Chain Elongation with Reactor Microbiomes: Open-Culture Biotechnology To Produce Biochemicals

Propanol as electron donor for efficient odd-chain carboxylate production by chain elongation with reactor microbiomes

Microbial consortia that catalyze chain elongation processes have been enriched using different selection strategies, for which the electron donor is an essential one. Propanol is an extraordinarily promising electron donor because it can be generated from renewable resources, including lignocellulosic biomass and protein wastes. Here, propanol was proven in detail to be an efficient electron donor, enhancing the production of odd medium-chain carboxylates during chain elongation. By exploring various electron acceptors, reactor conditions, and electron donor/electron acceptor mol ratios, our study highlights that acetate is the most suitable electron acceptor for the production of both odd- and even-chain carboxylates. The optimal conditions for propanol-based chain elongation were 30 °C and pH 6, achieving 82.8 % selectivity for odd-chain carboxylates. Another critical insight from our work is that a propanol/acetate mol ratio of 1:1 can minimize the inhibitory effect of propanol and maximize the yield of medium-chain carboxylates, with the highest concentration of n-heptanoate reaching 124.5 mmol C/L. This was further illustrated by 16S rRNA amplicon sequencing, which elucidated that the community composition and keystone species in a propanol-based reactor closely resembled that of the ethanol one. The dominant phylum of the propanol-based reactor, Firmicutes showed a significant positive correlation with the concentrations of n-caproate and n-valerate. Additionally, the co-occurrence of Clostridium sensu stricto 12 and Oscillibacter, known as typical chain elongators, was identified within the propanol-based reactor. These findings enhance our understanding of propanol-based chain elongation, offer guiding principles for reactor microbiota assembly, and support efficient odd medium-chain carboxylate production.

Advancements in chain elongation technology: Transforming lactic acid into caproic acid for sustainable biochemical production

This review provides an insight into the chain-elongation technology for the production of caproic acid, a chemical widely used in the food, pharmaceutical, and cosmetic industries, from lactic acid in waste organic matter. The evolution of the technology is traced, the reaction mechanism is elucidated, and the properties of key microbial agents capable of carrying out the chain-elongation technology are summarized and compared, including pure bacterial isolates and reactor-mixed microorganisms. Furthermore, the parameters that regulate caproic acid formation by influencing microbial activity, competitive pathways, product selection, and carbon flow distribution, such as pH, temperature, electron donor, electron acceptor, and hydrogen partial pressure, are highlighted and discussed. It is worth noting that various caproic acid product extraction technologies were also summarized and assessed. Finally, based on the perspective of interdisciplinary field, bold suggestions for the future research direction are put forward.

Enhanced ethanol-driven carboxylate chain elongation by MOF-808 from waste activated sludge: Process and mechanism

Carboxylate chain elongation can create value-added bioproducts from waste activated sludge (WAS). The bioconversion of WAS during anaerobic fermentation is often constrained by inefficient hydrolysis. The addition of MOF-808 (200 mg MOF-808/g volatile solids (VS)) increased caproate production and selectivity by approximately 38.9 % and 28.9 %, respectively. MOF-808 significantly promoted the hydrolysis of WAS, accelerated the degradation of extracellular polymeric substances, and enhanced acetate accumulation. Absolute quantitative metagenomics conducted during the acidification and chain elongation phases demonstrated that MOF-808 markedly improved enzymatic hydrolysis. The absolute gene abundance of protease and α-glucosidase increased by 168.9 % and 191.2 %, respectively, compared to the control trial. Furthermore, the reverse β-oxidation (RBO) pathway, the primary route for chain elongation, exhibited a 19.2 %-76.1 % increase in gene abundance for enzymes involved in this pathway in the presence of MOF-808. Notably, the absolute gene abundance of electron-bifurcating enzyme complexes, including butyryl-CoA dehydrogenase-electron transferring flavoprotein complex (Bcd-EtfAB), proton-translocating NAD(P)+ transhydrogenase, ATPase (subunits A-I), and NAD oxidoreductase (RnfA-E), was significantly elevated in the MOF-808 trial. These findings provide valuable insights into enhancing the efficiency of chain elongation fermentation of WAS using MOF-like materials.

Obligately Tungsten-Dependent Enzymes─Catalytic Mechanisms, Models and Applications

Tungsten-dependent enzymes incorporate a tungsten ion into their active site in the form of a complex with two pyranometallopterin (MPT) molecules, also known as tungsten cofactor (W-co). W-co-containing enzymes are found in several bacteria and archaea, predominantly in enzymes involved in anaerobic metabolism. While some enzymes occur with either molybdenum or tungsten in their active sites, we concentrate here on enzymes obligately depending on W-co, which are not functional as isoenzymes with Mo-co. These are represented by several subtypes of aldehyde oxidoreductases (AORs), class II benzoyl-CoA reductase (BCRs) and acetylene hydratase (AHs). They catalyze either low-potential redox reactions or the unusual hydration reaction of acetylene. In this review, we analyze the catalytic and structural properties of these enzymes and focus on various mechanistic hypotheses proposed to describe their catalytic action, including hypothetical mechanistic patterns common to all of these enzymes. The biochemical characterization of the enzymes is supported by studies with functional inorganic models that help in the elucidation of their spectroscopic and catalytic features. Finally, we discuss a range of ongoing biotechnological applications utilizing obligately tungsten-dependent enzymes in producing value-added chemicals, indicating the expected advantages of incorporating these enzymes into biotechnological processes despite their intrinsic oxygen-sensitivity and the requirement of special recombinant expression platforms.

Designing synthetic microbial communities for enhanced anaerobic waste treatment

Synthetic microbial communities (SynComs) are powerful tools for investigating microbial interactions and community assembly by focusing on minimal yet functionally representative members. Here, we will highlight key principles for designing SynComs, specifically emphasizing the anaerobic digestion (AD) microbiome for waste treatment and upcycling. The AD process has traditionally been used to reduce organic waste volume while producing biogas as a renewable energy source. Its microbiome features well-defined trophic layers and metabolic groups. There has been growing interest in repurposing the AD process to produce value-added products and chemical precursors, contributing to sustainable waste management and the goals of a circular economy. Optimizing the AD process requires a better understanding of microbial interactions and the influence of both biotic and abiotic parameters, where SynComs offer great promise. Focusing on AD microbiomes, we review the principles of SynComs' design, including keystone taxa and function, cross-feeding interactions, and metabolic redundancy, as well as how modeling approaches could guide SynComs design. Furthermore, we address practical considerations for working with AD SynComs and examine constructed SynComs designed for anaerobic waste digestion. Finally, we discuss the challenges associated with designing and applying SynComs to enhance our understanding of the AD process. This review aims to explore the use of synthetic communities in studying anaerobic digestion and highlights their potential for developing innovative biotechnological processes.

