Controlling Ethanol Use in Chain Elongation by CO2 Loading Rate

Hexanoic acid production and microbial community in anaerobic fermentation: Effects of inorganic carbon addition

Carbon dioxide (CO2) plays a crucial role in carbon chain elongation with ethanol serving as an electron donor. In this study, the impacts of various carbonates on CO2 concentration, hexanoic acid production, and microbial communities during ethanol-butyric acid fermentation were explored. The results showed that the addition of MgCO3 provided sustained inorganic carbon and facilitated interspecific electron transfer, thereby increasing hexanoic acid yield by 58%. MgCO3 and NH4HCO3 inhibited the excessive ethanol oxidation and decreased the yield of acetic acid by 51% and 42%, respectively. The yields of hexanoic acid and acetic acid in the CaCO3 group increased by 19% and 15%, respectively. The NaHCO3 group exhibited high headspace CO2 concentration, promoting acetogenic bacteria enrichment while reducing the abundance of Clostridium_sensu_stricto_12. The batch addition of NaHCO3 accelerated the synthesis of hexanoic acid and increased its production by 26%. The relative abundance of Clostridium_sensus_stricto_12 was positively correlated with hexanoic acid production.

Recent progress and prospects for chain elongation of transforming biomass waste into medium-chain fatty acids

Chain elongation technology utilises microorganisms in anaerobic digestion to transform waste biomass into medium-chain fatty acids that have greater economic value. This innovative technology expands upon traditional anaerobic digestion methods, requiring abundant substrates that serve as electron donors and acceptors, and inoculating microorganisms with chain elongation functions. While this process may result in the production of by-products and elicit competitive responses, toxicity suppression of microorganisms by substrates and products remains a significant obstacle to the industrialisation of chain elongation technology. This study provides a comprehensive overview of existing research on widely employed electron donors and their synthetic reactions, competitive reactions, inoculum selection, toxicity inhibition of substrates and products, and increased chain elongation approaches. Additionally, it presents actionable recommendations for future research and development endeavours in this domain, intending to inspire and guide researchers in advancing the frontiers of chain elongation technology.

CO2 supply is a powerful tool to control homoacetogenesis, chain elongation and solventogenesis in ethanol and carboxylate fed reactor microbiomes

Anaerobic fermentation technology enables the production of medium chain carboxylates and alcohols through microbial chain elongation. This involves steering reactor microbiomes to yield desired products, with CO2 supply playing a crucial role in controlling ethanol-based chain elongation and facilitating various bioprocesses simultaneously. In the absence of CO2 supply (Phase I), chain elongation predominantly led to n-caproate with a high selectivity of 96 Cmol%, albeit leaving approximately 80% of ethanol unconverted. During this phase, C. kluyveri and Proteiniphilum-related species dominated the reactors. In Phase II, with low CO2 input (2.0 NmL L-1 min-1), formation of n-butyrate, butanol, and hexanol was stimulated. Increasing CO2 doses in Phase III (6 NmL L-1 min-1) led to CO2 utilization via homoacetogenesis, coinciding with the enrichment of Clostridium luticellarii, a bacterium that can use CO2 as an electron acceptor. Lowering CO2 dose to 0.5 NmL L-1 min-1 led to a shift in microbiome composition, diminishing the dominance of C. luticellarii while increasing C. kluyveri abundance. Additionally, other Clostridia, Proteiniphilum, and Lactobacillus sakei-related species became prevalent. This decrease in CO2 load from 6 to 0.5 NmL L-1 min-1 minimized excessive ethanol oxidation from 30%-50% to 0%-3%, restoring a microbiome favoring net n-butyrate consumption and n-caproate production. The decreased ethanol oxidation coincided with the resurgence of hydrogen formation at partial pressures above 1%. High concentrations of butyrate, caproate, and ethanol in the reactor, along with low acetate concentration, promoted the formation of butanol and hexanol. It is evident that CO2 supply is indispensable for controlling chain elongation in an open culture and it can be harnessed to stimulate higher alcohol formation or induce CO2 utilization as an electron acceptor.

Influence of pH and temperature on the performance and microbial community during the production of medium-chain carboxylic acids using winery effluents as substrate

Winery effluents containing high ethanol concentrations and diverse organic matter are ideal substrates for producing medium-chain carboxylic acids via fermentation and chain elongation. However, the process needs to be better understood. This study presents novel insights into the bioconversion mechanisms of medium-chain carboxylic acids by correlating fermentation and chain elongation kinetic profiles with the study of microbial communities at different pH (5 to 7) conditions and temperatures (30 to 40 °C). It was found that high productivities of MCCA were obtained using a native culture and winery effluents as a natural substrate. Minor pH variations significantly affected the metabolic pathway of the microorganisms for MCCA production. The maximal productivities of hexanoic (715 mg/L/d) and octanoic (350 mg/L/d) acids were found at pH 6 and 35 °C. Results evidence that the presence of Clostridium, Bacteroides, and Negativicutes promotes the high productions of MCCA. The formation of heptanoic acid was favor when Mogibacterium and Burkholderia were present.

Research on the Resource Recovery of Medium-Chain Fatty Acids from Municipal Sludge: Current State and Future Prospects

The production of municipal sludge is steadily increasing in line with the production of sewage. A wealth of organic contaminants, including nutrients and energy, are present in municipal sludge. Anaerobic fermentation can be used to extract useful resources from sludge, producing hydrogen, methane, short-chain fatty acids, and, via further chain elongation, medium-chain fatty acids. By comparing the economic and use values of these retrieved resources, it is concluded that a high-value resource transformation of municipal sludge can be achieved via the production of medium-chain fatty acids using anaerobic fermentation, which is a hotspot for future research. In this study, the selection of the pretreatment method, the method of producing medium-chain fatty acids, the influence of the electron donor, and the technique used to enhance product synthesis in the anaerobic fermentation process are introduced in detail. The study outlines potential future research directions for medium-chain fatty acid production using municipal sludge. These acids could serve as a starting point for investigating other uses for municipal sludge.

Metagenomic analysis towards understanding the effects of ammonia on chain elongation process for medium chain fatty acids production

The production of medium chain fatty acids (MCFAs) through chain elongation (CE) from organic wastes/wastewater has attracted much attention, while the effects of a common inhibitor-ammonia has not been elucidated. The mechanism of ammonia affecting CE was studied by metagenomic. The lag phase duration of caproate production was increased, and the maximum caproate production rate was decreased by 43.4 % at 4 g-N/L, as compared to 0 g-N/L. And hydrochar (HC) alleviated the inhibition of ammonia at 4 g-N/L. Metagenomic analysis indicated that ammonia induced UBA4085 sp.FDU78 as the dominant microorganism, and metabolic reconstruction revealed its potential CE ability. Furthermore, ammonia inhibited the reverse β oxidation pathway and Acetyl-CoA production pathway. The tolerance of UBA4085 sp.FDU78 to ammonia was associated with the uptake of inorganic ions, energy conservation, and synthesis of osmoprotectants. The present study provided a deep-insight on the ammonia tolerance mechanism on the CE process.

