by Pseudomonas chlororaphis Using Cost-Effective Minimal Medium

Phenazine compounds are widely used in agricultural control and the medicine industry due to their high inhibitory activity against pathogens and antitumor activity. The green and sustainable method of synthesizing phenazine compounds through microbial fermentation often requires a complex culture medium containing tryptone and yeast extract, and its cost is relatively high, which greatly limits the large-scale industrial production of phenazine compounds by fermentation. The aim of this study was to develop a cost-effective minimal medium for the efficient synthesis of phenazine compounds by Pseudomonas chlororaphis. Through testing the minimum medium commonly used by Pseudomonas, an ME medium for P. chlororaphis with a high production of phenazine compounds was obtained. Then, the components of the ME medium and the other medium were compared and replaced to verify the beneficial promoting effect of Fe2+ and NH4+ on phenazine compounds. A cost-effective general defined medium (GDM) using glycerol as the sole carbon source was obtained by optimizing the composition of the ME medium. Using the GDM, the production of phenazine compounds by P. chlororaphis reached 1073.5 mg/L, which was 1.3 times that achieved using a complex medium, while the cost of the GDM was only 10% that of a complex medium (e.g., the KB medium). Finally, by engineering the glycerol metabolic pathway, the titer of phenazine-1-carboxylic acid reached the highest level achieved using a minimum medium so far. This work demonstrates how we systematically analyzed and optimized the composition of the medium and integrated a metabolic engineering method to obtain the most cost-effective fermentation strategy.

Valorisation of crude glycerol in the production of liquefied lignin bio-polyols for polyurethane formulations

Bio-polyols, produced by liquefying lignin with polyhydric alcohols, offer a promising alternative to conventional polyols for polyurethane production. To enhance the sustainability on the production of these bio-polyols, this study proposes the use of crude glycerol and microwave-assisted liquefaction as substitutes for conventional methods and commercial glycerol. This approach reduces the energy requirements of the reaction while also adding value to this by-product. The synthesis of bio-polyols with suitable properties to produce elastic and rigid polyurethane was carried out using previously optimised reaction conditions. Organosolv lignins obtained from Eucalyptus globulus and Pinus radiata were employed, using polyethylene glycol and crude glycerol as solvents and sulphuric acid as a catalyst. Several parameters of the bio-polyols were analysed, including hydroxyl number (IOH), acid number (An), and functionality (f), suggesting that the bio-polyols were suitable for polyurethane synthesis. Bio-polyols formulated to produce rigid polyurethanes exhibited IOH values of 554 and 383 (mg KOH/g), An values of 1.91 and 4.21 (mg KOH/g), and functionalities of 4.16 and 3.14 for Eucalyptus globulus and Pinus radiata lignin. In the case of bio-polyols for elastic polyurethanes, the values were 228 and 173 (mg KOH/g) (IOH), 20.94 and 25.09 (mg KOH/g) (An), and functionalities of 3.51 and 2.08.

High production of poly(3-hydroxybutyrate) in Escherichia coli using crude glycerol

Poly(3-hydroxybutyrate) (PHB) is a prominent bio-plastic and recognized as the potential replacement of petroleum-derived plastics. To make PHB cost-effective, the production scheme based on crude glycerol was developed using Escherichia coli. The heterogeneous synthesis pathway of PHB was introduced into the E. coli strain capable of efficiently utilizing glycerol. The central metabolism that links to the synthesis of acetyl-CoA and NADPH was further reprogrammed to improve the PHB production. Key genes were targeted for manipulation, involving those in glycolysis, the pentose phosphate pathway, and the tricarboxylic cycle. As a result, the engineered strain gained a 22-fold increase in the PHB titer. Finally, the fed-batch fermentation was conducted with the producer strain to give the PHB titer, content, and productivity reaching 36.3±3.0 g/L, 66.5±2.8%, and 1.2±0.1 g/L/h, respectively. The PHB yield on crude glycerol accounts for 0.3 g/g. The result indicates that the technology platform as developed is promising for the production of bio-plastics.

Water footprint and wastewater quality assessment of yeast single cell oil production: Gate to gate approach for industrial water sustainability

Effective water resource utilization and sustainability for industrial operations is a growing concern. With increased industrial water demand, abstraction and water quality changes are rising. In India, distilleries generate more than 40.4 billion litres of effluent daily within the fermentation industry. Water, a public good with market and opportunity costs, needs effective mapping and management. Emerging distillery processes such as yeast lipid fermentation, if developed along with water sustainability, could aid in advancing water resource management. In the scope of this idea, the present study focuses on assessing the water footprint and water quality mapping for Rhodotorula mucilaginosa IIPL32 lipid production using crude glycerol, a by-product of the biodiesel industry. The assessment was based on primary data generated during the 500 L plant scale operation. The process's blue water footprint was assessed by applying a chain-summation approach, and the grey water requirement was determined by measuring water quality parameters for the effluent streams. The process's net blue and grey water footprint were estimated to be 3.87 and 23.66 m³ water/kg of lipid, respectively. Water quality index ratings were identified for all the respective water streams within the processing system, and human risk factors were estimated. The results suggested proper treatment of the spent broth, whereas the secondary effluent stream from cleaning operations could be reutilized within the system. Quality mapping also suggested that the effluent's high organic and mineral load can be processed for water and material recovery, which may significantly reduce the process's grey water and pollution load.