Production of caproate from lactate by chain elongation under electro-fermentation: Dual role of exogenous ethanol electron donor

Ethanol and lactate as co-electron donors (co-EDs) on chain elongation (CE) have been studied, however, the effects of ethanol on lactate-driven CE remain unknown. This study evaluated the caproate production performance from lactate under different ethanol additions (7.67-23.01 g/L), and determined the optimal ethanol concentration. The results showed that the highest caproate yield of 4.51 g/L was obtained at 15.34 g/L ethanol concentration. The synergistic effects of co-EDs on caproate production were enhanced due to more stable pH and higher reducing power under appropriate ethanol stimulation. Functional bacteria related to caproate production, including Sporanaerobacter and Clostridium_senseu_stricto_12, were specifically enriched under appropriate ethanol stimulation. The ethanol toxicity caused by excessive concentration (more than 23.01 g/L) inhibited the growth of key CE bacteria and reduced the production of caproate. This study revealed the dual effects of ethanol in the co-EDs fermentation system for caproate production and determined the threshold of ethanol addition.

Enhanced production of ethyl caproate in strong-flavor Baijiu through a dual bacterial co-culture system and immobilization on natural luffa sponge

Ethyl caproate, a significant aromatic component in strong-flavor baijiu, is synthesized requiring caproic acid as an essential precursor. In this study, a novel caproic acid-producing bacteria Rummeliibacillus suwonensis J-1 was isolated from pit mud. Subsequently, a dual bacterial co-culture system (DBCS) was successfully established by combining J-1 with the acid-producing bacterium Enterococcus sp. D-1, resulting in a 21-fold increase in yield, which reached 4.41 g/L. A new immobilization strategy was developed, utilizing luffa sponge as a carrier for DBCS to facilitate pit mud-free strong-flavor baijiu production. The findings indicated a substantial increase in the ethyl caproate concentrations, with a 218 % increase, reaching 0.625 g/L. Transcriptomic analysis showed that in the two-bacterial system, crucial genes implicated in the biosynthesis pathway of caproic acid in J-1, including Crt, Scad, Ptb, pdxK, L-cysteine dehydrogenase, and l-serine decarboxylase were significantly upregulated, which enhanced the synthesis of caproic acid. These findings suggest that DBCS may have potential applications in fermentation without pit mud and could potentially enhance quality of strong-flavor baijiu.

Dynamic anaerobic digestion-based biorefineries for on-demand renewable energy and bioproducts in a circular bioeconomy

Anaerobic digestion (AD) is an important biotechnology for treating biodegradable residues and producing bioenergy, yet its full potential remains untapped. We investigate a two-phase AD system for biorefinery applications, producing valuable bioproducts, such as volatile fatty acids (VFAs) and biogas, from grass feedstock. We introduce a demand-driven operational approach to match market conditions, while minimising water use by reusing the process effluent. The proposed biorefinery model yields ~23 kg of VFAs and 75 kWh of biogas, with a potential gross revenue of €84 per tonne of grass. However, a preliminary economic analysis indicates that this biorefinery model is currently unprofitable. A sensitivity analysis suggests that reducing operating costs through technology advancements and policy support are vital to ensure economic viability. Such biorefineries offer opportunities for the diversification of farmers' incomes and the transition away from fossil resources. Our work exemplifies the role of AD as a key biotechnology in the circular bioeconomy.

Enhancing Gas Fermentation Efficiency via Bioaugmentation with Megasphaera sueciensis and Clostridium carboxidivorans

Gas fermentation aims to fix CO2 into higher-value compounds, such as short or medium-chain fatty acids or alcohols. In this context, the use of mixed microbial consortia presents numerous advantages, including increased resilience and adaptability. The current study aimed to improve the performance of an enriched mixed microbial population via bioaugmentation with Megasphaera sueciensis and Clostridium carboxidivorans to improve the metabolite spectrum. The initial fermentation in trickle-bed reactors mainly yielded acetate, a low-value compound. Introducing M. sueciensis, which converts acetate into higher-chain fatty acids, shifted production toward butyrate (up to 3.2 g/L) and caproate (1.1 g/L). The presence of M. sueciensis was maintained even after several media swaps, showing its ability to establish itself as a permanent part of the microbial community. Metataxonomic analysis confirmed the successful integration of M. sueciensis into the mixed culture, with it becoming a dominant member of the Veillonellaceae family. In contrast, bioaugmentation with C. carboxidivorans was unsuccessful. Although this strain is known for producing alcohols, such as butanol and hexanol, it did not significantly enhance alcohol production, as attempts to establish it within the microbial consortium were unsuccessful. Despite these mixed results, bioaugmentation with complementary microbial capabilities remains a promising strategy to improve gas fermentation efficiency. This approach may enhance the economic feasibility of industrial-scale renewable chemical production.

Carboxylic Acid Concentration in Downstream Bioprocessing Using High-Pressure Reverse Osmosis

During the production of many bio-based chemicals from fermentation and enzymatic processes, product separations frequently represent the most expensive and energy-intensive unit operations in an integrated process, often due to the low concentrations of target bioproducts. In this study, we integrated high-pressure reverse osmosis (HPRO) to concentrate an exemplary fermentation product, butyric acid, prior to downstream extraction. Through both modeling and experimental measurements, we identified the major factors limiting the maximum achievable concentration factor (CF) of 4.0 for butyric acid concentration with an HPRO membrane compared to the 2.6-3.2 range for conventional reverse osmosis (RO) membranes. The resulting concentrated aqueous stream underwent liquid-liquid extraction with an organic solvent and distillation for butyric acid purification and solvent recycling. The integration of HPRO product concentration into an in situ product recovery (ISPR) process leads to >5-fold increase in the final butyric acid concentration in the organic phase, and a concomitant 76% reduction in organic solvent usage. These improvements lead to an estimated 53 and 46% reduction in ISPR butyric acid production cost and greenhouse gas (GHG) emissions, respectively, considerably exceeding the process performance when integrating conventional RO product concentration. Overall, the integration of an HPRO membrane for product concentration enables more economical and sustainable bioproduct recovery from dilute aqueous streams.