Integrated biogas upgrading and medium-chain fatty acids production for more efficient resource recovery

The conversion of carbon dioxide (CO2) from biogas into medium-chain fatty acids (MCFAs) represents an eco-friendly resource recovery approach to reduce dependence on fossil fuels and combat global climate change. This study presented the novel concept of integrated resource recovery by coupling biogas upgrading and MCFAs production for the first time. Initially, the impact of different initial ethanol concentrations on chain elongation was examined, determining that an ethanol concentration of 160 mmol/L maximized MCFAs yield at 45.7 mmol/L. Subsequently, using this optimal ethanol supply, the integrated strategy was implemented by connecting two bioreactors in series and maintaining continuous operation for 160-day. The results were noteworthy: upgraded bio-methane purity reached 97.6 %, MCFAs production rate and carbon-flow distribution reached 24.5 mmol/L d-1 and 76.1 %, respectively. In summary, these promising outcomes pioneer a resource recovery approach, enabling the high-value utilization of biogas and the conversion of CO2 into valuable bio-chemicals.

Carbon dioxide as key player in chain elongation and growth of Clostridium kluyveri: Insights from batch and bioreactor studies

Chain elongation technology allows medium-chain fatty acids (MCFAs) production as an alternative to fossil resources. Clostridium kluyveri generates n-caproate primarily from ethanol and acetate, presumably requiring CO2 for growth. Here, the impact of CO2 on C. kluyveri was explored. Bottle studies revealed the bacterium's adaptability to low CO2 levels, even in conditions with minimal dissolved NaHCO3 (0.0003 M) and unfavorable pH (below 6) under 1 bar CO2. Bioreactor investigations demonstrated a direct correlation between CO2 availability and bacterial growth. The highest n-caproate production (11.0 g/L) with 90.1 % selectivity was achieved in a bioreactor with continuous CO2 supply at 3 mL/min. Additional bottle experiments pressurized with 1 bar CO2 and varying ethanol:acetate ratios (1:1, 2:1, 4:1) also confirmed CO2 consumption by C. kluyveri. However, increasing the ethanol:acetate ratio did not enhance n-caproate selectivity, likely due to overly acidic pH conditions. These findings provide insights into chain-elongators responses under diverse conditions.

Carbon dioxide reduction to high-value chemicals in microbial electrosynthesis system: Biological conversion and regulation strategies

Large emissions of atmospheric carbon dioxide (CO2) are causing climatic and environmental problems. It is crucial to capture and utilize the excess CO2 through diverse methods, among which the microbial electrosynthesis (MES) system has become an attractive and promising technology to mitigate greenhouse effects while reducing CO2 to high-value chemicals. However, the biological conversion and metabolic pathways through microbial catalysis have not been clearly elucidated. This review first introduces the main acetogenic bacteria for CO2 reduction and extracellular electron transfer mechanisms in MES. It then intensively analyzes the CO2 bioconversion pathways and carbon chain elongation processes in MES, together with energy supply and utilization. The factors affecting MES performance, including physical, chemical, and biological aspects, are summarized, and the strategies to promote and regulate bioconversion in MES are explored. Finally, challenges and perspectives concerning microbial electrochemical carbon sequestration are proposed, and suggestions for future research are also provided. This review provides theoretical foundation and technical support for further development and industrial application of MES for CO2 reduction.

CO2 uptake in ethanol-driven chain elongation system: Microbial metabolic mechanisms

CO2 as a byproduct of organic waste/wastewater fermentation has an important impact on the carboxylate chain elongation. In this study, a semi-continuous flow reactor was used to investigate the effects of CO2 loading rates (Low = 0.5 LCO2·L-1·d-1, Medium = 1.0 LCO2·L-1·d-1, High = 2.0 LCO2·L-1·d-1) on chain elongation system Ethanol and acetate were utilized as the electron donor and electron acceptor, respectively. The results demonstrate that low loading rate of CO2 has a positive effect on chain elongation. The maximum production of caproate and CH4 were observed at a low CO2 loading rate. Caproate production reached 1.88 g COD·L-1·d-1 with a selectivity of 62.55 %, while CH4 production reached 129.7 ml/d, representing 47.4 % of the total. Metagenomic analysis showed that low loading rate of CO2 favored the enrichment of Clostridium kluyveri, with its abundance being 3.8 times higher than at of high CO2 loading rate. Metatranscriptomic analysis revealed that high CO2 loading rate induced oxidative stress in microorganisms, as evidenced by increased expression of heat shock proteins and superoxide dismutase genes. Further investigation suggested that genes associated with the reverse β-oxidation pathway, CO2 uptake pathway and hydrogenotrophic methanogenesis pathway were reduced at high CO2 loading rate. These findings provide insight into the underlying mechanisms of how CO2 affects chain elongation, and it could be a crucial reason for the poor performance of chain elongation systems with high endogenous CO2 production.

Expanding the product portfolio of carbon dioxide and hydrogen-based gas fermentation with an evolved strain of Clostridium carboxidivorans

CO2:H2-based gas fermentation with acetogenic Clostridium species are at an early stage of development. This work exploited the Adaptive Laboratory Evolution technique to improve the growth of C. carboxidivorans P7 on CO2 and H2. An adapted strain with decreased growth lag phase and improved biomass production was obtained. Genomic analysis revealed a conserved frameshift mutation in the catalytic subunit of the hexameric hydrogenase gene. The resulted truncated protein variant, most likely lacking its functionality, suggests that other hydrogenases might be more efficient for H2-based growth of this strain. Furthermore, the adapted strain generated hexanol as primary fermentation product. For the first time, hexanol was produced directly from CO2:H2 blend, achieving the highest maximum productivity reported so far via gas fermentation. Traces of valerate, pentanol, eptanol and octanol were observed in the fermentation broth. The adapted strain shows promising to enrich the product spectrum targetable by future gas fermentation processes.

Co-electron donors driven medium-chain fatty acids production: Roles of electron donors, reaction kinetics and metabolic pathways

Energy conversion of waste activated sludge alkaline fermentation liquor (WASAFL) to medium-chain fatty acids (MCFAs) is promising for sludge treatment and carbon recovery. However, the single electron donor (ED) fermentation for MCFAs production has irreparable defects. To resolve the respective shortcomings of single electron donor (ED) and improve the MCFAs production efficiency from WASAFL, a novel biotechnical process utilizing ethanol and lactate as co-EDs within different combination ratios were investigated. The results verified that MCFAs production was highest with ethanol to lactate ratio of 1:3 (6988.54 ± 208.18 mg COD/L), being 1.46 and 1.87 times of that with ethanol and lactate as single ED. The kinetic analysis results confirmed that ethanol to lactate ratio of 1:3 resulted in the highest MCFAs yield and formation rate. The microbial taxa results uncovered that the relative abundance of Sphaerochaeta and Haloimpatiens showed positive correlation with MCFAs production. The metabolic pathway analysis indicated that the ethanol oxidization, lactate oxidization, acrylate pathway, reverse β oxidization and fatty acid biosynthesis pathway might take place in the WASAFL fermentation system, contributing to the WASAFL-to-MCFAs conversion.

Co-fermenting lactic acid and glucose towards caproic acid production

The functional role of lactate (H_Lac), as a co-substrate along with glucose (Glu) as well as an electron donor for the synthesis of caproic acid (H_Ca), a medium chain fatty acid (MCFAs) was studied. A varied H_Lac and Glu ratios were thus investigated in fed-batch anaerobic reactors (R1-R5) operated at pH 6 with a heat-treated anaerobic consortium. R1 and R5 were noted as controls and operated with sole Glu and H_Lac, respectively. Strategically, ethanol (H_Eth) was additionally supplemented as co-electron donor after the production of short chain carboxylic acids (SCCAs) for chain elongation in all the reactors. The reactor operated with H_Lac and Glu in a ratio of 0.25:0.75 (1.25 g/L (H_Lac) and 3.75 g/L (Glu)) showed the highest H_Ca production of 1.86 g/L. R5 operated with solely H_Lac yielded propionic acid (H_Pr) as the major product which further led to the higher valeric acid (H_Va) production of 1.1 g/L within the reactor. Butyric acid (H_Bu) was observed in R1, which used Glu as carbon source alone indicating the importance of H_Lac as electron co-donor. Clostridium observed as the most dominant genera in shotgun metagenome sequencing in R2 and R3, the reactors that produced the highest H_Ca in comparison to other studied reactors. The study thus provided insight into the importance of substrate and electron donor and their supplementation strategies during the production of MCFAs.