Polyhydroxybutyrate production from crude glycerol using a highly robust bacterial strain Halomonas sp. YLGW01

Petrochemical-based plastics are hardly biodegradable and a major cause of environmental pollution, and polyhydroxybutyrate (PHB) is attracting attention as an alternative due to its similar properties. However, the cost of PHB production is high and is considered the greatest challenge for its industrialization. Here, crude glycerol was used as a carbon source for more efficient PHB production. Among the 18 strains investigated, Halomonas taeanenisis YLGW01 was selected for PHB production due to its salt tolerance and high glycerol consumption rate. Furthermore, this strain can produce poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(3HB-co-3 HV)) with 17 % 3 HV mol fraction when a precursor is added. PHB production was maximized through medium optimization and activated carbon treatment of crude glycerol, resulting in 10.5 g/L of PHB with 60 % PHB content in fed-batch fermentation. Physical properties of the produced PHB were analyzed, i.e., weight average molecular weight (6.8 × 10⁵), number average molecular weight (4.4 × 10⁵), and the polydispersity index (1.53). In the universal testing machine analysis, the extracted intracellular PHB showed a decrease in Young's modulus, an increase in Elongation at break, greater flexibility than authentic film, and decreased brittleness. This study confirmed that YLGW01 is a promising strain for industrial PHB production using crude glycerol.

Economic and environmental evaluation for a closed loop of crude glycerol bioconversion to biodiesel

Crude glycerol, a byproduct of biodiesel production, was utilized as a carbon source to produce microbial lipids by the oleaginous yeast Rhodotorula toruloides in this study. The maximum lipid production and lipid content were 10.56 g/L and 49.52%, respectively, by optimizing fermentation conditions. The obtained biodiesel met the standards of China, the United States, and the European Union. The economic value of biodiesel produced from crude glycerol increased by 48% compared with the sale of crude glycerol. In addition, biodiesel production from crude glycerol could reduce 11,928 tons of carbon dioxide emissions and 55 tons of sulfur dioxide emissions. This study provides a strategy for a closed loop of crude glycerol to biofuel and ensures sustainable and stable development of the biodiesel industries.

Characterization of crude glycerol and glycerol pitch from palm-based residual biomass

Crude glycerol (CG) and glycerol pitch (GP) are highly alkaline residues from biodiesel and oleochemical plants, respectively, and have organic content which incurs high disposal cost and poses an environmental threat. Characterization of these residues for composition and properties could provide insight into their quality for proper disposal and can help the biodiesel industry to adopt more sustainable practices, such as reducing waste and improving the efficiency of the production process, hence minimizing the impact of the biodiesel supply chain to the environment. These data also allow the identification and exploration of new ways for their utilization and transformation into highly value-added products. In this study, we evaluated four CG samples (B, C, D, and E) and two GP samples (F and G) obtained from Malaysian palm oil refineries, and the results were compared with pure glycerol (A). Spectroscopic analysis was performed using FTIR, 1H-, and 13C-NMR. All samples had similar density to A (1.26 g/cm3), except for F (1.31 g/cm3), while the density for E and G could not be determined due to their physical states. The pH and viscosity largely varied in the range of 7.26-11.89 and 43-225 cSt, respectively. The glycerol content of CG (B, C, D, and E) was high and consistent (81.7-87.3%) whereas GP F and G had 71.5 and 63.9% glycerol content, respectively. Major contaminants in CG and GP were water and matter organic non-glycerol (MONG), respectively. The water, ash, soap, and salt content were considerably low, which varied from 3.4 to 14.1%, 3.9 to 13.0%, 0.1 to 5.7%, and 4.1 to 9.2% respectively. Thermal analysis of CG and GP exhibited four phases of decomposition attributed to the impurities compared to the single phase in A. All samples had calorific values lower than A (18.1 MJ/kg) between 9.0 and 17.7 MJ/kg. Based on the results, CG and GP have high glycerol content which reveals their potential to be used as feedstock in bioconversion and chemical or thermal treatment while impurities may be removed by pre-treatment if required. As palm oil is one of the main feedstocks for the oleochemical industry, this work underlines the importance of characterization of the residue generated to provide additional data and information on palm-based agricultural industry wastes, minimize the impact of palm oil supply chain on the environment, and explore its potential usage for value-addition.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13399-023-04003-4.

Establishing a growth-coupled mechanism for high-yield production of β-arbutin from glycerol in Escherichia coli

Biodiesel production has increased significantly in recent years, leading to an increase in the production of crude glycerol. In this study, a novel growth-coupled erythrose 4-phosphate (E4P) formation strategy that can be used to produce high levels of β-arbutin using engineered Escherichia coli and glycerol as the carbon source was developed. In the strategy, E4P formation was coupled with cell growth, and a growth-driving force made the E4P formation efficient. By applying this strategy, efficient microbial synthesis of β-arbutin was achieved, with 7.91 g/L β-arbutin produced in shaking flask, and 28.1 g/L produced in a fed batch fermentation with a yield of 0.20 g/g glycerol and a productivity of 0.39 g/L/h. This is the highest β-arbutin production through microbial fermentation ever reported to date. This study may have significant implications in the large-scale production of β-arbutin as well as other aromatic compounds of importance.