Mildly acidic pH boosts up CO2 conversion to isobutyrate in H2 driven gas fermentation system

As a greenhouse gas, massive carbon dioxide (CO2) has been generated due to organic matter degradation in wastewater treatment processes. Microbial gas fermentation offers a promising approach to capture CO2 and generate various valuable chemicals. However, limited studies have achieved branched or medium-chain fatty acids production via gas fermentation. This study reported the production of isobutyrate and hexanoate by feeding H2 and CO2 into membrane biofilm reactors (MBfRs). The gas fermentation product in the reactor with neutral pH (pH of 7) was dominated by acetate (accounting for 90 % of the product spectrum), whereas a mildly acidic pH (pH of 6) resulted in isobutyrate and hexanoate as the dominant products, with a selectivity of 57 % and 42 %, respectively. Notably, a remarkably high concentration of isobutyrate (266 mmol C/L) was produced in the reactor with pH of 6. Subsequent batch test results suggest that the isobutyrate production in this study is coupled with acetogenesis and ethanol-driven chain elongation processes, rather than via methanol-driven chain elongation reported previously. High-throughput 16S rRNA gene amplicon sequencing revealed that the microbial community under neutral pH was dominated by acetate-producing homoacetogens Acetobacterium. By contrast, a mildly acidic pH promoted the community shifting towards chain elongation microorganisms, dominated by Clostridium sensu stricto 12, Oscillibacter and Caproiciproducens. Collectively, this study demonstrates the significant role of mildly acidic pH in boosting up bioisomerization and chain elongation in gas fermentation systems, thus triggering isobutyrate and hexanoate production. The findings highlight gas fermentation as a new green alternative route for generating highly valuable isobutyrate and hexanoate.

Enhancement of caproate production via carboxylate chain elongation with sequential fermentation facilitated by biochar: A corn stover full-component utilization perspective

In this study, caproate was synthesized from corn stover through sequential fermentation, and biochar was prepared from unhydrolyzable corn stover by pyrolysis to utilize full-component of corn stover. The results indicate that the caproate concentration in the unhydrolyzable corn stover biochar (UCSB) group was 2.2 times higher than that of the control group, and the fermentation start-up time was shortened by 18 days. Mechanistic analysis suggested that the rough surface of UCSB facilitated microbial colonization and reduced product inhibition. Genes expression analysis further demonstrated that UCSB significantly upregulated crucial functional genes responsible for ethanol oxidation and the reverse β-oxidation pathway, ultimately resulting in enhanced caproate production. The successful utilization of UCSB derived from unhydrolyzable solid residue effectively boosted fermentation, leading to a 37 % increase in the carbon utilization efficiency of corn stover. This study offering valuable insights for the high-value and full-component utilization of lignocellulosic biomass.

Optimising medium chain carboxylate production in xylan mixed-culture monofermentation

This work investigates the optimization of medium-chain carboxylate (MCC) production through xylan mixed-culture monofermentation. The pH screening in batch assays showed that the hydrolysis stage and selectivity towards MCC precursors were optimised at pH 6. Subsequently, a continuous stirred tank reactor (CSTR) and a Sequential Batch Reactor (SBR) were operated at different Hydraulic Retention Times (HRT), revealing that the SBR at HRT 2 days yielded the highest caproic acid since lactic acid availability and chain elongation process were balanced. An enriched medium with yeast extract and vitamins favoured the growth of chain elongators, and therefore, the MCC production. Moreover, cross-feeding interaction between bacteria in xylan fermentation was observed, and Pseudoramibacter was present in the highest caproic acid yields. This work highlights the impact of selecting the proper operational window to optimise one-stage MCC production.

A powerful but frequently overlooked role of thermodynamics in environmental microbiology: inspirations from anammox

Thermodynamics has long been applied in predicting undiscovered microorganisms or analyzing energy flows in microbial metabolism, as well as evaluating microbial impacts on global element distributions. However, further development and refinement in this interdisciplinary field are still needed. This work endeavors to develop a whole-cycle framework integrating thermodynamics with microbiological studies, focusing on representative nitrogen-transforming microorganisms. Three crucial concepts (reaction favorability, energy balance, and reaction directionality) are discussed in relation to nitrogen-transforming reactions. Specifically, reaction favorability, which sheds lights on understanding the diversity of nitrogen-transforming microorganisms, has also provided guidance for novel bioprocess development. Energy balance, enabling the quantitative comparison of microbial energy efficiency, unravels the competitiveness of nitrogen-transforming microorganisms under substrate-limiting conditions. Reaction directionality, revealing the niche-differentiating patterns of nitrogen-transforming microorganisms, provides a foundation for predicting biogeochemical reactions under various environmental conditions. This review highlights the need for a more comprehensive integration of thermodynamics in environmental microbiology, aiming to comprehensively understand microbial impacts on the global environment from micro to macro scales.

Enhanced anaerobic digestion model no.1 for high solids fermentation: Integrating homoacetogenesis and chain elongation

The production of volatile fatty acids (VFA) through high-solids anaerobic fermentation of organic waste offers a promising route for resource recovery. This study used a batch-mode anaerobic leach bed reactor (LBR) with leachate circulation to ferment the organic fraction of municipal solid waste, producing high concentrations of butyric acid, along with notable amounts of lactic and caproic acids. These results provide valuable insights and underscore the need for process optimization within conventional fermentation systems. To better model the LBR's complex dynamics, the Anaerobic Digestion Model No.1 (ADM1) was modified and extended to account for slow hydrolysis in dry fermentation conditions, thermodynamic constraints imposed by Gibbs Free Energy and hydrogen partial pressures, as well as lactic acid production and chain elongation pathways including homoacetogenesis and caproic acid formation. These enhancements provided deeper insights into high-solids anaerobic fermentation, advancing strategies for improved process control and system optimization.