Enhanced ethanol-driven carboxylate chain elongation by Pt@C in simulated sequencing batch reactors: Process and mechanism

Carboxylate chain elongation can create value-added bioproducts from the organic waste. The effects of Pt@C on chain elongation and associated mechanisms were investigated in simulated sequencing batch reactors. 5.0 g/L of Pt@C greatly increased the synthesis of caproate, with an average yield of 21.5 g COD/L, which was 207.4% higher than the trial without Pt@C. Integrated metagenomic and metaproteomic analyses were used to reveal the mechanism of Pt@C-enhanced chain elongation. Pt@C enriched chain elongators by increasing the relative abundance of dominant species by 115.5%. The expression of functional genes related to chain elongation was promoted in the Pt@C trial. This study also demonstrates that Pt@C may promote overall chain elongation metabolism by enhancing CO₂ uptake of Clostridium kluyveri. The study provides insights into the fundamental mechanisms of how chain elongation can perform CO₂ metabolism and how it can be enhanced by Pt@C to upgrade bioproducts from organic waste streams.

A specific H2/CO2 consumption molar ratio of 3 as a signature for the chain elongation of carboxylates from brewer's spent grain acidogenesis

Brewer's spent grain (BSG) is an undervalorized organic feedstock residue composed of fermentable macromolecules, such as proteins, starch, and residual soluble carbohydrates. It also contains at least 50% (as dry weight) of lignocellulose. Methane-arrested anaerobic digestion is one of the promising microbial technologies to valorize such complex organic feedstock into value-added metabolic intermediates, such as ethanol, H₂, and short-chain carboxylates (SCC). Under specific fermentation conditions, these intermediates can be microbially transformed into medium-chain carboxylates through a chain elongation pathway. Medium-chain carboxylates are of great interest as they can be used as bio-based pesticides, food additives, or components of drug formulations. They can also be easily upgraded by classical organic chemistry into bio-based fuels and chemicals. This study investigates the production potential of medium-chain carboxylates driven by a mixed microbial culture in the presence of BSG as an organic substrate. Because the conversion of complex organic feedstock to medium-chain carboxylates is limited by the electron donor content, we assessed the supplementation of H₂ in the headspace to improve the chain elongation yield and increase the production of medium-chain carboxylates. The supply of CO₂ as a carbon source was tested as well. The additions of H₂ alone, CO₂ alone, and both H₂ and CO₂ were compared. The exogenous supply of H₂ alone allowed CO₂ produced during acidogenesis to be consumed and nearly doubled the medium-chain carboxylate production yield. The exogenous supply of CO₂ alone inhibited the whole fermentation. The supplementation of both H₂ and CO₂ allowed a second elongation phase when the organic feedstock was exhausted, which increased the medium-chain carboxylate production by 285% compared to the N₂ reference condition. Carbon- and electron-equivalent balances, and the stoichiometric ratio of 3 observed for the consumed H₂/CO₂, suggest an H₂- and CO₂-driven second elongation phase, converting SCC to medium-chain carboxylates without an organic electron donor. The thermodynamic assessment confirmed the feasibility of such elongation.

Upgrading dilute ethanol to odd-chain carboxylic acids by a synthetic co-culture of Anaerotignum neopropionicum and Clostridium kluyveri

BACKGROUND

Dilute ethanol streams generated during fermentation of biomass or syngas can be used as feedstocks for the production of higher value products. In this study, we describe a novel synthetic microbial co-culture that can effectively upgrade dilute ethanol streams to odd-chain carboxylic acids (OCCAs), specifically valerate and heptanoate. The co-culture consists of two strict anaerobic microorganisms: Anaerotignum neopropionicum, a propionigenic bacterium that ferments ethanol, and Clostridium kluyveri, well-known for its chain-elongating metabolism. In this co-culture, A. neopropionicum grows on ethanol and CO₂ producing propionate and acetate, which are then utilised by C. kluyveri for chain elongation with ethanol as the electron donor.

RESULTS

A co-culture of A. neopropionicum and C. kluyveri was established in serum bottles with 50 mM ethanol, leading to the production of valerate (5.4 ± 0.1 mM) as main product of ethanol-driven chain elongation. In a continuous bioreactor supplied with 3.1 g ethanol L^-1 d^-1, the co-culture exhibited high ethanol conversion (96.6%) and produced 25% (mol/mol) valerate, with a steady-state concentration of 8.5 mM and a rate of 5.7 mmol L^-1 d^-1. In addition, up to 6.5 mM heptanoate was produced at a rate of 2.9 mmol L^-1 d^-1. Batch experiments were also conducted to study the individual growth of the two strains on ethanol. A. neopropionicum showed the highest growth rate when cultured with 50 mM ethanol (μ_max = 0.103 ± 0.003 h^-1) and tolerated ethanol concentrations of up to 300 mM. Cultivation experiments with C. kluyveri showed that propionate and acetate were used simultaneously for chain elongation. However, growth on propionate alone (50 mM and 100 mM) led to a 1.8-fold reduction in growth rate compared to growth on acetate. Our results also revealed sub-optimal substrate use by C. kluyveri during odd-chain elongation, where excessive ethanol was oxidised to acetate.

CONCLUSIONS

This study highlights the potential of synthetic co-cultivation in chain elongation processes to target the production of OCCAs. Furthermore, our findings shed light on to the metabolism of odd-chain elongation by C. kluyveri.

Polyhydroxyalkanoates and 5-aminolevulinic acid production by a mixed phototrophic culture using medium-chain carboxylic acids from winery effluents

This work aimed to obtain polyhydroxyalkanoates (PHA) and 5-aminolevulinic acid (5-ALA) from medium-chain carboxylic acids (MCCA) using a mixed culture enriched in Rhodopseudomnas palustris. MCCA, obtained from residual wine lees, were tested in batch photofermentation experiments. First, the influence of individual MCCA (hexanoic, heptanoic, and octanoic acids) was evaluated; then, the MCCA coming directly from a fermentation reactor (LC-effluent) or after acids extraction (HC-effluent) were studied. Nutrient supplementation, bicarbonate, and acetic acid addition were also tested. Results showed that PHA production was higher in hexanoic (328 mg PHA/L) compared to heptanoic (152 mg PHA/L) and octanoic (164 mg PHA/L) acids. Bicarbonate addition and acetic acid as co-substrate improved the MCCA consumption, the PHA content and production rate. The HC-effluent, without nutrient supplementation, was allowed to increase 2.5 times the PHA content (reaching 40 % w/w and 584 mg/L) and to double 5-ALA production (7.6 µM) compared to the LC-effluent condition.