Second generation Pichia pastoris strain and bioprocess designs

Yeast was the first microorganism used by mankind for biotransformation processes that laid the foundations of industrial biotechnology. In the last decade, Pichia pastoris has become the leading eukaryotic host organism for bioproduct generation. Most of the P. pastoris bioprocess operations has been relying on toxic methanol and glucose feed. In the actual bioeconomy era, for sustainable value-added bioproduct generation, non-conventional yeast P. pastoris bioprocess operations should be extended to low-cost and renewable substrates for large volume bio-based commodity productions. In this review, we evaluated the potential of P. pastoris for the establishment of circular bioeconomy due to its potential to generate industrially relevant bioproducts from renewable sources and waste streams in a cost-effective and environmentally friendly manner. Furthermore, we discussed challenges with the second generation P. pastoris platforms and propose novel insights for future perspectives. In this regard, potential of low cost substrate candidates, i.e., lignocellulosic biomass components, cereal by-products, sugar industry by-products molasses and sugarcane bagasse, high fructose syrup by-products, biodiesel industry by-product crude glycerol, kitchen waste and other agri-food industry by products were evaluated for P. pastoris cell growth promoting effects and recombinant protein production. Further metabolic pathway engineering of P. pastoris to construct renewable and low cost substrate utilization pathways was discussed. Although, second generation P. pastoris bioprocess operations for valorisation of wastes and by-products still in its infancy, rapidly emerging synthetic biology tools and metabolic engineering of P. pastoris will pave the way for more sustainable environment and bioeconomy. From environmental point of view, second generation bioprocess development is also important for waste recycling otherwise disposal of carbon-rich effluents creates environmental concerns. P. pastoris high tolerance to toxic contaminants found in lignocellulosic biomass hydrolysate and industrial waste effluent crude glycerol provides the yeast with advantages to extend its applications toward second generation P. pastoris strain design and bioprocess engineering, in the years to come.

Production and recovery of polyhydroxyalkanoates (PHA) from waste streams - A review

Polyhydroxyalkanoates (PHA) are the more attractive sustainable green plastic, and it has the potential to replace petroleum-based plastics (PBP) in the global market. Recently, most of the developed and developing countries have banned the use of traditional PBP. This increases the demand for green plastic production and positively impacts the global market. Producing green plastic from various waste streams such as whey, animal, and crude glycerol will be eco-friendly and cost-effective. However, the factors influencing the environmental sustainability of PHA production from different waste streams are still unclear. This review could be reinforced concrete to researchers to gather deep knowledge on techno-economic analysis, life-cycle assessment, environmental and ecological risks caused during PHA production from different waste streams.

A co-fermentation strategy with wood hydrolysate and crude glycerol to enhance the lipid accumulation in Rhodosporidium toruloides-1588

Wood hydrolysate has been regarded as sustainable and renewable substrate to produce microbial lipids, a potential feedstock for the biodiesel industry. Moreover, the major by-product of biofuel industries is crude glycerol but its implementation as a carbon source is still constrained due to the presence of impurities resulting in low biomass production and low lipid titer. Thus, this study investigates the effect of different carbon ratios of hydrolysate and crude glycerol on R. toruloides-1588. Hydrolysate to crude glycerol ratio of 60:40 resulted in maximum lipid accumulation of 49% (w/w), more than 90% of sugars and glycerol consumption. Further, scale-up to bench-scale fermenter resulted in 12% higher lipid accumulation (56.3% w/w, 0.15 g/L∙h) in 50% less time than flask fermentation. Hence, the ability of R. toruloides-1588 to flourish on different carbohydrates and accumulate high lipid content will be beneficial for the further development of biorefinery industries.

Isopropanol biosynthesis from crude glycerol using fatty acid precursors via engineered oleaginous yeast Yarrowia lipolytica

Background

Isopropanol is widely used as a biofuel and a disinfectant. Chemical preparation of isopropanol destroys the environment, which makes biological preparation of isopropanol necessary. Previous studies focused on the use of expensive glucose as raw material. Therefore, the microbial cell factory that ferments isopropanol with cheap raw materials will provide a greener way to produce isopropanol.

Results

This study converted crude glycerol into isopropanol using Y. lipolytica. As a microbial factory, the active natural lipid and fatty acid synthesis pathway endows Y. lipolytica with high malonyl-CoA production capacity. Acetoacetyl-CoA synthase (nphT7) and isopropanol synthesis genes are integrated into the Y. lipolytica genome. The nphT7 gene uses the accumulated malonyl-CoA to synthesize acetoacetyl-CoA, which increases isopropanol production. After medium optimization, the best glycerol medium was found and resulted in a 4.47-fold increase in isopropanol production. Fermenter cultivation with pure glycerol medium resulted in a maximum isopropanol production of 1.94 g/L. In a crude glycerol fermenter, 1.60 g/L isopropanol was obtained, 82.53% of that achieved with pure glycerol. The engineered Y. lipolytica in this study has the highest isopropanol titer reported.

