Consistent 1,3-propanediol production from glycerol in mixed culture fermentation over a wide range of pH

Effect of loading rate and pH on glycerol fermentation and microbial population in an upflow anaerobic filter reactor

A reactor with silicone tubes as support medium was used for glycerol fermentation. The experimental set-up consisted of three phases. In P1, the applied glycerol loading rate (gly-LR) was in the range of 6-10 g.L-1.d-1 at an influent pH of 7.9 ± 0.4. In P2, gly-LR was kept constant (18.0 ± 1.8 g.L-1.d-1) with different doses of NaHCO3. Finally in P3, two different gly-LR (9 and 18 g.L-1.d-1) were evaluated, dosing 1 g-NaHCO3 per g-COD of glycerol. Glycerol consumption was close 90%. The main end-product was 1,3-propanediol (1,3-PDO) (0.40 mol.mol-gly-1), but ethanol was also generated, particularly at pH above 8 and low gly-LR (0.20 mol.mol-gly-1). After 1-year operation with glycerol as the only carbon source, a drastic shift in the bacterial community was observed. The 1,3-PDO producers Lacrimispora and Clostridium became dominant, although non-glycerol-degrading fermentative genera, e.g., Actinomyces and Eubacterium, thrived at the expense of cellular breakdown products.

Heterologous 1,3-Propanediol Production Using Different Recombinant Clostridium beijerinckii DSM 6423 Strains

1,3-propanediol (1,3-PDO) is a valuable basic chemical, especially in the polymer industry to produce polytrimethylene terephthalate. Unfortunately, the production of 1,3-PDO mainly depends on petroleum products as precursors. Furthermore, the chemical routes have significant disadvantages, such as environmental issues. An alternative is the biobased fermentation of 1,3-PDO from cheap glycerol. Clostridium beijerinckii DSM 6423 was originally reported to produce 1,3-PDO. However, this could not be confirmed, and a genome analysis revealed the loss of an essential gene. Thus, 1,3-PDO production was genetically reinstalled. Genes for 1,3-PDO production from Clostridium pasteurianum DSM 525 and Clostridium beijerinckii DSM 15410 (formerly Clostridium diolis) were introduced into C. beijerinckii DSM 6423 to enable 1,3-PDO production from glycerol. 1,3-PDO production by recombinant C. beijerinckii strains were investigated under different growth conditions. 1,3-PDO production was only observed for C. beijerinckii [pMTL83251_P_pta-ack_1,3-PDO.diolis], which harbors the genes of C. beijerinckii DSM 15410. By buffering the growth medium, production could be increased by 74%. Furthermore, the effect of four different promoters was analyzed. The use of the constitutive thlA promoter from Clostridium acetobutylicum led to a 167% increase in 1,3-PDO production compared to the initial recombinant approach.

Steering the product spectrum in high-pressure anaerobic processes: CO2 partial pressure as a novel tool in biorefinery concepts

BACKGROUND

Elevated CO₂ partial pressure (pCO₂) has been proposed as a potential steering parameter for selective carboxylate production in mixed culture fermentation. It is anticipated that intermediate product spectrum and production rates, as well as changes in the microbial community, are (in)directly influenced by elevated pCO₂. However, it remains unclear how pCO₂ interacts with other operational conditions, namely substrate specificity, substrate-to-biomass (S/X) ratio and the presence of an additional electron donor, and what effect pCO₂ has on the exact composition of fermentation products. Here, we investigated possible steering effects of elevated pCO₂ combined with (1) mixed substrate (glycerol/glucose) provision; (2) subsequent increments in substrate concentration to increase the S/X ratio; and (3) formate as an additional electron donor.

RESULTS

Metabolite predominance, e.g., propionate vs. butyrate/acetate, and cell density, depended on interaction effects between pCO₂-S/X ratio and pCO₂-formate. Individual substrate consumption rates were negatively impacted by the interaction effect between pCO₂-S/X ratio and were not re-established after lowering the S/X ratio and adding formate. The product spectrum was influenced by the microbial community composition, which in turn, was modified by substrate type and the interaction effect between pCO₂-formate. High propionate and butyrate levels strongly correlated with Negativicutes and Clostridia predominance, respectively. After subsequent pressurized fermentation phases, the interaction effect between pCO₂-formate enabled a shift from propionate towards succinate production when mixed substrate was provided.

CONCLUSIONS

Overall, interaction effects between elevated pCO₂, substrate specificity, high S/X ratio and availability of reducing equivalents from formate, rather than an isolated pCO₂ effect, modified the proportionality of propionate, butyrate and acetate in pressurized mixed substrate fermentations at the expense of reduced consumption rates and increased lag-phases. The interaction effect between elevated pCO₂ and formate was beneficial for succinate production and biomass growth with a glycerol/glucose mixture as the substrate. The positive effect may be attributed to the availability of extra reducing equivalents, likely enhanced carbon fixating activity and hindered propionate conversion due to increased concentration of undissociated carboxylic acids.

Improvement of 1,3-propanediol production from crude glycerol by co-cultivation of anaerobic and facultative microbes under non-strictly anaerobic conditions

Background

Natural microbial consortia could efficiently produce 1,3-propanediol (1,3-PDO), a most promising bulk biochemical derived from glycerol that can be used as a monomer in the synthesis of polytrimethylene terephthalate (PTT). While natural microbial communities are made up of a diverse range of microbes with frequently unknown functions, the construction of synthetic microbial consortia allows for the creation of more defined systems with lower complexity.