Acclimation of anaerobic fermentation microbiome with acetate and ethanol for chain elongation and the biochemical response

Medium chain fatty acids (MCFAs) production is a promising method for resource recovery from organic wastes. In this study, the microbial community structure shift along the long-term acclimation experiment and the concomitant effect of H₂ level on chain elongation performance was investigated. Chain elongation microbiome could be rapidly acclimated from traditional anaerobic fermentation consortia. Genera Caproiciproducens, Clostridium sensu stricto 12, Rummeliibacillus and Oscillibacter was found to be dominant during the operation. The H₂ was accumulated in the headspace by increasing the ethanol input, which inhibited oxidation of caproate and butyrate immediately, while its inhibition effect on chain elongation was delayed. H₂ level in the headspace was positively correlated to the MCFAs production related bacteria. However, too much H₂ accumulated might be suppressive for MCFAs production in the long term. It might result from the thermodynamic barrier for discarding excess reducing equivalents under high H₂ level, which further gave rise to ethanol accumulation in this system.

Microbial Recycling of Bioplastics via Mixed-Culture Fermentation of Hydrolyzed Polyhydroxyalkanoates into Carboxylates

Polyhydroxyalkanoates (PHA) polymers are emerging within biobased biodegradable plastic products. To build a circular economy, effective recycling routes should be established for these and other end-of-life bioplastics. This study presents the first steps of a potential PHA recycling route by fermenting hydrolyzed PHA-based bioplastics (Tianan ENMATTM Y1000P; PHBV (poly(3-hydroxybutyrate-co-3-hydroxyvalerate)) into carboxylates acetate and butyrate. First, three different hydrolysis pretreatment methods under acid, base, and neutral pH conditions were tested. The highest 10% (from 158.8 g COD/L to 16.3 g COD/L) of hydrolysate yield was obtained with the alkaline pretreatment. After filtration to remove the remaining solid materials, 4 g COD/L of the hydrolyzed PHA was used as the substrate with the addition of microbial nutrients for mixed culture fermentation. Due to microbial conversion, 1.71 g/L acetate and 1.20 g/L butyrate were produced. An apparent complete bioconversion from intermediates such as 3-hydroxybutyrate (3-HB) and/or crotonate into carboxylates was found. The overall yields of the combined processes were calculated as 0.07 g acetate/g PHA and 0.049 g butyrate/g PHA. These produced carboxylates can theoretically be used to reproduce PHA or serve many other applications as part of the so-called carboxylate platform.

Competitive Reactions during Ethanol Chain Elongation Were Temporarily Suppressed by Increasing Hydrogen Partial Pressure through Methanogenesis Inhibition

Organic waste streams can be converted into high-value platform chemicals such as medium-chain carboxylic acids (MCCAs) using mixed microbial communities via chain elongation. However, the heterogeneity of waste streams and the use of complex microbial communities can lead to undesirable reactions, thus decreasing process efficiency. We explored suppressing excessive ethanol oxidation to acetate (EEO) by increasing the hydrogen partial pressure (PH2) through hydrogenotrophic methanogenesis inhibition by periodically adding 2-bromoethanesulfonate (2-BES) to an MCCA-producing bioreactor to reach 10 mM of 2-BES upon addition. The bioreactor was fed with pretreated food waste and brewery waste containing high concentrations of short-chain carboxylic acids and ethanol, respectively. While 2-BES addition initially reduced EEO, some methanogens (Methanobrevibacter spp.) persisted and resistant populations were selected over time. Besides changing the methanogenic community structure, adding 2-BES also changed the bacterial community structure due to its impact on PH2. While we demonstrated that PH2 could be manipulated using 2-BES to control EEO, methods that do not require the addition of a chemical inhibitor should be explored to maintain optimum PH2 for long-term suppression of EEO.

Ferrate pretreatment-anaerobic fermentation enhances medium-chain fatty acids production from waste activated sludge: Performance and mechanisms

The rupture of cytoderm and extracellular polymeric substances (EPS), and competitive inhibition of methanogens are the main bottlenecks for medium-chain fatty acids (MCFAs) production from waste activated sludge (WAS). This study proposes a promising ferrate (Fe (VI))-based technique to enhance MCFAs production from WAS through accelerating WAS disintegration and substrates transformation, and eliminating competitive inhibition of methanogens, simultaneously. Results shows that the maximal MCFAs production attains 8106.3 mg COD/L under 85 mg Fe/g TSS, being 58.6 times that of without Fe (VI) pretreatment. Mechanism exploration reveals that Fe (VI) effectively destroys EPS and cytoderm through electron transfer, reactive oxygen species generation (i.e., OH, O₂^- and ¹O₂) and elevated alkalinity, resulting in the transfer of organics from solid to soluble phase and from macromolecules to intermediates. Generation and transformation of intermediates analyses illustrate that Fe (VI) facilitates hydrolysis, acidification and chain elongation (CE) but suppresses methanogenesis, promoting the targeted conversion of intermediates to MCFAs. Also, Fe (VI) pretreatment provides potential electron shuttles for chain elongation. Microbial community and functional genes encoding key enzymes analysis indicates that Fe (VI) screens key microorganisms and up-regulates functional genes expression involved in CE pathways. Overall, this technology avoids methanogens inhibitor addition and stimulates vivianite synthesis during MCFAs production from WAS.

Recent progress and challenges in biotechnological valorization of lignocellulosic materials: Towards sustainable biofuels and platform chemicals synthesis

Lignocellulosic materials (LCM) have garnered attention as feedstocks for second-generation biofuels and platform chemicals. With an estimated annual production of nearly 200 billion tons, LCM represent an abundant source of clean, renewable, and sustainable carbon that can be funneled to numerous biofuels and platform chemicals by sustainable microbial bioprocessing. However, the low bioavailability of LCM due to the recalcitrant nature of plant cell components, the complexity and compositional heterogeneity of LCM monomers, and the limited metabolic flexibility of wild-type product-forming microorganisms to simultaneously utilize various LCM monomers are major roadblocks. Several innovative strategies have been proposed recently to counter these issues and expedite the widespread commercialization of biorefineries using LCM as feedstocks. Herein, we critically summarize the recent advances in the biological valorization of LCM to value-added products. The review focuses on the progress achieved in the development of strategies that boost efficiency indicators such as yield and selectivity, minimize carbon losses via integrated biorefinery concepts, facilitate carbon co-metabolism and carbon-flux redirection towards targeted products using recently engineered microorganisms, and address specific product-related challenges, to provide perspectives on future research needs and developments. The strategies and views presented here could guide future studies in developing feasible and economically sustainable LCM-based biorefineries as a crucial node in achieving carbon neutrality.

Boosting ethanol production rates from carbon dioxide in MES cells under optimal solventogenic conditions

Microbial Electrosynthesis (MES) has been widely applied for acetic acid (HA) production from CO₂ and electricity. Ethanol (EtOH) has a higher market value than HA, and wide application in industry and as a biofuel. However, it has only been obtained sporadically and at low concentrations, probably due to sub-optimal operating conditions. This study aimed at enhancing EtOH productivity in MES cells by jointly optimising key operation parameters, including pH, H₂ and CO₂ partial pressure (pH₂ and pCO₂), and HA concentration, to promote solventogenesis. Two H-type cells were operated in fed-batch mode at -0.8 V vs. SHE with CO₂ as the sole carbon source. A mixed culture, enriched with Clostridium ljungdahlii was used as the biocatalyst. The combination of low pH (<4.5) and pCO₂ (<0.3 atm), along with high HA concentration (about 6 g L^-1) and pH₂ (>3 atm), were mandatory conditions for maintaining an efficient solventogenic culture, dominated by Clostridium sp., capable of high-rate EtOH production. The maximum EtOH production rate was 10.95 g m^-2 d^-1, and a concentration of 5.28 g L^-1 was achieved. Up to 30 % of the electrons and 15.2 % of the carbon provided were directed towards EtOH production, and 28.1 kWh were required for the synthesis of 1 kg of EtOH from CO₂. These results highlight that strict conditions are required for a continuous, reliable, EtOH production in MES cells. Future investigation should focus on improving cell configuration to achieve EtOH production at higher current densities while minimizing the electric energy input.