Conclusions

The engineered Y. lipolytica successfully produced isopropanol by using crude glycerol as a cheap carbon source. This is the first study demonstrating the use of Y. lipolytica as a cell factory to produce isopropanol. In addition, this is also a new attempt to accumulate lipid synthesis precursors to synthesize other useful chemicals by integrating exogenous genes in Y. lipolytica.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01890-6.

Identification and characterization of chemical and physical stability of insulin formulations utilizing degraded glycerol after repeated use and storage

Insulin treatment is currently considered to be the main strategy for controlling diabetes. Although the recombinant insulin formulation is relatively mature, we found that a batch of insulin formulation exhibited an unusual degradation rate in the stability experiment. The main purposes of this article are to identify the root cause for this phenomenon and characterize of chemical and physical degradation products. We compared the chemical and physical stability of two batches of insulin formulations prepared separately with simulated repeated use and freshly opened glycerol. The chemical stability of insulin was identified by liquid chromatography coupled with tandem mass spectrometry (LC- MS/MS). Micro-flow imaging (MFI), far-ultraviolet circular dichroism (Far-UV CD) and Thioflavin T (ThT) fluorescent assays were used to reveal protein aggregation and fibrosis. The chemical and physical stability of the insulin formulation with newly opened glycerol was much better than that with degraded glycerol, and both groups of formulations were extremely sensitive to light. The results indicated that the original batch insulin formulation with abnormal stability was indeed caused by the excipient glycerol after long-term storage and repeated usage. More attention should be paid to the quality changes of excipients during repeated usage and storage of excipients for the practical purpose. Moreover, we have discovered a novel degradation pathway for insulin and peptides in general. In addition, LC-MS/MS results suggested that the N-terminus of insulin B-chain was prone to chemical degradation which enlightens that it could be potentially modified to improve the stability of insulin formulations.

Exploring Yeast Diversity to Produce Lipid-Based Biofuels from Agro-Forestry and Industrial Organic Residues

Exploration of yeast diversity for the sustainable production of biofuels, in particular biodiesel, is gaining momentum in recent years. However, sustainable, and economically viable bioprocesses require yeast strains exhibiting: (i) high tolerance to multiple bioprocess-related stresses, including the various chemical inhibitors present in hydrolysates from lignocellulosic biomass and residues; (ii) the ability to efficiently consume all the major carbon sources present; (iii) the capacity to produce lipids with adequate composition in high yields. More than 160 non-conventional (non-Saccharomyces) yeast species are described as oleaginous, but only a smaller group are relatively well characterised, including Lipomyces starkeyi, Yarrowia lipolytica, Rhodotorula toruloides, Rhodotorula glutinis, Cutaneotrichosporonoleaginosus and Cutaneotrichosporon cutaneum. This article provides an overview of lipid production by oleaginous yeasts focusing on yeast diversity, metabolism, and other microbiological issues related to the toxicity and tolerance to multiple challenging stresses limiting bioprocess performance. This is essential knowledge to better understand and guide the rational improvement of yeast performance either by genetic manipulation or by exploring yeast physiology and optimal process conditions. Examples gathered from the literature showing the potential of different oleaginous yeasts/process conditions to produce oils for biodiesel from agro-forestry and industrial organic residues are provided.

Valorization of Low-Cost Substrates for the Production of Odd Chain Fatty Acids by the Oleaginous Yeast Yarrowia lipolytica

Odd-chain fatty acids (OCFAs) have recently gained interest as target compounds in microbial production due to their diverse applications in the medical, pharmaceutical and chemical industries for the production of biofuels. Yarrowia lipolytica is a promising oleaginous yeast that has the ability to accumulate high quantities of fatty acids. However, the use of Y. lipolytica oils is still under research, in order to decrease the production costs related to the fermentation process and improve economic feasibility. In this work, sugar beet molasses (10–50 g/L) and crude glycerol (30 g/L) were used as the main carbon sources to reduce the processing costs of oil production from a genetically engineered Y. lipolytica strain. The effects of medium composition were studied on biomass production, lipid content, and OCFAs profile. Lipid production by yeast growing on molasses (20 g/L sucrose) and crude glycerol reached 4.63 ± 0.95 g/L of culture medium. OCFAs content represented 58% of the total fatty acids in lipids, which corresponds to ≈2.69 ± 0.03 g/L of culture medium. The fermentation was upscaled to 5 L bioreactors and fed-batch co-feeding increased OCFA accumulation in Y. lipolytica by 56% compared to batch cultures.

Production, Biosynthesis, and Commercial Applications of Fatty Acids From Oleaginous Fungi

Oleaginous fungi (including fungus-like protists) are attractive in lipid production due to their short growth cycle, large biomass and high yield of lipids. Some typical oleaginous fungi including Galactomyces geotrichum, Thraustochytrids, Mortierella isabellina, and Mucor circinelloides, have been well studied for the ability to accumulate fatty acids with commercial application. Here, we review recent progress toward fermentation, extraction, of fungal fatty acids. To reduce cost of the fatty acids, fatty acid productions from raw materials were also summarized. Then, the synthesis mechanism of fatty acids was introduced. We also review recent studies of the metabolic engineering strategies have been developed as efficient tools in oleaginous fungi to overcome the biochemical limit and to improve production efficiency of the special fatty acids. It also can be predictable that metabolic engineering can further enhance biosynthesis of fatty acids and change the storage mode of fatty acids.