Enhancement of medium-chain fatty acids production from sewage sludge fermentation by zero-valent iron

The effect of zero-valent iron (ZVI) dosage on medium-chain fatty acids (MCFAs) production from sewage sludge fermentation was explored. ZVI within a dosage of 2-20 g/L favored MCFAs production. Adding 20 g/L ZVI (ZVI20) increased the MCFAs and long-chain alcohols (LCAs) production to 4079.0 mg/L and 93.1 mg/L, the electron transfer efficiency of MCFAs and MCFAs selectivity were also increased by over 40% and 25% than the control. This may be due to the enriched MCFAs-producing genera, like Romboutsia and Paraclostridium. 2 g/L ZVI favorably strengthened the RBO pathway and facilitated intracellular electron generation. Moreover, ZVI facilitated the extracellular electron transfer, and cytochrome C was most enriched by ZVI20. The low MCFAs production in the ZVI50 group might be due to the inhibition of acetyl-CoA and ATP synthesis. These results provided a deep insight into the effects of ZVI dosage on MCFAs production and the specific mechanisms.

Unraveling temperature effects on caproate and caprylate production from waste activated sludge

This study explored the impact of different temperatures on the continuous production of medium-chain fatty acids (MCFAs) from waste activated sludge (WAS). Experimental results showed that there was almost no MCFAs accumulation at 55 °C. Both 25 °C and 37 °C were suitable for MCFAs production, with 25 °C favoring high-value caprylate production. The metagenomic and metatranscriptomic analysis highlighted reverse β-oxidization as the main chain elongation (CE) cycle. The lack of CE-related microorganisms and enzymes at 55 °C hindered MCFAs production, in contrast to the heightened activity observed at 25 °C and 37 °C, with peak activity at 25 °C leading to increased longer-chain MCFAs synthesis. 37 °C promoted hydrolysis and acidification, resulting in a accumulation of higher short-chain fatty acids, but further elongation to MCFAs would be hindered by product toxicity. This research concludes that 25 °C is the most effective temperature for the production of WAS-derived MCFAs, offering significant economic advantages.

Overexpression of Umellularia californica FatB thioesterase affects plant growth and lipid metabolome leading to improved drought tolerance in Arabidopsis and tomato

Frequent and extreme drought exerts profound effects on vegetation growth and production worldwide. It is imperative to identify key genes that regulate plant drought resistance and to investigate their underlying mechanisms of action. Long-chain fatty acids and their derivatives have been demonstrated to participate in various stages of plant growth and stress resistance; however, the effects of medium-chain fatty acids on related functions have not been thoroughly studied. Here, we integrate lipidomic, transcriptomic, and genetic analyses to elucidate the roles of the medium-chain acyl-acyl carrier protein thioesterase of Umellularia californica FatB (UcFatB) in drought tolerance and plant growth. Arabidopsis and tomato transgenic lines overexpressing UcFatB showed that the medium chain fatty acids mainly affect the male reproductive process of plant development. Transcriptomic and non-targeted lipid metabolomic combination analysis revealed significant changes in lauric acid-related metabolic pathways, as evidenced by increased phosphatidylcholine accumulation and upregulated stress-response gene expression. Consistent with the thicker waxy cutin layer and increased membrane integrity, UcFatB-overexpression enhanced drought tolerance in both Arabidopsis and tomato. Furthermore, methyl laurate and phosphatidylcholine application improved tomato drought resistance and fruit yield. These findings provide new insights into the potential genetic resources and cost-effective chemicals for enhancing drought resistance in crops.

pH-dependent medium-chain fatty acid synthesis in waste activated sludge fermentation: Metabolic pathway regulation

Transforming waste activated sludge (WAS) into medium-chain fatty acids (MCFAs) via chain elongation (CE) technology is sustainable, yet pH effects on this process are poorly understood. In this study, semi-continuous flow experiments demonstrated that WAS degradation was highest under alkaline pH (10) but unsuitable for CE. Continuous output of MCFAs indicated that CE could be successfully performed under acidic pH (5) and neutral pH (7). Moreover, neutral pH optimized MCFAs production, achieving higher MCFAs yield (8.9 ± 1.2 g COD/L), MCFAs selectivity (51.2 ± 7.3%), and WAS degradation (25.4 ± 0.4%) than acidic pH. Further metagenomic and metatranscriptomic analysis revealed that the reverse β-oxidation cycle was the primary CE pathway. The absence of CE-related microorganisms and enzymes under alkaline pH hindered MCFAs synthesis, while under acidic pH, carboxylate accumulation may reduce cellular protection capabilities and affect energy metabolism, thereby inhibiting anaerobic fermentation. Conversely, neutral pH enhanced amino acid and butyrate metabolic pathways, facilitating WAS degradation and SCFAs production, providing precursor substrates for MCFAs production. Additionally, neutral pH promoted the enrichment and activity of CE-related microorganisms and enzymes, contributing to the accumulation of high-concentration MCFAs. Notably, Clostridium_kluyveri and Sporanaerobacter_acetigenes were key CE-functional bacteria at neutral pH.

The enhancement of caproic acid synthesis from organic solid wastes: A review

Organic solid waste (OSW) significantly harms the environment and threatens human health. Producing caproic acid (CA) from OSW presents a cost-effective, sustainable, and resource-efficient solution. This study comprehensively examines the various methods for synthesizing CA from OSW, focusing on waste material selection, pretreatment processes to improve dissolution and hydrolysis of OSW, key substrates, and optimization strategies. Using OSW resources has been extensively studied and applied across numerous industries, presenting a promising solution for reducing environmental pollution. This study provides insights into CA synthesis pathways and substrate selection while emphasizing the optimization of CA production from OSW. It also highlights key areas for future research.