Regulation of acidogenic fermentation through exogenous additives for promoting carbon conversion of food waste in two-phase anaerobic system

In this study, exogenous Megasphaera elsdenii inoculum and acetate supplementation were introduced at the acidogenic phase to regulate the acidogenic fermentation pathway and assess their effects on food waste (FW) carbon conversion in two-phase anaerobic digestion (AD) system. These two additives significantly accelerated organic removal efficiency and subsequently increased FW hydrolysis and acidogenesis by 16% and 35%, respectively. As expected, two exogenous additives promoted butyrate fermentation during FW acidogenesis. With regard to the role of exogenous additives, both hydrogen and butyrate yields increased by over 60%. This desired increment resulted in a 25% increase in methane production. The overall carbon conversion from FW in the integrated two-phase AD system was enhanced by biochemical additives, which was 1.3-fold higher than that in control without any additives. Collectively, findings demonstrate the feasibility of regulating acidogenic fermentation via exogenous biochemical additives and its benefits on FW carbon conversion during AD process.

Influence of lactate to acetate ratio on biological production of medium chain carboxylates via open culture fermentation

Waste valorisation via biological production of widely used in the industry medium chain carboxylates (MCCs) via open culture fermentation (OCF) could be a promising alternative to the commonly used anaerobic digestion. Lactate-rich waste streams are considered as valuable substrates for carboxylate chain elongation (CE), however, there are certain limitations related to the production efficiency. Acetate produced and accumulated in the acetogenesis plays an important role in CE, i.e. acetate is elongated to butyrate and then to caproate which is most popular MCC. Henceforth, it was investigated whether the ratio of lactate to acetate (L:A) affected carboxylates yields and product distribution in the lactate-based CE in OCF. The tested L:A ratios influenced carboxylates selectivity in batch trials. In the ones with lactate as the sole carbon source, propionate production was predominant but when a higher relative acetate concentration was used, the production of butyrate and CE to caproate was favored. The co-utilization of lactate and acetate in a continuous process increased the production of butyrate and caproate compared to the phase with lactate as the sole carbon source, however, controlling the relative concentration of lactate and acetate during co-utilization was not an effective strategy for increasing caproate production. 16S rRNA gene amplicon reads mapping to Caproiciproducens were the most abundant in samples collected throughout the continuous processes regardless of the L:A ratios.

Enrichment of homoacetogens converting H2/CO2 into acids and ethanol and simultaneous methane production

An anaerobic granular sludge was enriched to utilize H₂/CO₂ in a continuous gas-fed up-flow anaerobic sludge reactor by applying operating conditions expected to produce acetic acid, butyric acid, and ethanol. Three stages of fermentation were found: Stage I with acetic acid accumulation with the highest concentration of 35 mM along with a pH decrease from initial 6 to 4.5. In Stage II, H₂/CO₂ was replaced by 100% H₂ to induce solventogenesis, whereas butyric acid was produced with the highest concentration of 2.5 mM. At stage III with 10 µM tungsten (W) addition, iso-valeric acid, valeric acid, and caproic acid were produced at pH 4.5-5.0. In the batch tests inoculated with the enriched sludge taken from the bioreactor (day 70), however, methane production occurred at pH 6. Exogenous 15 mM acetate addition enhanced both the H₂ and CO₂ consumption rate compared to exogenous 10, 30, and 45 mM acetate by the enriched sludge. Exogenous acetate was failed to be converted to ethanol using H₂ as electron donor by the enriched acetogens.

Enhancing the selective synthesis of butyrate in microbial electrosynthesis system by gas diffusion membrane composite biocathode

The reduction of carbon dioxide (CO₂) to high value-added multi-carbon compounds at the cathode is an emerging application of microbial electrosynthesis system (MES). In this study, a composite cathode consisting of hollow fiber membrane (HFM) and the carbon felt is designed to enhance the CO₂ mass transfer of the cathode. The result shows that the main products are acetate and butyrate without other substances. The electrochemical performance of the electrode is significantly improved after biofilm becomes matures. The composite cathode significantly reduces the "threshold" for the synthesis of butyrate. Moreover, CO₂ is dissolved and protons are consumed by synthesizing volatile fatty acids (VFAs) to maintain a stable pH inside the composite electrode. The synthesis mechanism of butyrate is that CO₂ is converted sequentially into acetate and butyrate. The microenvironment of the composite electrode enriches Firmicute. This composite electrode provides a novel strategy for regulating the microenvironment.

Strategy of electron acceptors for ethanol-driven chain elongation from kitchen waste

A two-phase kitchen waste (KW) fermentation was proposed in the current study to enhance medium-chain fatty acids (MCFAs) production from kitchen waste. In particular, effect of acetate to butyrate ratio (ABR) on MCFAs production was investigated which can be regulated by different pH and organic loading during the acidification phase. Medium ABR (1.00) was obtained when pH is 5.5 and organic loading is 20 g VS/L in FW acidification fermentation. Subsequent chain elongation fermentation demonstrated that the highest yield of caproate 9.67 g/L with selectivity of 79 %, and highest ethanol conversion efficiency of 1.11 was achieved in medium ABR system. Microbial community study showed that medium ABR significantly enrich the functional bacteria especially Clostridium kluyveri. The study provides a new method for chain elongation enhancement without addition of other additives in kitchen waste fermentation system and gives a guide for the regulation of the short-chain fatty acids distribution in its acidification phase.

Potentiality of recovering bioresource from food waste through multi-stage Co-digestion with enzymatic pretreatment

Food waste (FW) is not only a major social, nutritional and environmental issue, but also an underutilized resource with significant energy, which has not been fully explored currently. Considering co-digestion can adjust carbon to nitrogen ratio (C/N) of the feedstock and improve the synergetic interactions among microorganisms, anaerobic co-digestion (AnCoD) is then becoming an emerging approach to achieve higher energy recovery from FW while ensuring the stability of the system. To obtain higher economic gain from such biodegradable wastes, increasing attention has been paid on optimizing the system configuration or applying enzymatic hydrolysis before digesting FW. A better understanding on the potentiality of correlating enzymatic pretreatment and AnCoD operated in various system configuration would enhance the bioresource recovery from FW and increase revenue through treating this organic waste. Specifically, the biobased chemicals outputs from FW-related co-digestion system with different configuration were firstly compared in this review. A deep discussion concerning the challenges for achieving bioresources recovery from FW co-digestion systems with enzymatic pretreatment was then given. Recommendations for future studies regarding FW co-digestion were then proposed at last.

Current Trends in Biological Valorization of Waste-Derived Biomass: The Critical Role of VFAs to Fuel A Biorefinery

The looming climate and energy crises, exacerbated by increased waste generation, are driving research and development of sustainable resource management systems. Research suggests that organic materials, such as food waste, grass, and manure, have potential for biotransformation into a range of products, including: high-value volatile fatty acids (VFAs); various carboxylic acids; bioenergy; and bioplastics. Valorizing these organic residues would additionally reduce the increasing burden on waste management systems. Here, we review the valorization potential of various sustainably sourced feedstocks, particularly food wastes and agricultural and animal residues. Such feedstocks are often micro-organism-rich and well-suited to mixed culture fermentations. Additionally, we touch on the technologies, mainly biological systems including anaerobic digestion, that are being developed for this purpose. In particular, we provide a synthesis of VFA recovery techniques, which remain a significant technological barrier. Furthermore, we highlight a range of challenges and opportunities which will continue to drive research and discovery within the field. Analysis of the literature reveals growing interest in the development of a circular bioeconomy, built upon a biorefinery framework, which utilizes biogenic VFAs for chemical, material, and energy applications.