Bioprocesses for the Biodiesel Production from Waste Oils and Valorization of Glycerol

The environmental context causes the use of renewable energy to increase, with the aim of finding alternatives to fossil-based products such as fuels. Biodiesel, an alternative to diesel, is now a well-developed solution, and its production from renewable resources makes it perfectly suitable in the environmental context. In addition, it is biodegradable, non-toxic and has low greenhouse gas emissions: reduced about 85% compared to diesel. However, the feedstock used to produce biodiesel competes with agriculture and the application of chemical reactions is not advantageous with a “green” process. Therefore, this review focuses only on bioprocesses currently taking an important place in the production of biodiesel and allow high yields, above 90%, and with very few produced impurities. In addition, the use of waste oils as feedstock, which now accounts for 10% of feedstocks used in the production of biodiesel, avoids competition with agriculture. To present a complete life-cycle of oils in this review, a second part will focus on the valorization of the biodiesel by-product, glycerol. About 10% of glycerol is generated during the production of biodiesel, so it should be recovered to high value-added products, always based on bioprocesses. This review will also present existing techniques to extract and purify glycerol. In the end, from the collection of feedstocks to the production of CO2 during the combustion of biodiesel, this review presents the steps using the “greener” possible processes.

Microbial cell factories for the production of three-carbon backbone organic acids from agro-industrial wastes

At present, mass production of basic and valuable commodities is dependent on linear petroleum-based industries, which ultimately makes the depletion of finite natural reserves and accumulation of non-biodegradable and hazardous wastes. Therefore, an ecofriendly and sustainable solution should be established for a circular economy where infinite resources, such as agro-industrial wastes, are fully utilized as substrates in the production of target value-added chemicals. Hereby, recent advances in metabolic engineering strategies and techniques used in the development of microbial cell factories for enhanced production of three-carbon platform chemicals such as lactic acid, propionic acid, and 3-hydroxypropionic acid are discussed. Further developments and future perspectives in the production of these organic acids from agro-industrial wastes from the dairy, sugar, and biodiesel industries are also highlighted to demonstrate the importance of waste-based biorefineries for organic acid production.

The reliance of glycerol utilization by Cupriavidus necator on CO2 fixation and improved glycerol catabolism

While crude glycerol is a cheap carbon source for industrial-scale cultivation of microorganisms, its application relies on fast growth and conversion. The biopolymer producing Cupriavidus necator H16 (synonym: Ralstonia eutropha H16) grows poorly on glycerol. The heterologous expression of glycerol facilitator glpF, glycerol kinase glpK, and glycerol dehydrogenase glpD from E. coli accelerated the growth considerably. The naturally occurring glycerol utilization is inhibited by low glycerol kinase activity. A limited heterotrophic growth promotes the dependency on autotrophic growth by carbon dioxide (CO₂) fixation and refixation. As mixotrophic growth occurs in the wildtype due to low consumption rates of glycerol, CO₂ fixation by the Calvin-Benson-Bassham (CBB) cycle is essential. The deletion of both cbbX copies encoding putative RuBisCO-activases (AAA + ATPase) resulted in a sharp slowdown of growth and glycerol consumption. Activase activity is necessary for functioning carboxylation by RuBisCO. Each of the two copies compensates for the loss of the other, as suggested by observed expression levels. The strong tendency towards autotrophy supports previous investigations of glycerol growth and emphasizes the versatility of the metabolism of C. necator H16. Mixotrophy with glycerol-utilization and CO₂ fixation with a high dependence on the CBB is automatically occurring unless transportation and degradation of glycerol are optimized. Parallel engineering of CO₂ fixation and glycerol degradation is suggested towards application for value-added production from crude glycerol. KEY POINTS: • Growth on glycerol is highly dependent on efficient carbon fixation via CBB cycle. • CbbX is essential for the efficiency of RuBisCO in C. necator H16. • Expression of glycerol degradation pathway enzymes accelerates glycerol utilization.

Use of hydrous ABE-glycerin-diesel microemulsions in a nonroad diesel engine - Performance and unignorable emissions