Acetate Shock Loads Enhance CO Uptake Rates of Anaerobic Microbiomes

Pyrolysis of lignocellulosic biomass commonly produces syngas, a mixture of gases such as CO, CO2 and H2, as well as an aqueous solution generally rich in organic acids such as acetate. In this study, we evaluated the impact of increasing acetate shock loads during syngas co-fermentation with anaerobic microbiomes at different pH levels (6.7 and 5.5) and temperatures (37°C and 55°C) by assessing substrates consumption, metabolites production and microbial community composition. The anaerobic microbiomes revealed to be remarkably resilient and were capable of converting syngas even at high acetate concentrations of up to 64 g/L and pH 5.5. Modifying process parameters and acetate loads resulted in a shift of the product spectrum and microbiota composition. Specifically, a pH of 6.7 promoted methanogens such as Methanosarcina, whereas lowering the pH to 5.5 with lower acetate loads promoted the enrichment of syntrophic acetate oxidisers such as Syntrophaceticus, alongside hydrogenotrophic methanogens. Increasing acetate loads intensified the toxicity of undissociated acetic acid, thereby inhibiting methanogenic activity. Under non-methanogenic conditions, high acetate concentrations suppressed acetogenesis in favour of hydrogenogenesis and the production of various carboxylates, including valerate, with product profiles and production rates being contingent upon temperature. A possible candidate for valerate production was identified in Oscillibacter. Across all tested conditions, acetate supplementation provided additional carbon and energy to the mixed cultures and consistently increased carboxydotrophic conversion rates up to about 20-fold observed at pH 5.5, 55°C and 48 g/L acetate compared to control experiments. Species of Methanobacterium, Methanosarcina and Methanothermobacter may have been involved in CO biomethanation. Under non-methanogenic conditions, the bacterial species responsible for CO conversion remain unclear. These results offer promise for integrating process streams, such as syngas and wastewater, as substrates for mixed culture fermentation allowing for enhanced resource circularity, mitigation of environmental impacts and decreased dependence on fossil fuels.

Carbon chain elongation characterizations of electrode-biofilm microbes in electro-fermentation

The higher efficiency of electro-fermentation in synthesizing medium-chain fatty acids (MCFAs) compared to traditional fermentation has been acknowledged. However, the functional mechanisms of electrode-biofilm enhancing MCFAs synthesis remain research gaps. To address this, this study proposed a continuous flow electrode-biofilm reactor for chain elongation (CE). After 225 days of operation, stable electrode-biofilms formed and notably improved caproate yield by more than 38 %. The electrode-biofilm was enriched with more CE microorganisms and electroactive bacteria compared to the suspended sludge microorganisms, including Caproicibacterium, Oscillibacter and Pseudoramibacter. Besides, the upregulated CE pathways were evaluated by metagenomic analysis, and the results indicated that the pathways such as acetyl-CoA and malonyl-[acp] formation, reverse beta-oxidation, and fatty acid biosynthesis pathway were all markedly enhanced in cathodic biofilm, more than anodic biofilm and suspended microorganisms. Moreover, microbial community regulated processes like bacterial chemotaxis, flagellar assembly and quorum sensing, crucial for electrode-biofilm formation. Electron transfer, energy metabolism, and microbial interactions were found to be prominently upregulated in the cathodic biofilm, surpassing levels observed in anodic biofilm and suspended sludge microorganisms, which further enhanced CE efficiency. In addition, the statistical analyses further highlighted key microbial functions and interactions within the cathodic biofilm. Oscillospiraceae_bacterium was identified to be the most active microbe, alongside pivotal roles played by Caproiciproducens_sp._NJN-50, Clostridiales_bacterium, Prevotella_sp. and Pseudoclavibacter_caeni. Eventually, the proposed microbial collaboration mechanisms of cathodic biofilm were ascertained. Overall, this study uncovered the biological effects of the electrode-biofilm on MCFAs electrosynthesis, thereby advancing biochemicals production and filling the knowledge gaps in CE electroactive biofilm reactors.

Biochar confers significant microbial resistance to ammonia toxicity in n-caproic acid production

Microbial chain elongation integrating innovative bioconversion technologies with organic waste utilization can transition current energy-intensive n-caproic acid production to sustainable circular bioeconomy systems. However, ammonia-rich waste streams, despite their suitability, pose inhibitory challenges to these bioconversion processes. Herein, biochar was employed as an additive to enhance the activity of chain elongating microbes under ammonia inhibition conditions, with an objective to detail underlying mechanisms of improvements. Biochar addition significantly improved chain elongation performance even under severe ammonia stress (exceeding 8 g N/L), increasing n-caproic acid yields by 40 % to 158 % and reducing lag times by 51 % to 90 %, compared with the best-performing group without biochar addition. The material contribution to n-caproic production reached up to 94.3 % (at 4 g N/L). These enhancements were mainly attributed to the new electron syntrophy induced by biochar, which improved electron transfer system activity and electrical conductivity of the fermentation system. This is further substantiated by increased relative abundances of the genus Sporanaerobacter, electroactive bacteria, and up-regulated direct electron transfer-related genes including conductive pili and c-type cytochrome. This study demonstrates that biochar can confer robust resilience to ammonia toxicity in functional microbes, paving a way for efficient and sustainable n-caproic acid production.

Impact of cell voltage on synthesis of caproic acid from carbon dioxide and ethanol in direct current powered microbial electrosynthesis cell

Production of medium chain fatty acids (MCFAs) from CO2 through microbial electrosynthesis (MES) holds great potential. The present study investigated the effect of cathode voltages of - 0.8 V (MES-1), -1.0 V (MES-2) and -1.2 V vs Ag/AgCl (MES-3), on the production of MCFAs from CO2 and ethanol using an enriched culture. Direct current (DC) power supply was used to maintain constant cathode voltages. The highest amounts of caproic acid were produced in MES-2 at an average concentration of 1.51 ± 0.14 g/L with a maximum selectivity of 68 ± 7 %. Microbial diversity analysis showed abundance of the Clostridiaceae family that allowed chain elongation in all MES reactors. This study shows that potentiostatic control approach for MCFA synthesis, can be replaced by DC power supply in future MES setups. Using selective culture enrichment, MES efficiently produces MCFAs from CO2 and ethanol, with -1.0 V yielding the highest caproic acid.

Research on the conversion of biowaste to MCCAs: A review of recent advances in the electrochemical synergistic anaerobic pathway

Medium-chain carboxylic acids (MCCAs) show great promise as commercial chemicals due to their high energy density, significant product value, and wide range of applications. The production of MCCAs from waste biomass through coupling chain extension with anaerobic fermentation represents a new and innovative approach to biomass utilization. This review provides an overview of the principles of MCCAs production through coupled chain extension and anaerobic fermentation, as well as the extracellular electron transfer pathways and microbiological effects involved. Emphasis is placed on the mechanisms, limitations, and microbial interactions in MCCAs production, elucidating metabolic pathways, potential influencing factors, and the cooperative and competitive relationships among various microorganisms. Additionally, this paper delves into a novel technology for the bio-electrocatalytic generation of MCCAs, which promotes electron transfer through the use of different three-dimensional electrodes, various electrical stimulation methods, and hydrogen-assisted approaches. The insights and conclusions from previous studies, as well as the identification of existing challenges, will be valuable for the further development of high-product-selectivity strategies and environmentally friendly treatments.