Caproic Acid-Producing Bacteria in Chinese Baijiu Brewing

Caproic acid can be used as spices, preservatives, animal feed additives, and biofuels. At the same time, caproic acid plays an important role in Chinese Baijiu. It is the precursor substance for the synthesis of ethyl caproate, which directly affects the quality of Chinese Baijiu. Caproic acid-producing bacteria are the main microorganisms that synthesize caproic acid in Chinese Baijiu, and the most common strain is Clostridium kluyveri. Caproic acid-producing bacteria synthesize n-caproic acid through reverse β-oxidation to extend the carboxylic acid chain. This method mainly uses ethanol and lactic acid as substrates. Ethanol and lactic acid are converted into acetyl-CoA, and acetyl-CoA undergoes a series of condensation, dehydrogenation, dehydration, and reduction to extend the carboxylic acid chain. This review addresses the important issues of caproic acid-producing bacteria in the brewing process of Baijiu: the common caproic acid-producing bacteria that have been reported metabolic pathways, factors affecting acid production, biological competition pathways, and the effect of mixed bacteria fermentation on acid production. It is hoped that this will provide new ideas for the study of caproic acid-producing bacteria in Chinese Baijiu.

Caproate production from xylose via the fatty acid biosynthesis pathway by genus Caproiciproducens dominated mixed culture fermentation

Caproate production from organic wastes is deemed as a novel strategy in mixed culture fermtation (MCF). However, producing caproate from natural sugar of xylose by MCF is seldom reported and the metabolic pathway is still unclear. Thus, the caproate production from xylose was investigated in this study by mesophilic MCF. The results showed that the caproate concentration from xylose (10 g/L) was 1.2 ± 0.17 g/L (equal to 2.7 gCOD/L) under pH 5.0. Dosing extra ethanol of 5 g/L could slightly increase the caproate production by ∼ 30% (i.e., 1.6 g/L). While dosing extra acetate of 5 g/L negatively affected the caproate production, which was just 0.2 g/L. The microbial analysis illustrated that genus Caproiciproducens was a main identified caproate producer, occupying over 80% of enriched mixed culture. The fatty acid biosynthesis pathway was identified via metagenomic analysis. These unexpected differences extended the understanding of caproate production from organic wastes.

Efficient production of medium chain fatty acids in microbial electrosynthesis with simultaneous bio-utilization of carbon dioxide and ethanol

Microbial electrosynthesis (MES) is a promising technology for chemicals production driven by renewable energy. However, how the medium chain fatty acids (MCFAs) production in MES is affected by the method of chain elongation is not clear, and no direct evidence is provided yet for a simultaneous bio-utilization of CO₂ and ethanol. In this study, different methods of chain elongation in MES reactors were investigated. During in-situ chain elongation, a maximum caproate concentration of 11.9 ± 0.6 g L^-1 was achieved, while the C6 specificity (56.4% ± 0.5%) was much lower than that of ex-situ chain elongation (78.7% ± 1.5%). Carbon distribution and reduction degree balance indicated a simultaneous bio-utilization of CO₂ and ethanol, and it was validated by the isotope tracer technique. MCFAs-forming microbes, acetogens, and electrochemically active microorganisms were enriched. This study provides fundamental insights relevant to the carbon and electron fluxes driven by electricity.

Produce individual medium chain carboxylic acids (MCCA) from swine manure: Performance evaluation and economic analysis

Environmental issues caused by untreated animal manure require the development of resource recovery from waste through a circular economy approach. Producing medium chain carboxylic acids (MCCA) with higher value than biogas from manure has become promising. The objective of this study was to develop an effective individual MCCA produce process utilizing manure. In this study, animal manure was firstly anaerobic fermentation into short chain fatty acids (SCFA), then acidified manure and ethanol were fed into the chain elongation reactor with gradually increasing the organic loading rate (OLR) from 7.0 to 18.5 gCOD/L/d, and the mixed MCCA was separated individually via a fractional distillation process. The SCFA fermentation occurred mainly at the first 10 days, and the optimum concentrations of SCFA for treatments at 2 %VS, 4 %VS and 6 %VS were 6.58, 10.40 and 14.10 g/L, respectively. For the chain elongation reactor, the maximum concentrations of n-caproate and n-caprylate were 10.25 and 0.63 g/L, respectively, which were comparable with that obtained from other complex wastes. Over 90% MCCA can be recovered from the fermentation broth via the optimized extractant of methyl tert-butyl ether (MTBE) and the fractional distillation system. Preliminary economic analysis shows that this MCCA production process presented a higher economic benefit (9.25 $/m³ manure) than traditional biogas production (2.65 $/m³ manure), making MCCA production from swine manure economically competitive. This work provides a new route for manure resource recovery besides the biogas process.

Performance and Mechanism of Fe3O4 Improving Biotransformation of Waste Activated Sludge into Liquid High-Value Products

This study demonstrated that Fe₃O₄ simultaneously improves the total production and formation rate of medium-chain fatty acids (MCFAs) and long-chain alcohols (LCAs) from waste activated sludge (WAS) in anaerobic fermentation. Results revealed that when Fe₃O₄ increased from 0 to 5 g/L, the maximal MCFA and LCA production increased significantly, and the optimal fermentation time was also remarkably shortened from 24 to 9 days. Moreover, Fe₃O₄ also enhanced WAS degradation, and the corresponding degradation rate in the fermentation system increased from 43.86 to 72.38% with an increase in Fe₃O₄ from 0 to 5 g/L. Further analysis showed that Fe₃O₄ promoted the microbe activities of all the bioprocesses (including hydrolysis, acidogenesis, and chain elongation processes) involved in the MCFA and LCA production from WAS. Microbial community analysis indicated that Fe₃O₄ increased the abundances of key microbes involved in abovementioned bioprocesses correspondingly. Mechanistic investigations showed that Fe₃O₄ increased the conductivity of the fermented sludge, providing a better conductive environment for the anaerobic microbes. The redox cycle of Fe(II) and Fe(III) existed in the fermentation system with Fe₃O₄, which was likely to act as electron shuttles to conduct electron transfer (ET) from the electron donor to the acceptor, thus increasing ET efficiency. This study provides an effective method for enhancing the biotransformation of WAS into high-value products, potentially bringing economic benefits to WAS treatment.

Transforming waste activated sludge into medium chain fatty acids in continuous two-stage anaerobic fermentation: Demonstration at different pH levels

Bioenergy recovery in the form of medium-chain fatty acids (MCFAs) from waste activated sludge (WAS) is increasingly attractive, which are valuable building blocks for fuel production. This study experimentally demonstrated the long-term MCFAs (C6-C8) production from WAS in two-stage anaerobic sludge fermentation at different pH conditions, using continuously operated bench-scale anaerobic reactors. The WAS was continuously converted to short chain fatty acids (SCFAs, 3500-3800 mg chemical oxygen demand (COD)/L) at the first stage via alkaline anaerobic fermentation, which was directly fed into the second stage as both substrates and inoculum for MCFAs production through chain elongation (CE). The productions of MCFAs at the second stage were continuously studied under three different pH conditions (i.e., 10, 7 and 5.5). The results demonstrated that there was no significant MCFAs production at pH 10 during the steady state, whereas the MCFAs productions were clearly observed at both pH 7 and pH 5.5, with much higher MCFAs production from WAS at pH 7 (i.e., 10.32 g COD/L MCFAs) than that at pH 5.5 (i.e., 8.73 g COD/L MCFAs) during the steady state. A higher MCFAs selectivity of 62.3% was also achieved at pH 7. The relatively lower MCFAs production and selectivity at pH 5.5 was likely due to the higher undissociated MCFAs generated at pH 5.5, which would pose toxicity impact on CE microbes and thus inhibit the CE process. Microbial community analysis confirmed that the relative abundances of CE related microbes (e.g., Clostridium sensu stricto 12 sp. and Clostridium sensu stricto 1) increased at pH 7 compared to those at pH 5.5, which enabled more efficient MCFAs production from WAS.