Results

In this study, the synthetic microbial consortia were constructed by combining facultative microbes of Klebsiella pneumoniae DUT2 (KP) and/or Escherichia coli DUT3 (EC) cultures with the strictly anaerobic microbe of Clostridium butyricum DUT1 (CB) cultures under micro-aerobic conditions. The function of EC and KP during the fermentation process was to deplete oxygen and create an anaerobic environment for CB. Furthermore, KP competes with CB for the consumption of crude glycerol and the production of 1,3-PDO. The interaction of commensalism and competition resulted in the construction of synthetic microbial consortia capable of efficiently converting crude glycerol to 1,3-PDO even under micro-aerobic conditions. In a batch fermentation, the synthetic CB:KP co-culture at an initial abundance ratio of 92.5:7.5, yielded a maximum 1,3-PDO concentration of 52.08 g/L, with a yield of 0.49 g/g and a productivity of 1.80 g/(L.h), which increased by 10%, 9%, and 12%, respectively, when compared to the CB mono-culture under strictly anaerobic conditions. The final 1,3-PDO concentration, yield, and productivity by the synthetic CB:KP consortia increased by 16%, 19%, and 84%, respectively, when compared to the KP mono-culture. At an initial abundance ratio of 85:7.5:7.5, the synthetic CB:KP:EC co-culture achieved the highest 1,3-PDO flux of 49.17%, while 7.43%, 5.77%, 3.15% 4.24%, and 2.13% of flux was distributed to butyric acid, acetic acid, lactic acid, ethanol, and succinic acid pathways. In a fed-batch fermentation, the synthetic CB:KP:EC co-culture demonstrated a maximum 1,3-PDO concentration of 77.68 g/L with a yield of 0.51 g/g which is 30% and 13% higher than the production by the CB mono-culture at 0.02 vvm (nitrogen volume/culture volume/min) N2 supply. The initial abundance of CB, which is guaranteed to be at least 85%, enables efficient 1,3-PDO production from crude glycerol via the development of synthetic microbial consortia.

Conclusion

The synthetic microbial consortia demonstrated excellent performance on 1,3-propanediol production under micro-aerobic conditions through the interaction of commensalism and competition. The experimental results demonstrated the potential benefit of using synthetic microbial consortia to produce 1,3-propanediol from crude glycerol.

Glycerol conversion by aerobic granular sludge

Glycerol is abundantly present in wastewater from industries such as biodiesel production facilities. Glycerol is also a potential carbon source for microbes that are involved in wastewater nutrient removal processes. The conversion of glycerol in biological phosphorus removal of aerobic granular sludge processes has not been explored to date. The current study describes glycerol utilization by aerobic granular sludge and enhanced biological phosphorus removal (EBPR). Robust granules with good phosphorus removal capabilities were formed in an aerobic granular sludge sequencing batch reactor fed with glycerol. The interaction between the fermentative conversion of glycerol and product uptake by polyphosphate accumulating organisms (PAO) was studied using stoichiometric and microbial community analysis. Metagenomic, metaproteomic and microscopic analysis identified a community dominated by Actinobacteria (Tessaracoccus and Micropruina) and a typical PAO known as Ca. Accumulibacter. Glycerol uptake facilitator (glpF) and glycerol kinase (glpK), two proteins involved in the transport of glycerol into the cellular metabolism, were only observed in the genome of the Actinobacteria. The anaerobic conversion appeared to be a combination of a substrate fermentation and product uptake-type reaction. Initially, glycerol fermentation led mainly to the production of 1,3-propanediol (1,3-PDO) which was not taken up under anaerobic conditions. Despite the aerobic conversion of 1,3-PDO stable granulation was observed. Over time, 1,3-PDO production decreased and complete anaerobic COD uptake was observed. The results demonstrate that glycerol-containing wastewater can effectively be treated by the aerobic granular sludge process and that fermentative and polyphosphate accumulating organisms can form a food chain in glycerol-based EBPR processes.

Lactic acid production from food waste using a microbial consortium: Focus on key parameters for process upscaling and fermentation residues valorization

In this study, the production of lactic acid from food waste in industrially relevant conditions was investigated. Laboratory assays were first performed in batch conditions to determine the suitable operational parameters for an efficient lactic acid production. The use of compost as inoculum, the regulation of temperature at 35 °C and pH at 5 enhanced the development of Lactobacillus sp. resulting in the production of 70 g/L of lactic acid with a selectivity of 89% over the other carboxylic acids. Those parameters were then applied at pilot scale in successive fed-batch fermentations. The subsequent high concentration (68 g/L), yield (0.38 g/gTS) and selectivity (77%) in lactic acid demonstrated the applicability of the process. To integrate the process into a complete value chain, fermentation residues were then converted into biogas through anaerobic digestion. Lastly, the experiment was successfully replicated using commercial and municipal waste collected in France.

Microbial community redundance in biomethanation systems lead to faster recovery of methane production rates after starvation

The Power-to-Gas concept corresponds to the use of the electric energy surplus to produce H₂ by water electrolysis, that can be further converted to methane by biomethanation. However, the fluctuant production of renewable energy sources can lead to discontinuous H₂ injections into the reactors, that may interfere with the adaptation of the microbial community to high H₂ partial pressures. In this study, the response of the microbial community to H₂ and organic feed starvation was evaluated in in-situ and ex-situ biomethanation. The fed-batch reactors were fed with acetate or glucose and H₂, and one or four weeks of starvation periods were investigated. Methane productivity was mostly affected by the four-week starvation period. However, both in-situ and ex-situ biomethanation reactors recovered their methane production rate after starvation within approximately one-week of normal operation, while the anaerobic digestion (AD) reactors did not recover their performances even after 3 weeks of normal operation. The recovery failure of the AD reactors was probably related to a slow growth of the syntrophic and methanogen microorganisms, that led to a VFA accumulation. On the contrary, the faster recovery of both biomethanation reactors was related to the replacement of Methanoculleus sp. by Methanobacterium sp., restoring the methane production in the in-situ and ex-situ biomethanation reactors. This study has shown that biomethanation processes can respond favourably to the intermittent H₂ addition without compromising their CH₄ production performance.