Explainable machine learning-driven predictive performance and process parameter optimization for caproic acid production

In this study, four machine learning (ML) prediction models were developed to predict and optimize the production performance of caproic acid based on substrates, products, and process parameters. The XGBoost outperformed others, with a high R2 of 0.998 on the training set and 0.885 on the test set. Feature importance analysis revealed hydraulic retention time (HRT) and butyric acid concentration are decisive. The SHAP method offered profound insights into the interplay and cumulative effects of substrate composition, identified the synergistic effects between butyric acid and lactic acid, and emphasized adding glucose can benefit caproic with lactic acid co-fermentation. By integrating the Adaptive Variation Particle Swarm Optimization (AVPSO) algorithm, the optimal process conditions to achieve a maximum caproic acid production of 8.64 g/L was obtained. This study not only advances caproic acid production but contributes a versatile ML-driven strategy applicable to bioprocess optimizations, potentially transformative for sustainable and economically viable bioproduction.

Nano zero valent iron stimulated acetaldehyde oxidation, electron transfer, and RBO pathway for enhanced medium-chain carboxylic acids production

Nano zero-valent iron (NZVI) has been shown to effectively enhance the chain elongation (CE) process, addressing the issue of limited yield of medium-chain carboxylic acids (MCCA) from organic wastewater. However, the specific impact of NZVI on the metabolism of CE bacteria (CEB) is not well understood. In this study, it was aimed to investigate the mechanism by which an optimal concentration of NZVI influences CE metabolism, particularly in relation to ethanol oxidation, electron transfer, and MCCA synthesis. This was achieved through single-factor influence experiments and metagenomic analysis. The results showed that the addition of 1 g/gVSS NZVI achieved the highest MCCA yield (n-caproic acid + n-octanoic acid) at 2.02 g COD/L, which was 4.9 times higher than the control. This improvement in MCCA production induced by NZVI was attributed to several factors. Firstly, NZVI facilitated the oxidation of acetaldehyde, leading to its reduced accumulation in the system (from 18.4 % to 5.8 %), due to the optimized chemical environment created by NZVI corrosion, including near-neutral pH and a more reductive oxidation-reduction potential (ORP). Additionally, the inherent conductivity property of NZVI and the additional Fe ions released during corrosion improved the electron transfer efficiency between CEB. Lastly, both the composition of microbial communities and the abundance of unique enzyme genes confirmed the selective stimulation of NZVI on the reverse β-oxidation (RBO) pathway. These findings provide valuable insights into the role of NZVI in CEB metabolism and its potential application for enhancing MCCA production in CE bioreactors.

Advancements in medium chain fatty acids production through chain elongation: Key mechanisms and innovative solutions for overcoming rate-limiting steps

The depletion of fossil fuels has prompted an urgent search for alternative chemicals from renewable sources. Current technology in medium chain fatty acids (MCFAs) production though chain elongation (CE) is becoming increasingly sustainable, hence the motivation for this review, which provides the detailed description, insights and analysis of the metabolic pathways, substrates type, inoculum and fermentation process. The main rate-limiting steps of microbial MCFAs production were comprehensively revealed and the corresponding innovative solutions were also critically evaluated. Innovative strategies such as substrate pretreatment, electrochemical regulation, product separation, fermentation parameter optimization, and electroactive additives have shown significant advantages in overcoming the rate-limiting steps. Furthermore, novel regulatory strategies such as quorum sensing and electronic bifurcation are expected to further increase the MCFAs yield. Finally, the techno-economic analysis was carried out, and the future research focuses were also put forward.

Comprehensive comparative study on n-caproate production by Clostridium kluyveri: batch vs. continuous operation modes

Recently, there has been notable interest in researching and industrially producing medium-chain carboxylic acids (MCCAs) like n-caproate and n-caprylate via chain elongation process. This study presents a comprehensive assessment of the behavior and MCCA production profiles of Clostridium kluyveri in batch and continuous modes, at different ethanol:acetate molar ratios (1.5:1, 3.5:1 and 5.5:1). The highest n-caproate concentration, 12.9 ± 0.67 g/L (92.9 ± 1.39 % MCCA selectivity), was achieved in batch mode at a 3.5:1 ratio. Interestingly, higher ratios favored batch mode selectivity over continuous mode when this was equal or higher to 3.5:1. Steady state operation yielded the highest n-caproate (9.5 ± 0.13 g/L) and n-caprylate (0.35 ± 0.020 g/L) concentrations at the 3.5:1 ratio. Increased ethanol:acetate ratios led to a higher excessive ethanol oxidation (EEO) in both operational modes, potentially limiting n-caproate production and selectivity, especially at the 5.5:1 ratio. Overall, this study reports the efficient MCCA production of both batch and continuous modes by C. kluyveri.

Gut microbiota derived short-chain fatty acids in physiology and pathology: An update

Short-chain fatty acids (SCFAs) are essential molecules produced by gut bacteria that fuel intestinal cells and may also influence overall health. An imbalance of SCFAs can result in various acute and chronic diseases, including diabetes, obesity and colorectal cancer (CRC). This review delves into the multifaceted roles of SCFAs, including a brief discussion on their source and various gut-residing bacteria. Primary techniques used for detection of SCFAs, including gas chromatography, high-performance gas chromatography, nuclear magnetic resonance and capillary electrophoresis are also discussed through this article. This review study also compiles various synthesis pathways of SCFAs from diverse substrates such as sugar, acetone, ethanol and amino acids. The different pathways through which SCFAs enter cells for immune response regulation are also highlighted. A major emphasis is the discussion on diseases associated with SCFA dysregulation, such as anaemia, brain development, CRC, depression, obesity and diabetes. This includes exploring the relationship between SCFA levels across ethnicities and their connection with blood pressure and CRC. In conclusion, this review highlights the critical role of SCFAs in maintaining gut health and their implications in various diseases, emphasizing the need for further research on SCFA detection, synthesis and their potential as diagnostic biomarkers. Future studies of SCFAs will pave the way for the development of novel diagnostic tools and therapeutic strategies for optimizing gut health and preventing diseases associated with SCFA dysregulation.