Upgrading the value of anaerobic fermentation via renewable chemicals production: A sustainable integration for circular bioeconomy

The single bioprocess approach has certain limitations in terms of process efficiency, product synthesis, and effective resource utilization. Integrated or combined bioprocessing maximizes resource recovery and creates a novel platform to establish sustainable biorefineries. Anaerobic fermentation (AF) is a well-established process for the transformation of organic waste into biogas; conversely, biogas CO₂ separation is a challenging and expensive process. Biological fixation of CO₂ for succinic acid (SA) mitigates CO₂ separation issues and produces commercially important renewable chemicals. Additionally, utilizing digestate rich in volatile fatty acid (VFA) to produce medium-chain fatty acids (MCFAs) creates a novel integrated platform by utilizing residual organic metabolites. The present review encapsulates the advantages and limitations of AF along with biogas CO₂ fixation for SA and digestate rich in VFA utilization for MCFA in a closed-loop approach. Biomethane and biohydrogen processes CO₂ utilization for SA production is cohesively deliberated along with the role of biohydrogen as an alternative reducing agent to augment SA yields. Similarly, MCFA production using VFA as a substrate and functional role of electron donors namely ethanol, lactate, and hydrogen are comprehensively discussed. A road map to establish the fermentative biorefinery approach in the framework of AF integrated sustainable bioprocess development is deliberated along with limitations and factors influencing for techno-economic analysis. The discussed integrated approach significantly contributes to promote the circular bioeconomy by establishing carbon-neutral processes in accord with sustainable development goals.

Chain elongation process for caproate production using lactate as electron donor in Megasphaera hexanoica

Megasphaera hexnaoica is anaerobic bacteria who has well running reverse β-oxidation pathway. In previous study, the strain showed excellent production of medium chain carboxylic acids (MCCAs) using fructose as electron donor. In this study, chain elongation process study using lactate instead of fructose was conducted in M. hexnaoica fermentation. It was found that M. hexanoica can use lactate as electron donor in chain elongation process. 8.9 g/L caproate production was achieved in fermentation using lactate as sole electron donor. Compare to fructose condition, lactate as electron donor showed more than 3 times higher specific titer and specific productivity. In addition, when fructose and lactate were used as electron donor simultaneously, further improvement of MCCAs production was observed to achieve maximum caproate productivity of 20.9 g/L/day. Utilization of lactate as electron donor in M. hexanoica showed potential opportunity in chain elongation process.

Effect of external acetate on lactate-based carboxylate platform: Shifted lactate overloading limit and hydrogen co-production

Chain elongation is an anaerobic biotechnological process that converts short chain carboxylates and an electron donor (e.g. ethanol, lactate) into more valuable medium chain carboxylates. Caproate production in lactate-based chain elongation is gaining popularity, however, the relation between lactate (electron donor) and acetate (electron acceptor) has not yet been fully elucidated. Herein, for the first time, the effect of an external acetate on the lactate-based chain elongation in a continuously-fed bioreactor was tested to verify how the external acetate would affect the product spectrum, gas production, as well as stability and efficiency of carboxylates production. Periodic fluctuations in caproate production were observed in bioreactor continuously fed with lactate as a sole carbon source due to the lack of an electron acceptor (acetate) and low chain elongation performance. The recovery of stable caproate production (68.9 ± 2.2 mmol C/L/d), total lactate consumption, and high hydrogen co-production (748 ± 76 mL_H2/d) was observed as an effect of the addition of an external acetate. The lactate conversion with the external acetate in the second bioreactor ensured stable and dominant caproate production from the beginning of the process. Moreover, despite the continuous lactate overloading in the process with external acetate, stable caproate production was achieved (71.7 ± 2.4 mmol C/L/d) and previously unobserved hydrogen production occurred (213 ± 30 mL_H2/d). Thus, external electron acceptor addition (i.e. acetate) was proposed as an effective method for stable lactate-based caproate production. Microbiological analysis showed the dominance of microbes closely related to Ruminococcaceae bacterium CPB6 and Acinetobacter throughout the process. Co-occurrence networks based on taxon abundances and process parameters revealed microbial sub-networks responding to lactate concentrations.

Metabolic cascade of complex organic wastes to medium-chain carboxylic acids: A review on the state-of-the-art multi-omics analysis for anaerobic chain elongation pathways

Medium-chain carboxylic acid (MCCA) production from organic wastes has attracted much attention because of their higher energy contents and diverse applications. Anaerobic reactor microbiomes are stable and resilient and have resulted in efficient performance during many years of operation for thousands of full-scale anaerobic digesters worldwide. The method underlying how the relevant microbial pathways contribute to elongate carbon chains in reactor microbiomes is important. In particular, the reverse β-oxidation pathway genes are critical to upgrading short-chain fermentation products to MCCAs via a chain elongation (CE) process. Diverse genomics and metagenomics studies have been conducted in various fields, ranging from intracellular metabolic pathways to metabolic cascades between different strains. This review covers taxonomic approach to culture processes depending on types of organic wastes and the deeper understanding of genome and metagenome-scale CE pathway construction, and the co-culture and multi-omics technology that should be addressed in future research.

Biological production of medium-chain carboxylates through chain elongation: An overview

Medium chain carboxylates (MCCs) have wide applications in various industries, but the traditional MCCs production methods are costly and unsustainable. Anaerobic fermentation offers a more scalable, economical and eco-friendly platform for producing MCCs through chain elongation which converts short chain carboxylates and electron donor into more valuable MCCs. However, the underlying microbial pathways are not well understood. In this review, biological production of MCCs through chain elongation is introduced elaborately, including the metabolic pathways, electron donor and substrates, microorganisms and influencing factors. Then, the strategies for enhancing MCCs production are extensively analyzed and summarized, along with the technologies for MCCs separation from the fermentation broth. Finally, challenges and perspectives concerning the large-scale MCCs production are proposed, providing suggestions for the future research. Extensive review demonstrated that anaerobic fermentation has great potential in achieving economical and sustainable MCCs production from complex organic substrates, including organic waste streams, which would significantly broaden the application of MCCs, especially in the renewable energy field. An interdisciplinary approach with knowledge from microbiology and biochemistry to chemical separations and environmental engineering is required to use this promising technology as a valorization method for converting organic biomass or organic wastes into valuable MCCs.

Reactor Designs and Configurations for Biological and Bioelectrochemical C1 Gas Conversion: A Review

Microbial C1 gas conversion technologies have developed into a potentially promising technology for converting waste gases (CO₂, CO) into chemicals, fuels, and other materials. However, the mass transfer constraint of these poorly soluble substrates to microorganisms is an important challenge to maximize the efficiencies of the processes. These technologies have attracted significant scientific interest in recent years, and many reactor designs have been explored. Syngas fermentation and hydrogenotrophic methanation use molecular hydrogen as an electron donor. Furthermore, the sequestration of CO₂ and the generation of valuable chemicals through the application of a biocathode in bioelectrochemical cells have been evaluated for their great potential to contribute to sustainability. Through a process termed microbial chain elongation, the product portfolio from C1 gas conversion may be expanded further by carefully driving microorganisms to perform acetogenesis, solventogenesis, and reverse β-oxidation. The purpose of this review is to provide an overview of the various kinds of bioreactors that are employed in these microbial C1 conversion processes.