Phenotype analysis of cultivation processes via unsupervised machine learning: Demonstration for Clostridium pasteurianum

A novel approach of phenotype analysis of fermentation‐based bioprocesses based on unsupervised learning (clustering) is presented. As a prior identification of phenotypes and conditional interrelations is desired to control fermentation performance, an automated learning method to output reference phenotypes (defined as vector of biomass‐specific rates) was developed and the necessary computing process and parameters were assessed. For its demonstration, time series data of 90 Clostridium pasteurianum cultivations were used which feature a broad spectrum of solventogenic and acidogenic phenotypes, while 14 clusters of phenotypic manifestations were identified. The analysis of reference phenotypes showed distinct differences, where potential conditionalities were exemplary isolated. Further, cluster‐based balancing of carbon and ATP or the use of reference phenotypes as indicator for bioprocess monitoring were demonstrated to highlight the perks of this approach. Overall, such analysis depends strongly on the quality of the data and experimental validations will be required before conclusions. However, the automated, streamlined and abstracted approach diminishes the need of individual evaluation of all noisy dataset and showed promising results, which could be transferred to strains with comparably wide‐ranging phenotypic manifestations or as indicators for repeated bioprocesses with clearly defined target.

Hermetia illucens fat affects the gastrointestinal tract selected microbial populations, their activity, and the immune status of broiler chickens

The present study investigated the effect of Hermetia illucens larvae (BSFL) fat, derived using supercritical CO2 extraction and added to broiler chickens’ diets as a partial (50%) or total replacement for commonly used soybean oil, on the gastrointestinal tract (GIT) microbial population, its activity, and selected physiological and immune traits. A total of 576 one-dayold female Ross 308 chicks were randomly assigned to 3 dietary treatments with 16 replicates each. The following treatments were applied: SO – 100% soybean oil, BSFL50 – a mixture of BSFL and soybean oils in a 50:50 ratio, and BSFL100 – 100% BSFL fat. Digesta samples from the crop, jejunum and ceca were collected for further analyses, i.e., pH measurements, fluorescent in situ hybridization, and short-chain fatty acid (SCFA) concentrations. Additionally, the selected plasma biochemical parameters and immunological traits were assessed. In general, the implementation of BSFL fat in broilers’ diets resulted in increased proliferation of potentially pathogenic bacterial populations in the crop, such as Enterobacteriaceae, Bacteroides – Prevotella cluster, and Clostridium perfringens. Furthermore, BSFL100 enhanced microbial activity via total SCFA production and lowered the pH in this segment. However, no detrimental effects were observed in terms of other GIT segments, i.e., the jejunal and cecal microecosystems. The strongest impact on reduction of select components of the microbial population in the cecum was observed with the BSFL50 treatment for potentially pathogenic bacteria such as Enterobacteriaceae, Bacteroides – Prevotella cluster, while commensal populations were also limited, i.e., Bacillus spp., C. leptum subgroup, and C. coccoides – Eubacterium rectale cluster. Additionally, BSFL100 reduced the cholesterol concentration in the blood, while both experimental treatments decreased the ALT level. In conclusion, due to the insufficient release of lauric acid from the BSFL fat in the crop, an adverse shift in the microbiota can be noted. However, a positive suppressive effect on the select components of the cecal microbiota, as well as improvement of liver health suggests implying the BSFL fat in broiler nutrition.

Psychrophile-based simple biocatalysts for effective coproduction of 3-hydroxypropionic acid and 1,3-propanediol

3-Hydroxypropionic acid (3-HP) and 1,3-propanediol (1,3-PDO) have tremendous potential markets in many industries. This study evaluated the simultaneous biosynthesis of the 2 compounds using the new psychrophile-based simple biocatalyst (PSCat) reaction system. The PSCat method is based on the expression of glycerol dehydratase, 1,3-propanediol dehydrogenase, and aldehyde dehydrogenase from Klebsiella pneumoniae in Shewanella livingstonensis Ac10 and Shewanella frigidimarina DSM 12253, individually. Heat treatment at 45 °C for 15 min deactivated the intracellular metabolic flux, and the production process was started after adding substrate, cofactor, and coenzyme. In the solo production process after 1 h, the maximum production of 3-HP was 62.0 m m. For 1,3-PDO, the maximum production was 25.0 m m. In the simultaneous production process, productivity was boosted, and the production of 3-HP and 1,3-PDO increased by 13.5 and 4.9 m m, respectively. Hence, the feasibility of the individual production and the simultaneous biosynthesis system were verified in the new PSCat approach.

Co-fermentation of glycerol and glucose by a co-culture system of engineered Escherichia coli strains for 1,3-propanediol production without vitamin B12 supplementation

The necessity of costly co-enzyme B₁₂ for the activity of glycerol dehydratase (GDHt) is considered as a major bottleneck in sustainable bioproduction of 1,3-propanediol (1,3-PD) from glycerol. Here, an E. coil Rosetta-dhaB1-dhaB2 strain was constructed by overexpressing a B₁₂-independent GDHt (dhaB1) and its activating factor (dhaB2) from Clostridium butyricum. Subsequently, it was used in designing a co-culture with E. coli BL21-dhaT that overexpressed 1,3-PD oxidoreductase (dhaT), to produce 1,3-PD during co-fermentation of glycerol and glucose. The optimum initial ratio of BL21-dhaT to Rosetta-dhaB1-dhaB2 strains in the co-culture was 1.5. Compared to the fermentation of glycerol alone, co-fermentation approach provided 1.3-folds higher 1,3-PD. Finally, co-fermentation was done in a 10 L bioreactor that produced 41.65 g/L 1,3-PD, which corresponded to 0.69 g/L/h productivity and 0.67 mol/mol yield of 1,3-PD. Hence, the developed co-culture could produce 1,3-PD cost-effectively without requiring vitamin B₁₂.