Oversupply, extra energy consumption, and CO₂ emissions from the refinery of biodiesel-derived glycerin (G) led to the consideration of its use as an alternative fuel. In this study, a nonroad diesel engine generator was employed to represent potential emissions under stringent regulated standards. G-diesel has been reported to reduce nitrogen oxides (NOx) and soot levels but increase CO and hydrocarbon emissions. A bio-producible acetone-butanol-ethanol (ABE) solution with multiple polarities was added to stabilize the glycerin and water in diesel examined in this study. A series of ABE-G-diesel blends were prepared to form the thermostable microemulsions. Four blends with small and well-dispersed bubbles were tested in the engine generator. The specific thermal efficiencies of the engine were slightly improved by using ABE-G from regular diesel due to better spray quality, longer ignition delay, and fuel-oxygen content that would enhance combustion. Meanwhile, the PM-NOx-CO emission trade-off in the previous study has been overcome by using ABE-G-diesel since the better fuel atomization and more premixed combustion were approached, as well as the lower and homogeneous in-cylinder temperature caused by water content and micro-explosion. However, the condensable particulate matter and nitro-PAHs were also observed and realized their unignorable contribution, which has not been regulated and even researched for the generators. Fortunately, the new fuels could inhibit both of them to a certain degree. Consequently, this study proposes using recyclable glycerin with a simple pretreatment mixed with ABE and diesel for greener nonroad diesel engine especially those equipped with low-grade aftertreatment.

Biochemical biorefinery: A low-cost and non-waste concept for promoting sustainable circular bioeconomy

The transition from a fossil-based linear economy to a circular bioeconomy is no longer an option but rather imperative, given worldwide concerns about the depletion of fossil resources and the demand for innovative products that are ecocompatible. As a critical component of sustainable development, this discourse has attracted wide attention at the regional and international levels. Biorefinery is an indispensable technology to implement the blueprint of the circular bioeconomy. As a low-cost, non-waste innovative concept, the biorefinery concept will spur a myriad of new economic opportunities across a wide range of sectors. Consequently, scaling up biorefinery processes is of the essence. Despite several decades of research and development channeled into upscaling biorefinery processes, the commercialization of biorefinery technology appears unrealizable. In this review, challenges limiting the commercialization of biorefinery technologies are discussed, with a particular focus on biofuels, biochemicals, and biomaterials. To counteract these challenges, various process intensification strategies such as consolidated bioprocessing, integrated biorefinery configurations, the use of highly efficient bioreactors, simultaneous saccharification and fermentation, have been explored. This study also includes an overview of biomass pretreatment-generated inhibitory compounds as platform chemicals to produce other essential biocommodities. There is a detailed examination of the technological, economic, and environmental considerations of a sustainable biorefinery. Finally, the prospects for establishing a viable circular bioeconomy in Nigeria are briefly discussed.

Identification of a Novel Alditol Oxidase from Thermopolyspora flexuosa with Potential Application in D-Glyceric Acid Production

Glycerol is a potential sustainable feedstock, and biorefining processes to convert glycerol into value-added chemicals have been developed over the past decade. Alditol oxidase (AldO) is capable of selectively oxidizing the primary hydroxyl groups of alditols such as glycerol. In this study, a new FAD-binding protein from Thermopolyspora flexuosa was expressed and identified as a novel alditol oxidase (AldO_{T. fle}). AldO_{T. fle} displayed the optimal activity at pH 8.0 and 25 °C. AldO_{T. fle} was not metal-dependent, but the activity was completely inhibited by Fe³⁺. AldO_{T. fle} had a wide substrate specificity and high catalytic efficiency for glycerol. Furthermore, the recombinant AldO_{T. fle} could produce D-glyceric acid from glycerol with a conversion rate ranging from 86.6% (5 mM glycerol) to 20.5% (500 mM glycerol). The recombinant E. coli with AldO_{T. fle} could also produce 23.8 mM D-glyceric acid from 100 mM glycerol. The recombinant AldO_{T. fle} had the potential to produce other aldehyde products by selectively oxidizing the hydroxyl groups of alditols and many other commodity chemicals by redesigning glycerol metabolism.

An Overview of the Latest Advances in the Catalytic Synthesis of Glycerol Carbonate

In recent years, the development of renewable energy alternatives to traditional fossil fuels has become one of the major challenges all over the world, due to the decline of fossil fuel reserves and their effect on global warming. Biodiesel has become a popular alternative energy source to reduce gas emissions compared to traditional fossil fuels. According to statistics, a nine-fold increase in global biofuel production between 2000 and 2020 was observed. However, its production generates a large amount of glycerol as a by-product, posing an environmental problem when disposed directly in landfills or by incineration. Therefore, low-value glycerol should be converted into high value-added derivatives. As glycerol carbonate is one of the most important derivatives of glycerol, this review aims to discuss the studies over the last ten years about glycerol carbonate synthetic methods, including the typical routes such as phosgene, esterification reaction, urea, oxidative and direct carbonylation as well as several rare synthetic procedures. At the same time, it summarizes the different catalytic reaction systems of each route comparing the advantages and disadvantages of various catalysts and evaluating their catalytic activity. Finally, the future development of glycerol carbonate synthesis is prospected from the point of view of development, technology research and industrialization.

Production of biosurfactants from agro-industrial waste and waste cooking oil in a circular bioeconomy: An overview

Waste generation is becoming a global concern owing to its adverse effects on environment and human health. The utilization of waste as a feedstock for production of value-added products has opened new avenues contributing to environmental sustainability. Microorganisms have been employed for production of biosurfactants as secondary metabolites by utilizing waste streams. Utilization of waste as a substrate significantly reduces the cost of overall process. Biosurfactant(s) derived from these processes can be utilized in environmental and different industrial sectors. This review focuses on global market of biosurfactants followed by discussion on production of biosurfactants from waste streams such as agro-industrial waste and waste cooking oil. The need for waste stream derived circular bioeconomy and scale up of biosurfactant production have been narrated with applications of biosurfactants in environment and industrial sectors. Road blocks and future directions for research have also been discussed.