Opportunities and challenges in microbial medium chain fatty acids production from waste biomass

Medium chain fatty acids (MCFAs) that produced from affordable waste biomass via chain elongation (CE) technology are recognized as the potential alternatives to part fossil-derived chemicals, contributing to the sustainable development of economy and environment. The purpose of this review is to provide comprehensive analyses on the opportunities and challenges of MCFAs production and application. First, both two microbial MCFAs synthesis pathways of reverse β-oxidation and fatty acid biosynthesis were introduced/compared in detail to give readers a thorough understanding of the CE process, with the expectation of further boosting MCFAs production by well distinguishing them. Furthermore, the six key MCFAs production bottlenecks, corresponding research progresses, and possible solutions were analyzed. Five major MCFAs production strategies with their production mechanism, performances, and characteristics were also critically assessed. Additionally, the commercial production status was introduced, and future alternative production mode and research priorities were also recommended.

Phenol promoted caproate production via two-stage batch anaerobic fermentation of organic substance with ethanol as electron donor for chain elongation

The conversion of organic wastes/wastewater into medium chain fatty acids (MCFAs) such as caproate has attracted much attention, while the effects of toxic compounds on the process have rarely been studied. The present study investigated the effects of phenol (0-1.5 g/L), which is a toxicant and present in various organic wastes, on the caproate production in the chain elongation (CE) process with ethanol as electron donor via two-stage batch anaerobic fermentation of glucose. The results showed phenol ≤ 1 g/L did not affect short chain fatty acids (SCFAs) production, while 1 g/L phenol increased caproate production by 59.9% in the following CE process. The higher selectivity of caproate and higher consumption of ethanol contributed to the higher caproate production at 1 g/L phenol. It was also shown 1 g/L phenol had more positive effect on CE of butyrate than acetate. 1.5 g/L phenol inhibited both SCFAs production and CE processes. 16S rRNA genes analysis showed phenol had slight effect on the microbial communities for SCFAs production, while it obviously changed the dominant microbes in CE process. For CE process, metagenomic analysis was further conducted and phenol mainly affected fatty acid biosynthesis (FAB) pathway, but not reverse β-oxidization (RBO) pathway. 1 g/L phenol increased the abundances of genes in FAB pathway, which could be related with the higher caproate production. Genome reconstruction identified the dominant microbial species in CE process, which were changed with different concentrations of phenol. Most of the dominant species were new microbial species potentially involved in CE. The syntrophic cooperation between Petrimonas mucosa FDU058 and Methanofollis sp. FDU007 might play important role in increased caproate production at 1 g/L phenol, and their adaption to phenol could be due to the presence of genes relating with active efflux system and refolding of proteins.

Open Culture Ethanol-Based Chain Elongation to Form Medium Chain Branched Carboxylates and Alcohols

Chain elongation fermentation allows for the synthesis of biobased chemicals from complex organic residue streams. To expand the product spectrum of chain elongation technology and its application range we investigated 1) how to increase selectivity towards branched chain elongation and 2) whether alternative branched carboxylates such as branched valerates can be used as electron acceptors. Elongation of isobutyrate elongation towards 4-methyl-pentanoate was achieved with a selectivity of 27% (of total products, based on carbon atoms) in a continuous system that operated under CO₂ and acetate limited conditions. Increasing the CO₂ load led to more in situ acetate formation that increased overall chain elongation rate but decreased the selectivity of branched chain elongation. A part of this acetate formation was related to direct ethanol oxidation that seemed to be thermodynamically coupled to hydrogenotrophic carboxylate reduction to corresponding alcohols. Several alcohols including isobutanol and n-hexanol were formed. The microbiome from the continuous reactor was also able to form small amounts of 5-methyl-hexanoate likely from 3-methyl-butanoate and ethanol as substrate in batch experiments. The highest achieved concentration of isoheptanoate was 6.4 ± 0.9 mM Carbon, or 118 ± 17 mg/L, which contributed for 7% to the total amount of products (based on carbon atoms). The formation of isoheptanoate was dependent on the isoform of branched valerate. With 3-methyl-butanoate as substrate 5-methylhexanoate was formed, whereas a racemic mixture of L/D 2-methyl-butanoate did not lead to an elongated product. When isobutyrate and isovalerate were added simultaneously as substrates there was a large preference for elongation of isobutyrate over isovalerate. Overall, this work showed that chain elongation microbiomes can be further adapted with supplement of branched-electron acceptors towards the formation of iso-caproate and iso-heptanoate as well as that longer chain alcohol formation can be stimulated.

Effect of pH on volatile fatty acid production and the microbial community during anaerobic digestion of Chinese cabbage waste

The effects of pH on the production of volatile fatty acids (VFAs) and the evolution of microbial community structure were studied via anaerobic fermentation of Chinese cabbage waste. The results showed that the concentration of total VFAs was highest at 20,241.4 mg COD/L at pH 6.0, followed by pH 7.0. Ethanol, acetate and butyrate were dominant under the acidic condition. The main products at pH 7.0 were acetate, propionate, and butyrate. Ethanol, acetate and butyrate were rapidly produced during the initial stage. The hexanoate concentration increased quickly from day 6 due to the chain extension between ethanol and butyrate, and was 4,885.1 mg COD/L on day 8, accounting for 30.4% of the total VFAs. As fermentation was extended, Bacteroidia and Clostridia were dominant at pH 6.0 and the uncontrolled pH, respectively. Clostridium IV, Ruminococcus, and Candida, were suspected to be related to hexanoate production.

Process Engineering Aspects for the Microbial Conversion of C1 Gases

Industrially applied bioprocesses for the reduction of C1 gases (CO₂ and/or CO) are based in particular on (syn)gas fermentation with acetogenic bacteria and on photobioprocesses with microalgae. In each case, process engineering characteristics of the autotrophic microorganisms are specified and process engineering aspects for improving gas and electron supply are summarized before suitable bioreactor configurations are discussed for the production of organic products under given economic constraints. Additionally, requirements for the purity of C1 gases are summarized briefly. Finally, similarities and differences in microbial CO₂ valorization are depicted comparing gas fermentations with acetogenic bacteria and photobioprocesses with microalgae.

Nano zero-valent iron: A pH buffer, electron donor and activator for chain elongation

Chain elongation produce medium chain carboxylates, which are important precursors to many pharmaceuticals, antimicrobials and biofuels. Results in the presented investigations show that the supply of nano zero-valent iron (NZVI) can enhance caproate production. The highest caproate concentration achieved amounted to 27.2 mmol/L when 5 g/L NZVI were added, which was about 100% higher than the control. The study also showed increase of ethanol oxidation and decrease of butyrate and butanol with NZVI addition. Mechanism study showed NZVI can stimulate caproate production by preventing pH to fall below 5.4 through displacement reaction. Electron balance analysis displayed that NZVI provides extra electron by promoting ethanol oxidation and its dissolution. H₂ was the potential electron shuttle between NZVI and chain elongators; High throughput sequencing showed function of NZVI on reshaping of microbial communities, especially enriching Oscillibacter Marseille-P3260, a kind of chain elongator and Corynebacterium which possesses fatty acid biosynthesis and iron utilization.