1,3-Propanediol production from glycerol in polyurethane foam containing anaerobic reactors: performance and biomass cultivation and retention

The use of batch and upflow anaerobic reactors filled with polyurethane foam for pure glycerol fermentation was evaluated. The best reactor operational conditions to obtain high yield and productivity of 1,3-propanediol (1,3-PDO) as the main product and the role of the polyurethane foam in the growth and retention of suspended and attached biomass in the reactors were investigated. In the experiment at 30 °C with a batch reactor (700 mL), biomass growth was mostly as immobilized attached cells, and the achieved 1,3-PDO yield was up to 0.58 mol mol-gly^-1. In the experiment (30 °C) with an upflow anaerobic reactor (717 mL), glycerol loading rates (gly-LR) ranging from 6.94 to 15.47 g gly L^-1 day^-1 were applied during a 102-day period. During the operation, average 1,3-PDO yield was 0.47 mol mol-gly^-1, reaching a maximum of 0.51 mol mol-gly^-1 at gly-LR of 13.57 g gly L^-1 day^-1. High 1,3-PDO productivity (5.35 to 5.44 g L^-1 day^-1) was obtained when gly-LR was 13.57 to 15.47 g gly L^-1 day^-1. Comparing the close yield values in both batch and continuous reactors and based on microbial evaluation, it is concluded that most of the 1,3-PDO generated in the continuous reactor was due to the suspended biomass retained by the foam cubes. The Clostridium genus was the predominant 1,3-PDO producer. Good yields and productivities with packed reactors were attributed to polyurethane foam used for mixed culture growth and retention. Consequently, they are worth considering for 1,3-PDO production from pure glycerol.

Temperature and Inoculum Origin Influence the Performance of Ex-Situ Biological Hydrogen Methanation

The conversion of H₂ into methane can be carried out by microorganisms in a process so-called biomethanation. In ex-situ biomethanation H₂ and CO₂ gas are exogenous to the system. One of the main limitations of the biomethanation process is the low gas-liquid transfer rate and solubility of H₂ which are strongly influenced by the temperature. Hydrogenotrophic methanogens that are responsible for the biomethanation reaction are also very sensitive to temperature variations. The aim of this work was to evaluate the impact of temperature on batch biomethanation process in mixed culture. The performances of mesophilic and thermophilic inocula were assessed at 4 temperatures (24, 35, 55 and 65 °C). A negative impact of the low temperature (24 °C) was observed on microbial kinetics. Although methane production rate was higher at 55 and 65 °C (respectively 290 ± 55 and 309 ± 109 mL CH₄/L.day for the mesophilic inoculum) than at 24 and 35 °C (respectively 156 ± 41 and 253 ± 51 mL CH₄/L.day), the instability of the system substantially increased, likely because of a strong dominance of only Methanothermobacter species. Considering the maximal methane production rates and their stability all along the experiments, an optimal temperature range of 35 °C or 55 °C is recommended to operate ex-situ biomethanation process.

Biomethanation processes: new insights on the effect of a high H2 partial pressure on microbial communities

BACKGROUND

Biomethanation is a promising solution to upgrade the CH₄ content in biogas. This process consists in the injection of H₂ into an anaerobic digester, using the capacity of indigenous hydrogenotrophic methanogens for converting the injected H₂ and the CO₂ generated from the anaerobic digestion process into CH₄. However, the injection of H₂ could cause process disturbances by impacting the microbial communities of the anaerobic digester. Better understanding on how the indigenous microbial community can adapt to high H₂ partial pressures is therefore required.

RESULTS

Seven microbial inocula issued from industrial bioprocesses treating different types of waste were exposed to a high H₂ partial pressure in semi-continuous reactors. After 12 days of operation, even though both CH₄ and volatile fatty acids (VFA) were produced as end products, one of them was the main product. Acetate was the most abundant VFA, representing up to 94% of the total VFA production. VFA accumulation strongly anti-correlated with CH₄ production according to the source of inoculum. Three clusters of inocula were distinguished: (1) inocula leading to CH₄ production, (2) inocula leading to the production of methane and VFA in a low proportion, and (3) inocula leading to the accumulation of mostly VFA, mainly acetate. Interestingly, VFA accumulation was highly correlated to a low proportion of archaea in the inocula, a higher amount of homoacetogens than hydrogenotrophic methanogens and, the absence or the very low abundance in members from the Methanosarcinales order. The best methanogenic performances were obtained when hydrogenotrophic methanogens and Methanosarcina sp. co-dominated all along the operation.

CONCLUSIONS

New insights on the microbial community response to high H₂ partial pressure are provided in this work. H₂ injection in semi-continuous reactors showed a significant impact on microbial communities and their associated metabolic patterns. Hydrogenotrophic methanogens, Methanobacterium sp. or Methanoculleus sp. were highly selected in the reactors, but the presence of co-dominant Methanosarcinales related species were required to produce higher amounts of CH₄ than VFA.

Monitoring steady production of 1,3-propanediol during bioprospecting of glycerol-assimilating soil microbiome using dye-based pH-stat method

AIM

In this investigation, a dye-based pH-stat method was devised for monitoring steady production of 1,3-propanediol (1,3-PDO) during bioprospecting of glycerol-assimilating soil microbiome.