Lactate-mediated medium-chain fatty acid production from expired dairy and beverage waste

Fruits, vegetables, and dairy products are typically the primary sources of household food waste. Currently, anaerobic digestion is the most used bioprocess for the treatment of food waste with concomitant generation of biogas. However, to achieve a circular carbon economy, the organics in food waste should be converted to new chemicals with higher value than energy. Here we demonstrate the feasibility of medium-chain carboxylic acid (MCCA) production from expired dairy and beverage waste via a chain elongation platform mediated by lactate. In a two-stage fermentation process, the first stage with optimized operational conditions, including varying temperatures and organic loading rates, transformed expired dairy and beverage waste into lactate at a concentration higher than 900 mM C at 43 °C. This lactate was then used to produce >500 mM C caproate and >300 mM C butyrate via microbial chain elongation. Predominantly, lactate-producing microbes such as Lactobacillus and Lacticaseibacillus were regulated by temperature and could be highly enriched under mesophilic conditions in the first-stage reactor. In the second-stage chain elongation reactor, the dominating microbes were primarily from the genera Megasphaera and Caproiciproducens, shaped by varying feed and inoculum sources. Co-occurrence network analysis revealed positive correlations among species from the genera Caproiciproducens, Ruminococcus, and CAG-352, as well as Megasphaera, Bacteroides, and Solobacterium, indicating strong microbial interactions that enhance caproate production. These findings suggest that producing MCCAs from expired dairy and beverage waste via lactate-mediated chain elongation is a viable method for sustainable waste management and could serve as a chemical production platform in the context of building a circular bioeconomy.

Variability in n-caprylate and n-caproate producing microbiomes in reactors with in-line product extraction

Medium-chain carboxylates (MCCs) are used in various industrial applications. These chemicals are typically extracted from palm oil, which is deemed not sustainable. Recent research has focused on microbial chain elongation using reactors to produce MCCs, such as n-caproate (C6) and n-caprylate (C8), from organic substrates such as wastes. Even though the production of n-caproate is relatively well-characterized, bacteria and metabolic pathways that are responsible for n-caprylate production are not. Here, three 5 L reactors with continuous membrane-based liquid-liquid extraction (i.e., pertraction) were fed ethanol and acetate and operated for an operating period of 234 days with different operating conditions. Metagenomic and metaproteomic analyses were employed. n-Caprylate production rates and reactor microbiomes differed between reactors even when operated similarly due to differences in H2 and O2 between the reactors. The complete reverse β-oxidation (RBOX) pathway was present and expressed by several bacterial species in the Clostridia class. Several Oscillibacter spp., including Oscillibacter valericigenes, were positively correlated with n-caprylate production rates, while Clostridium kluyveri was positively correlated with n-caproate production. Pseudoclavibacter caeni, which is a strictly aerobic bacterium, was abundant across all the operating periods, regardless of n-caprylate production rates. This study provides insight into microbiota that are associated with n-caprylate production in open-culture reactors and provides ideas for further work.IMPORTANCEMicrobial chain elongation pathways in open-culture biotechnology systems can be utilized to convert organic waste and industrial side streams into valuable industrial chemicals. Here, we investigated the microbiota and metabolic pathways that produce medium-chain carboxylates (MCCs), including n-caproate (C6) and n-caprylate (C8), in reactors with in-line product extraction. Although the reactors in this study were operated similarly, different microbial communities dominated and were responsible for chain elongation. We found that different microbiota were responsible for n-caproate or n-caprylate production, and this can inform engineers on how to operate the systems better. We also observed which changes in operating conditions steered the production toward and away from n-caprylate, but more work is necessary to ascertain a mechanistic understanding that could be predictive. This study provides pertinent research questions for future work.

Continuous chain elongation process for carbon resource recovery from excess sludge: Enhanced n-caprylate production and specific microbial functionalities

Thermal hydrolyzed sludge (THS) exhibits considerable promise in generating medium-chain fatty acids (MCFAs) through chain elongation (CE) technology. This study developed a novel continuous CE process using THS as the substrate, achieving an optimal ethanol loading rate (5.8 g COD/L/d) and stable MCFA production at 10.9 g COD/L, with a rate of 3.6 g COD/L/d. The MCFAs primarily comprised n-caproate and n-caprylate, representing 41.5 % and 54.3 % of the total MCFAs, respectively. Utilization efficiencies for ethanol and acetate were nearly complete at 100 % and 92.8 %, respectively. Key microbial taxa identified under these optimal conditions included Alcaligenes, SRB2, Sporanaerobacter, and Kurthia, which were instrumental in critical pathways such as the generation of acetyl-CoA, the initial carboxylation of acetyl-CoA, the fatty acid biosynthesis cycle, and energy metabolism. This research provides a theoretical and technical blueprint for converting waste sludge into valuable MCFAs, promoting sustainable waste-to-resource strategies.

Upgrading soybean dreg to caproate via intermediate of lactate and mediator of biochar

To address the environmental hazards posed by high-yield soybean dreg (SD), a high-value strategy is firstly proposed by synthesizing caproate through chain elongation (CE). Optimized conditions for lactate-rich broth as intermediate, utilizing 50 % inoculum ratio, 40 g/L substrate concentration, and pH 5, resulting in 2.05 g/L caproate from direct fermentation. Leveraging lactate-rich broth supplemented with ethanol, caproate was optimized to 2.76 g/L under a refined electron donor to acceptor of 2:1. Furthermore, incorporating 20 g/L biochar elevated caproate production to 3.05 g/L and significantly shortened the lag phase. Mechanistic insights revealed that biochar's surface-existed quinone and hydroquinone groups exhibit potent redox characteristics, thereby facilitating electron transfer. Moreover, biochar up-regulated the abundance of key genes involved in CE process (especially fatty acids biosynthesis pathway), also enriching Lysinibacillus and Pseudomonas as an unrecognized cooperation to CE. This study paves a way for sustainable development of SD by upgrading to caproate.