Chemical looping steam reforming of glycerol for hydrogen production over NiO–Fe2O3/Al2O3 oxygen carriers

Fe-based oxygen carriers (OCs) are widely used in chemical looping steam reforming (CLSR) due to excellent resistance to carbon buildup, low toxicity, and high activity. In this study, a type of nano NiO–Fe₂O₃/Al₂O₃ Fe-based OC that can easily be reduced by fuels and re-oxidized by air was developed for use in glycerol CLSR. It was synthesized by co-precipitation and impregnation. Based on the quadratic regression orthogonal model, a quadratic polynomial function was established to investigate the effects of temperature (T), water/carbon ratio (S/C), and loading (M) on hydrogen content (HL) and hydrogen selectivity (S). The OCs were characterized by XRD, XPS, SEM/EDX-mapping, TEM, and H₂-TPR to determine their physicochemical properties. XPS shows the Fe phase highly interacted with the Al₂O₃ supporting matrix by forming Fe aluminates in NiO–Fe₂O₃/Al₂O₃. The S (85.33%) and HL (78.41%) were obtained under the optimal conditions T = 600 °C, S/C = 1.0 mol mol⁻¹ and M = 0. A hydrogen content fluctuation within 4% was obtained under T = 700 °C, S/C = 1.0 mol mol⁻¹, and M = 2.5%, which means the cycle stability is perfect because of the addition of Ni. This study provides a basis for the development of efficient oxygen carriers in the CLSR system.

A nano NiO–Fe₂O₃/Al₂O₃ Fe-based OC was developed for CLSR hydrogen production experiments in a modified device. Ni promoted Fe-Al2O3 is effective and stable.

Recovery and utilization of crude glycerol, a biodiesel byproduct

Biodiesel production has increased significantly in the past decade because it has been demonstrated to be a viable alternative and renewable fuel. Consequently, the production of crude glycerol, the main byproduct of the transesterification of lipids to biodiesel, has risen as well. Therefore, the effective recovery and utilization of crude glycerol can provide biodiesel with additional value. In this review, we first summarized the state-of-the-art progress on crude glycerol recovery and purification. Subsequently, numerous approaches have been reviewed for the utilization of crude glycerol, including use as animal feeds, for combustion, anaerobic fermentation, and chemical conversion. Finally, an extensive discussion and outlook is presented in relation to the techniques and processes in the chemical conversion of crude glycerol.

In this review, we summarize the latest technologies for the recovery and purification of crude glycerol and applications of crude glycerol, with focus on its chemical conversion.

Assessing main process mechanism and rates of sulfate reduction by granular biomass fed with glycerol under sulfidogenic conditions

Sulfate-reducing bioreactors for sulfide production are the initial stage of processes targeting elemental sulfur recovery from sulfate-rich effluents. In this work, the principal reactions involved in glycerol fermentation and sulfate reduction using glycerol and its fermentation products as electron donors were assessed together with their specific consumption/production rates. A battery of batch activity tests with and without sulfate were performed with glycerol and with each fermentation product using a non-methanogenic but sulfidogenic granular sludge from an up-flow anaerobic sludge blanket (UASB) reactor operated under long-term while fed with crude glycerol. As a result, a mechanistic approach based on the experimental observations is proposed in this work. Glycerol was mainly fermented to 1,3-propanediol, ethanol, formate, propionate and acetate by fermentative bacteria. All organic intermediates were found to be further used by sulfate reducing bacteria (SRB) for sulfate reduction except for acetate. The most abundant genus detected under sulfidogenic conditions were Propionispora (15.2%), Dysgonomonas (13.2%), Desulfobulbus (11.6%) and Desulfovibrio (10.8%). The last two SRB genera accounted for 22.4% of the total amount of retrieved sequences, which were probably performing an incomplete oxidation of the carbon source in the sulfidogenic UASB reactor. As single substrates, specific sulfate reduction rates (SRRs) using low molecular weight (MW) carbon sources (formate and ethanol) were 39% higher than those using high-MW ones (propionate, 1,3-propanediol and butanol). However, SRRs in glycerol-fed tests showed that 1,3-propanediol played a major role in sulfate reduction in addition to formate and ethanol.