METHODS AND RESULTS

Soil samples were collected from two potential sites of CSIR-IIP, India. Selective enrichment of microbial consortia was done using the glycerol-based medium at initial stage, followed by purification to isolated colonies, after positive high-performance liquid chromatography detection of 1,3-PDO in the fermentation broth. When the purified isolated were re-tested for 1,3-PDO production, only two isolates namely Isolate 1 and Isolate 3 were capable of producing the targeted product preferably under anaerobic conditions. Based on better 1,3-PDO fermentation efficiency (Isolate 3, 22% vs Isolate 1, 4·48%) and acetic acid as the only major by-product, Isolate 3 was shortlisted for further studies. A dye-based technique was devised in which bromothymol blue was incorporated into the medium to monitor the pH drop due to acetic acid formation and hence change in colour. Visual change in colour helped in intermittent pH restoration. During fermentation, with pH stat being 8-8·5, Isolate 3 at 32°C yielded 0·67 mol mol^-1 1,3-PDO within a short span of 12 h only with an initial concentration of glycerol being 20 g l^-1 . Phylogenetic analysis revealed that Isolate 3 shared 95·8% homology with Citrobacter freundii CFNIH1 and hence designated as C. freundii IIP DR3.

CONCLUSION

This study demonstrated that during bioprospecting glycerol-assimilating microbiome, dye-based technique can be successfully employed. This technique can further be exploited to monitor consistent production of all microbial secondary metabolites that accompanies acid production.

SIGNIFICANCE AND IMPACT OF THE STUDY

Incorporation of 'Bromothymol blue' can visually help in the identification of pH drop in the medium, so that pH stat can be easily maintained during 1,3-PDO production from glycerol especially under shake flask conditions.

Novel insights on the versatility of biohydrogen production from sugarcane vinasse via thermophilic dark fermentation: Impacts of pH-driven operating strategies on acidogenesis metabolite profiles

An innovative application of the anaerobic structured-bed reactor (AnSTBR) in thermophilic dark fermentation of sugarcane vinasse targeting biohydrogen (bioH₂) production was assessed. A detailed metabolite monitoring program identified the major substrates and primary metabolic pathways within the system. Increasing the applied organic loading rate positively affected bioH₂ production, reaching 2074 N mL-H₂ L^-1 d^-1 and indicating an optimal load of approximately 70 kg-COD m^-3 d^-1. Controlling the fermentation pH (5.0-5.5) was the primary strategy to maintain bioH₂-producing conditions, offsetting negative impacts associated with the compositional variability of vinasse. Metabolic correlations pointed out lactate as the primary substrate for bioH₂ production, indicating its accumulation as evidence of impaired reactors. The versatility of the acidogenic system was confirmed by identifying three major metabolic pathways according to the pH, i.e., lactate-producing (pH <5.0), bioH₂-/butyrate-producing (pH = 5.0-5.5) and bioH₂-producing/sulfate-reducing (pH >6.0) systems, which enables managing the operation of the reactors for diversified purposes in practical aspects.

Production of 1,3-propanediol from pure and crude glycerol using a UASB reactor with attached biomass in silicone support

The 1,3-propanediol (1,3-PDO) yield and productivity from glycerol were studied over a 155-day period. A UASB reactor that also contained silicone support for biomass attachment was used to evaluate the optimal operational conditions and microbiota development. The highest average 1,3-PDO yield was 0.54 and 0.48 mol.mol-gly^-1 when reactor pH was 5.0-5.5 and the applied loading rate was 18 and 20 g-gly.L^-1.d^-1 using the pure and crude substrate, respectively. The productivity was close to 7.5 g.L^-1.d^-1 for both substrates; therefore, the direct use of crude glycerol can be valorized in practice. Clostridium was the predominant genus for 1,3-PDO production and C. pasteurianum was dominant in the biofilm. Using crude glycerol, C. beijerinckii dropped strongly; some Clostridium population was then replaced by Klebsiella pneumoniae and Lactobacillus spp. The good process performance and the advances in the microbiota knowledge are steps forward to obtain a more cost-effective system in practice.

Shaping microbial consortia in coupling glycerol fermentation and carboxylate chain elongation for Co-production of 1,3-propanediol and caproate: Pathways and mechanisms

Glycerol is presently being generated in surplus with the rapid growth of the biodiesel industry and seeks ways to be upcycled, rather than to be treated with costs. Glycerol for the co-production of 1,3-propanediol (1,3-PDO) and caproate has a great prospect. Yet, its technical difficulty lies in the enhancement of caproate productivity, which requires the presence of ethanol as a co-substrate and necessitates the co-existence of functional microbes for glycerol fermentation and chain elongation. This study successfully achieved 6.38 mM C 1,3-PDO d^-1 and 2.95 mM C caproate d^-1 in a 2-L mixed-cultured semi-continuous fermenter with a glycerol-ethanol-acetate stoichiometric ratio of 4:3:1. Such conversions were mainly facilitated by a microbial community of Eubacterium limosum, Clostridium kluyveri and Massilibacterium senegalense. With such a synergistic microbiome, the co-production of 1,3-PDO and caproate was achieved from glycerol without ethanol addition. Based on metagenomics, E. limosum is capable of converting glycerol to 1,3-PDO, ethanol and H₂, and also redirecting the electron potential of H₂ into acetate via the Wood-Ljungdahl pathway, which is then used for chain elongation. C. kluyveri worked synergistically with E. limosum by consuming ethanol and acetate for caproate production. M. senegalense encodes for ethanol oxidation to acetate and butyrate, facilitating the generation of these intermediates for C. kluyveri elongation to caproate. During the transition between fermentation and elongation, an unexpected observation of poly-β-hydroxybutyrate (PHB) formation and reutilization by M. senegalense may be associated with butyrate formation for further caproate generation. The knowledge gleaned from the substrate constitute, microbial consortium and their synergetic metabolism demonstrates a resource upgrade potential for crude glycerol or glycerol-containing wastewater generated from the biodiesel industry.

Enhancement of corn stover conversion to carboxylates by extrusion and biotic triggers in solid-state fermentation

Solid-state fermentation is a potential technology for developing lignocellulosic biomass-based biorefineries. This work dealt with solid-state fermentation for carboxylates production from corn stover, as building blocks for a lignocellulosic feedstock-based biorefinery. The effect of extrusion pretreatment, together with the action of a microbial consortia and hydrolytic enzymes as biotic triggers, was investigated on corn stover conversion, microbial metabolic pathways, and populations. The extrusion caused changes in the physical and morphological characteristics, without altering the biochemical composition of the corn stover. Extrusion also led to remarkable differences in the composition of the indigenous microbial population of the substrate. Consequently, it affected the structure of community developed after fermentation and the substrate conversion yield, which increased by 118% (from 23 ± 4 gCOD/kgVSi obtained with raw substrate to 51 ± 1 gCOD/kgVSi with extruded corn stover) with regard to self-fermentation experiments. The use of activated sludge as inoculum further increased the total substrate conversion into carboxylates, up to 60 ± 2 gCOD/kgVSi, and shaped the microbial communities (mainly composed of bacteria from the Clostridia and Bacteroidia classes) with subsequent homogenization of the fermentation pathways. The addition of hydrolytic enzymes into the reactors further increased the corn stover conversion, leading to a maximum yield of 142 ± 1 gCOD/kgVSi. Thus, extrusion pretreatment combined with the use of an inoculum and enzyme addition increased by 506% corn stover conversion into carboxylates. Beside biomass pretreatment, the results of this study indicated that biotic factor greatly impacted solid-state fermentation by shaping the microbial communities and related metabolic pathways.

Mixed bacterial culture mediated direct conversion of bio-glycerol to diols

Direct and economic transformation of biodiesel derived crude glycerol is gaining more significance. During screening of bacterial cultures Klebsiella pneumoniae and Enterobacter aerogenes were able to convert crude bio-glycerol to 2,3-butanediol (2,3-BDO) and 1,3-propanediol (1,3-PDO), as major compounds, ethanol and acetoin as minor compounds, with a conversion of 69% and 79% respectively. Process optimization could achieve maximum conversion at pH 7.0, 37 °C, 30-40 g/L glycerol and 1.5 g of inoculum until 120 h. Mixed cultures led to complete glycerol conversion with optimal yield and productivity. An innovative approach of using crude glycerol for sustained growth and tolerance of bacteria as source of carbon and energy makes this study more significant. In addition to this, a mixed culture concept introduced here is expected to make impact in process economics for industrial scale synthesis for direct transformation of glycerol into C3 and specifically, C4 diols.

Kinetic study of dry anaerobic co-digestion of food waste and cardboard for methane production

Dry anaerobic digestion is a promising option for food waste treatment and valorization. However, accumulation of ammonia and volatile fatty acids often occurs, leading to inefficient processes and digestion failure. Co-digestion with cardboard may be a solution to overcome this problem. The effect of the initial substrate to inoculum ratio (0.25 to 1gVS·gVS^-1) and the initial total solids contents (20-30%) on the kinetics and performance of dry food waste mono-digestion and co-digestion with cardboard was investigated in batch tests. All the conditions produced methane efficiently (71-93% of the biochemical methane potential). However, due to lack of methanogenic activity, volatile fatty acids accumulated at the beginning of the digestion and lag phases in the methane production were observed. At increasing substrate to inoculum ratios, the initial acid accumulation was more pronounced and lower cumulative methane yields were obtained. Higher amounts of soluble organic matter remained undegraded at higher substrate loads. Although causing slightly longer lag phases, high initial total solids contents did not jeopardize the methane yields. Cardboard addition reduced acid accumulation and the decline in the yields at increasing substrate loads. However, cardboard addition also caused higher concentrations of propionic acid, which appeared as the most last acid to be degraded. Nevertheless, dry co-digestion of food waste and cardboard in urban areas is demonstrated asan interesting feasible valorization option.

Revealing extracellular electron transfer mediated parasitism: energetic considerations

Extracellular electron transfer (EET) is a mechanism that allows energetic coupling between two microorganisms or between a microorganism and an electrode surface. EET is either supported by direct physical contacts or mediated by electron shuttles. So far, studies dealing with interspecies EET (so-called IET) have mainly focused on possible syntrophic interactions between microorganisms favoured by this mechanism. In this article, the case of fermentative bacteria receiving extracellular electrons while fermenting a substrate is considered. A thermodynamical analysis based on metabolic energy balances was applied to re-investigate experimental data from the literature. Results suggest that the observations of a decrease of cell biomass yields of fermentative electron-accepting species, as mostly reported, can be unravelled by EET energetics and correspond to parasitism in case of IET. As an illustration, the growth yield decrease of Propionibacterium freudenreichii (−14%) observed in electro-fermentation experiments was fully explained by EET energetics when electrons were used by this species at a potential of −0.12 ± 0.01 V vs SHE. Analysis of other cases showed that, in addition to EET energetics in Clostridium pasteurianum, biological regulations can also be involved in such biomass yield decrease (−33% to −38%). Interestingly, the diminution of bacterial biomass production is always concomitant with an increased production of reduced compounds making IET-mediated parasitism and electro-fermentation attractive ways to optimize carbon fluxes in fermentation processes.

Electro-fermentation triggering population selection in mixed-culture glycerol fermentation

Electro‐fermentation is a new technique that could be used to influence the global metabolism in mixed‐culture fermentation. In this study, a mixed‐culture cathodic electro‐fermentation of glycerol was investigated. Both microbial community structure and metabolic patterns were altered when compared to standard fermentation. This microbial population shift was more significant when the working electrodes were pre‐colonized by Geobacter sulfurreducens, before electro‐fermentation. The electro‐fermenting microbial community was more efficient for producing 1,3‐propanediol with an improved yield of 10% when compared with fermentation controls. Such improvement did not require high energy and total electron input represented < 1% of the total electron equivalents provided only by glycerol. A linear model was developed to estimate the individual metabolic pattern of each operational taxonomic unit. Application of this model compared to the experimental results suggests that the changes in global metabolism were supported by bacterial population selection rather than individual metabolism shift. This study shows for the first time that both fermentation pattern and bacterial community composition can be influenced by electro‐fermentation conditions.