Engineering of Corynebacterium glutamicum for high-level γ-aminobutyric acid production from glycerol by dynamic metabolic control

Synthetic biology seeks to reprogram microbial cells for efficient production of value-added compounds from low-cost renewable substrates. A great challenge of chemicals biosynthesis is the competition between cell metabolism and target product synthesis for limited cellular resource. Dynamic regulation provides an effective strategy for fine-tuning metabolic flux to maximize chemicals production. In this work, we created a tunable growth phase-dependent autonomous bifunctional genetic switch (GABS) by coupling growth phase responsive promoters and degrons to dynamically redirect the carbon flux for metabolic state switching from cell growth mode to production mode, and achieved high-level GABA production from low-value glycerol in Corynebacterium glutamicum. A ribosome binding sites (RBS)-library-based pathway optimization strategy was firstly developed to reconstruct and optimize the glycerol utilization pathway in C. glutamicum, and the resulting strain CgGly2 displayed excellent glycerol utilization ability. Then, the initial GABA-producing strain was constructed by deleting the GABA degradation pathway and introducing an exogenous GABA synthetic pathway, which led to 5.26 g/L of GABA production from glycerol. In order to resolve the conflicts of carbon flux between cell growth and GABA production, we used the GABS to reconstruct the GABA synthetic metabolic network, in which the competitive modules of GABA biosynthesis, including the tricarboxylic acid (TCA) cycle module and the arginine biosynthesis module, were dynamically down-regulated while the synthetic modules were dynamically up-regulated after sufficient biomass accumulation. Finally, the resulting strain G7-1 accumulated 45.6 g/L of GABA with a yield of 0.4 g/g glycerol, which was the highest titer of GABA ever reported from low-value glycerol. Therefore, these results provide a promising technology to dynamically balance the metabolic flux for the efficient production of other high value-added chemicals from a low-value substrate in C. glutamicum.

Production of bacterial cellulose from glycerol: the current state and perspectives

Current research in industrial microbiology and biotechnology focuses on the production of biodegradable microbial polymers as an environmentally friendly alternative to the still dominant fossil hydrocarbon-based plastics. Bacterial cellulose (BC) is important among microbial polymers due to its valuable properties and broad applications in variety of fields from medical to industrial technologies. However, the increase in BC production and its wider deployment is still limited by high costs of traditionally used raw materials. It is therefore necessary to focus on less expensive inputs, such as agricultural and industrial by-products or waste including the more extended use of glycerol. It is the environmentally harmful by-product of biofuel production and reducing it will also reduce the risk of environmental pollution. The experimental data obtained so far confirm that glycerol can be used as the renewable carbon source to produce BC through more efficient and environmentally friendly bioprocesses. This review summarizes current knowledge on the use of glycerol for the production of commercially prospective BC, including information on producer cultures, fermentation modes and methods used, nutrient medium composition, cultivation conditions, and bioprocess productivity. Data on the use of some related sugar alcohols, such as mannitol, arabitol, xylitol, for the microbial synthesis of cellulose are also considered, as well as the main methods and applications of glycerol pre-treatment briefly described.

Enhanced In Vitro Cascade Catalysis of Glycerol into Pyruvate and Acetoin by Integration with Dihydroxy Acid Dehydratase from Paralcaligenes ureilyticus

Recently, an in vitro enzymatic cascade was constructed to transform glycerol into the high-value platform chemical pyruvate. However, the low activity of dihydroxy acid dehydratase from Sulfolobus solfataricus (SsDHAD) limited the efficiency. In this study, the enzymatic reduction of pyruvate catalyzed by d-lactate dehydrogenase from Pseudomonas aeruginosa PAO1 was used to assay the activities of dihydroxy acid dehydratases. Dihydroxy acid dehydratase from Paralcaligenes ureilyticus (PuDHT) was identified as the most efficient candidate for glycerate dehydration. After the optimization of the catalytic temperature for the enzymatic cascade, comprising alditol oxidase from Streptomyces coelicolor A3, PuDHT, and catalase from Aspergillus niger, 20.50 ± 0.27 mM of glycerol was consumed in 4 h to produce 18.95 ± 0.97 mM of pyruvate with a productivity 12.15-fold higher than the previous report using SsDHAD. The enzymatic cascade was further coupled with the pyruvate decarboxylase from Zymomonas mobile for the production of another platform compound, acetoin. Acetoin at a concentration of 8.52 ± 0.12 mM was produced from 21.62 ± 0.19 mM of glycerol with a productivity of 1.42 ± 0.02 mM h−1.

Towards valorization of pectin-rich agro-industrial residues: Engineering of Saccharomyces cerevisiae for co-fermentation of d-galacturonic acid and glycerol

Pectin-rich plant biomass residues represent underutilized feedstocks for industrial biotechnology. The conversion of the oxidized monomer d-galacturonic acid (d-GalUA) to highly reduced fermentation products such as alcohols is impossible due to the lack of electrons. The reduced compound glycerol has therefore been considered an optimal co-substrate, and a cell factory able to efficiently co-ferment these two carbon sources is in demand. Here, we inserted the fungal d-GalUA pathway in a strain of the yeast S. cerevisiae previously equipped with an NAD-dependent glycerol catabolic pathway. The constructed strain was able to consume d-GalUA with the highest reported maximum specific rate of 0.23 g g_CDW^-1 h^-1 in synthetic minimal medium when glycerol was added. By means of a ¹³C isotope-labelling analysis, carbon from both substrates was shown to end up in pyruvate. The study delivers the proof of concept for a co-fermentation of the two 'respiratory' carbon sources to ethanol and demonstrates a fast and complete consumption of d-GalUA in crude sugar beet pulp hydrolysate under aerobic conditions. The future challenge will be to achieve co-fermentation under industrial, quasi-anaerobic conditions.