Preeminent productivity of 1,3-propanediol by Clostridium butyricum JKT37 and the role of using calcium carbonate as pH neutraliser in glycerol fermentation

Calcium carbonate was evaluated as a replacement for the base during the fermentation of glycerol by a highly productive strain of 1,3-propanediol (PDO), viz., Clostridium butyricum JKT37. Due to its high specific growth rate (µ_max=0.53h^-1), 40g/L of glycerol was completely converted into 19.6g/L of PDO in merely 7h of batch fermentation, leaving only acetate and butyrate as the by-products. The accumulation of these volatile fatty acids was circumvented with the addition of calcium carbonate as the pH neutraliser before the fermentation was inoculated. An optimal amount of 15g/L of calcium carbonate was statistically determined from screening with various glycerol concentrations (20-120g/L). By substituting potassium hydroxide with calcium carbonate as the pH neutraliser for fermentation in a bioreactor, a similar yield (Y_PDO/glycerol=0.6mol/mol) with a constant pH was achieved at the end of the fermentation.

Dry anaerobic digestion of food waste and cardboard at different substrate loads, solid contents and co-digestion proportions

The increasing food waste production calls for developing efficient technologies for its treatment. Anaerobic processes provide an effective waste valorization. The influence of the initial substrate load on the performance of batch dry anaerobic co-digestion reactors treating food waste and cardboard was investigated. The load was varied by modifying the substrate to inoculum ratio (S/X), the total solids content and the co-digestion proportions. The results showed that the S/X was a crucial parameter. Within the tested values (0.25, 1 and 4gVS·gVS^-1), only the reactors working at 0.25 produced methane. Methanosarcina was the main archaea, indicating its importance for efficient methanogenesis. Acidogenic fermentation was predominant at higher S/X, producing hydrogen and other metabolites. Higher substrate conversions (≤48%) and hydrogen yields (≤62mL·gVS^-1) were achieved at low loads. This study suggests that different value-added compounds can be produced in dry conditions, with the initial substrate load as easy-to-control operational parameter.

Cooperative growth of Geobacter sulfurreducens and Clostridium pasteurianum with subsequent metabolic shift in glycerol fermentation

Interspecies electron transfer is a common way to couple metabolic energy balances between different species in mixed culture consortia. Direct interspecies electron transfer (DIET) mechanism has been recently characterised with Geobacter species which couple the electron balance with other species through physical contacts. Using this mechanism could be an efficient and cost-effective way to directly control redox balances in co-culture fermentation. The present study deals with a co-culture of Geobacter sulfurreducens and Clostridium pasteurianum during glycerol fermentation. As a result, it was shown that Geobacter sulfurreducens was able to grow using Clostridium pasteurianum as sole electron acceptor. C. pasteurianum metabolic pattern was significantly altered towards improved 1,3-propanediol and butyrate production (+37% and +38% resp.) at the expense of butanol and ethanol production (−16% and −20% resp.). This metabolic shift was clearly induced by a small electron uptake that represented less than 0.6% of the electrons consumed by C. pasteurianum. A non-linear relationship was found between G. sulfurreducens growth (i.e the electrons transferred between the two species) and the changes in C. pasteurianum metabolite distribution. This study opens up new possibilities for controlling and increasing specificity in mixed culture fermentation.

Genetically Engineered Strains: Application and Advances for 1,3-Propanediol Production from Glycerol

1,3-Propanediol (1,3-PD) is one of the most important chemicals widely used as monomers for synthesis of some commercially valuable products, including cosmetics, foods, lubricants and medicines. Although 1,3-PD can be synthesized both chemically and biosynthetically, the latter offers more merits over chemical approach as it is economically viable, environmentally friendly and easy to carry out. The biosynthesis of 1,3-PD can be done by transforming glycerol or other similar substrates using some bacteria, such as Clostridium butyricum and Klebsiella pneumoniae. However, these natural microorganisms pose some bottlenecks like low productivity and metabolite inhibition. To overcome these problems, recent research efforts have been focused more on the development of new strains by modifying the genome through different techniques, such as mutagenesis and genetic engineering. Genetically engineered strains obtained by various strategies cannot only gain higher yield than wild types, but also overcome some of the barriers in production by the latter. This review paper presents an overview on the recent advances in the technological approaches to develop genetically engineered microorganisms for efficient biosynthesis of 1,3-PD.

Comprehensive characterization of the liquid fraction of digestates from full-scale anaerobic co-digestion

Waste management by anaerobic digestion generates a final byproduct, the digestate, which is usually separated into solid and liquid fractions to reduce the volume for transportation. The composition of the solid fraction has been recently studied to allow its valorization. However, full composition of liquid fraction of digestate and its size fractionation are less considered in the literature for efficient post treatment and valorization purposes. Therefore, here we characterized in detail liquid fraction of digestate obtained after solid-liquid separation from 11 full-scale co-digestion plants. The liquid fraction has a high concentration in organic matter with Chemical Oxygen Demand (COD) from 9.2 to 78g/L with 60-96% of COD in suspended particles (>1.2μm), 2-27% in colloids (1.2μm to 1kDa) and 2-18% in dissolved matter (<1kDa). Besides, it contained from 1.5 to 6.5g/L total nitrogen and high ions concentrations (0.5-3.1g/L NH₄⁺, 1.05-5.48g/L K⁺, 0-2.13g/L PO₄^3-). In addition, liquid fraction of digestate has poor biodegradability due to presence of humic substances making aerobic treatment inefficient. Only physico-chemical post treatment can be proposed for organic matter removal.

Enhanced Product Recovery from Glycerol Fermentation into 3-Carbon Compounds in a Bioelectrochemical System Combined with In Situ Extraction

Given the large amount of crude glycerol formed as a by-product in the biodiesel industries and the concomitant decrease in its overall market price, there is a need to add extra value to this biorefinery side stream. Upgrading can be achieved by new biotechnologies dealing with recovery and conversion of glycerol present in wastewaters into value-added products, aiming at a zero-waste policy and developing an economically viable process. In microbial bioelectrochemical systems (BESs), the mixed microbial community growing on the cathode can convert glycerol reductively to 1,3-propanediol (1,3-PDO). However, the product yield is rather limited in BESs compared with classic fermentation processes, and the synthesis of side-products, resulting from oxidation of glycerol, such as organic acids, represents a major burden for recovery of 1,3-PDO. Here, we show that the use of an enriched mixed-microbial community of glycerol degraders and in situ extraction of organic acids positively impacts 1,3-PDO yield and allows additional recovery of propionate from glycerol. We report the highest production yield achieved (0.72 mol_1,3-PDO mol⁻¹_glycerol) in electricity-driven 1,3-PDO biosynthesis from raw glycerol, which is very close to the 1,3-PDO yield reported thus far for a mixed-microbial culture-based glycerol fermentation process. We also present a combined approach for 1,3-PDO production and propionate extraction in a single three chamber reactor system, which leads to recovery of additional 3-carbon compounds in BESs. This opens up further opportunities for an economical upgrading of biodiesel refinery side or waste streams.

High-specificity synthesis of novel monomers by remodeled alcohol hydroxylase

BACKGROUND

Diols are important monomers for the production of plastics and polyurethanes, which are widely used in our daily life. The medium-chain diols with one hydroxyl group at its subterminal end are able to confer more flexibility upon the synthesized materials. But unfortunately, this type of diols has not been synthesized so far. The strong need for advanced materials impelled us to develop a new strategy for the production of these novel diols. In this study, we use the remodeled P450BM3 for high-specificity production of 1,7-decanediol.

RESULTS

The native P450BM3 was capable of converting medium-chain alcohols into corresponding α, ω1-, α, ω2- and α, ω3-diols, with each of them accounting for about one third of the total diols, but it exhibited a little or no activity on the short-chain alcohols. Greatly improved regiospecificity of alcohol hydroxylation was obtained by laboratory evolution of P450BM3. After substitution of 12 amino acid residues (J2-F87A), the ratio of 1,7-decanediol (ω-3 hydroxylation) to total decanediols increased to 86.8 % from 34.0 %. Structure modeling and site-directed mutagenesis demonstrated that the heme end residues such as Ala(78), Phe(87) and Arg(255) play a key role in controlling the regioselectivity of the alcohol hydroxylation, while the residues at the mouth of substrate binding site is not responsible for the regioselectivity.

CONCLUSIONS

Herein we employ an engineered P450BM3 for the first time to enable the high-specificity biosynthesis of 1,7-decanediol, which is a promising monomer for the development of advanced materials. Several key amino acid residues that control the regioselectivity of alcohol hydroxylation were identified, providing some new insights into how to improve the regiospecificity of alcohol hydroxylation. This report not only provides a good strategy for the biosynthesis of 1,7-decanediol, but also gives a promising approach for the production of other useful diols.

Elucidation of the co-metabolism of glycerol and glucose in Escherichia coli by genetic engineering, transcription profiling, and (13)C metabolic flux analysis

BACKGROUND

Glycerol, a byproduct of biodiesel, has become a readily available and inexpensive carbon source for the production of high-value products. However, the main drawback of glycerol utilization is the low consumption rate and shortage of NADPH formation, which may limit the production of NADPH-requiring products. To overcome these problems, we constructed a carbon catabolite repression-negative ΔptsGglpK* mutant by both blocking a key glucose PTS transporter and enhancing the glycerol conversion. The mutant can recover normal growth by co-utilization of glycerol and glucose after loss of glucose PTS transporter. To reveal the metabolic potential of the ΔptsGglpK* mutant, this study examined the flux distributions and regulation of the co-metabolism of glycerol and glucose in the mutant.

RESULTS

By labeling experiments using [1,3-(13)C]glycerol and [1-(13)C]glucose, (13)C metabolic flux analysis was employed to decipher the metabolisms of both the wild-type strain and the ΔptsGglpK* mutant in chemostat cultures. When cells were maintained at a low dilution rate (0.1 h(-1)), the two strains showed similar fluxome profiles. When the dilution rate was increased, both strains upgraded their pentose phosphate pathway, glycolysis and anaplerotic reactions, while the ΔptsGglpK* mutant was able to catabolize much more glycerol than glucose (more than tenfold higher). Compared with the wild-type strain, the mutant repressed its flux through the TCA cycle, resulting in higher acetate overflow. The regulation of fluxomes was consistent with transcriptional profiling of several key genes relevant to the TCA cycle and transhydrogenase, namely gltA, icdA, sdhA and pntA. In addition, cofactor fluxes and their pool sizes were determined. The ΔptsGglpK* mutant affected the redox NADPH/NADH state and reduced the ATP level. Redox signaling activated the ArcA regulatory system, which was responsible for TCA cycle repression.

CONCLUSIONS

This work employs both (13)C-MFA and transcription/metabolite analysis for quantitative investigation of the co-metabolism of glycerol and glucose in the ΔptsGglpK* mutant. The ArcA regulatory system dominates the control of flux redistribution. The ΔptsGglpK* mutant can be used as a platform for microbial cell factories for the production of biofuels and biochemicals, since most of fuel molecule (e.g., alcohols) synthesis requires excess reducing equivalents.