Energy recovery from syngas and pyrolysis wastewaters with anaerobic mixed cultures

The anaerobic digestion of aqueous condensate from fast pyrolysis is a promising technology for enhancing carbon and energy recovery from waste. Syngas, another pyrolysis product, could be integrated as a co-substrate to improve process efficiency. However, limited knowledge exists on the co-fermentation of pyrolysis syngas and aqueous condensate by anaerobic cultures and the effects of substrate toxicity. This work investigates the ability of mesophilic and thermophilic anaerobic mixed cultures to co-ferment syngas and the aqueous condensate from either sewage sludge or polyethylene plastics pyrolysis in semi-batch bottle fermentations. It identifies inhibitory concentrations for carboxydotrophic and methanogenic reactions, examines specific component removal and assesses energy recovery potential. The results show successful co-fermentation of syngas and aqueous condensate components like phenols and N-heterocycles. However, the characteristics and load of the aqueous condensates affected process performance and product formation. The toxicity, likely resulting from the synergistic effect of multiple toxicants, depended on the PACs' composition. At 37 °C, concentrations of 15.6 gCOD/gVSS and 7.8 gCOD/gVSS of sewage sludge-derived aqueous condensate inhibited by 50% carboxydotrophic and methanogenic activity, respectively. At 55 °C, loads between 3.9 and 6.8 gCOD/gVSS inhibited by 50% both reactions. Polyethylene plastics condensate showed higher toxicity, with 2.8 gCOD/gVSS and 0.3 gCOD/gVSS at 37 °C decreasing carboxydotrophic and methanogenic rates by 50%. At 55 °C, 0.3 gCOD/gVSS inhibited by 50% CO uptake rates and methanogenesis. Increasing PAC loads reduced methane production and promoted short-chain carboxylates formation. The recalcitrant components in sewage sludge condensate hindered e-mol recovery, while plastics condensate showed high e-mol recoveries despite the stronger toxicity. Even with challenges posed by substrate toxicity and composition variations, the successful conversion of syngas and aqueous condensates highlights the potential of this technology in advancing carbon and energy recovery from anthropogenic waste streams.

Engineered acetogenic bacteria as microbial cell factory for diversified biochemicals

Acetogenic bacteria (acetogens) are a class of microorganisms with conserved Wood-Ljungdahl pathway that can utilize CO and CO2/H2 as carbon source for autotrophic growth and convert these substrates to acetate and ethanol. Acetogens have great potential for the sustainable production of biofuels and bulk biochemicals using C1 gases (CO and CO2) from industrial syngas and waste gases, which play an important role in achieving carbon neutrality. In recent years, with the development and improvement of gene editing methods, the metabolic engineering of acetogens is making rapid progress. With introduction of heterogeneous metabolic pathways, acetogens can improve the production capacity of native products or obtain the ability to synthesize non-native products. This paper reviews the recent application of metabolic engineering in acetogens. In addition, the challenges of metabolic engineering in acetogens are indicated, and strategies to address these challenges are also discussed.

Exploiting the Thermotolerance of Clostridium Strain M1NH for Efficient Caproic Acid Fermentation from Ethanol and Acetic Acid

A novel thermotolerant caproic acid-producing bacterial strain, Clostridium M1NH, was successfully isolated from sewage sludge. Ethanol and acetic acid at a molar ratio of 4:1 proved to be the optimal substrates, yielding a maximum caproic acid production of 3.5 g/L. Clostridium M1NH exhibited remarkable tolerance to high concentrations of ethanol (up to 5% v/v), acetic acid (up to 5% w/v), and caproic acid (up to 2% w/v). The strain also demonstrated a wide pH tolerance range (pH 5.5-7.5) and an elevated temperature optimum between 35 and 40 °C. Phylogenetic analysis based on 16S rRNA gene sequences revealed that Clostridium M1NH shares a 98% similarity with Clostridium luticellarii DSM 29923 T. The robustness of strain M1NH and its efficient caproic acid production from low-cost substrates highlight its potential for sustainable bio-based chemical production. The maximum caproic acid yield achieved by Clostridium M1NH was 1.6-fold higher than that reported for C. kluyveri under similar fermentation conditions. This study opens new avenues for valorizing waste streams and advancing a circular economy model in the chemical industry.

Life cycle assessment of n-caproic acid production via chain elongation from food waste: Comparison of shunting and staged technology

n-Caproic acid is a widely used biochemical that can be produced from organic waste through chain elongation technology. This study aims to evaluate the environmental impacts of n-caproic acid production through chain elongation by two processes (i.e., shunting and staged technology). The Open-life cycle assessment (LCA) model was used to calculate the environmental impacts of both technologies based on experimental data. Results showed that the shunting technology had higher environmental impacts than the staged technology. Water and electricity made bigger contribution to the environmental impacts of both technologies. Reusing chain elongation effluent substituting for water and using electricity produced by wind power could reduce the environmental impacts of water and electricity effectively. Using ethanol from food waste had higher global warming potential than fossil ethanol, which suggested that a cradle-to-grave LCA is needed to be carried out for specific raw materials and chain elongation products in the future.

Medium-chain carboxylates production from plant waste: kinetic study and effect of an enriched microbiome

BACKGROUND

The need for addition of external electron donors such as ethanol or lactate impairs the economic viability of chain elongation (CE) processes for the production of medium-chain carboxylates (MCC). However, using feedstocks with inherent electron donors such as silages of waste biomass can improve the economics. Moreover, the use of an appropriate inoculum is critical to the overall efficiency of the CE process, as the production of a desired MCC can significantly be influenced by the presence or absence of specific microorganisms and their metabolic interactions. Beyond, it is necessary to generate data that can be used for reactor design, simulation and optimization of a given CE process. Such data can be obtained using appropriate mathematical models to predict the dynamics of the CE process.

RESULTS

In batch experiments using silages of sugar beet leaves, cassava leaves, and Elodea/wheat straw as substrates, caproate was the only MCC produced with maximum yields of 1.97, 3.48, and 0.88 g/kgVS, respectively. The MCC concentrations were accurately predicted with the modified Gompertz model. In a semi-continuous fermentation with ensiled sugar beet leaves as substrate and digestate from a biogas reactor as the sole inoculum, a prolonged lag phase of 7 days was observed for the production of MCC (C6-C8). The lag phase was significantly shortened by at least 4 days when an enriched inoculum was added to the system. With the enriched inoculum, an MCC yield of 93.67 g/kgVS and a productivity of 2.05 gMCC/L/d were achieved. Without the enriched inoculum, MCC yield and productivity were 43.30 g/kgVS and 0.95 gMCC/L/d, respectively. The higher MCC production was accompanied by higher relative abundances of Lachnospiraceae and Eubacteriaceae.

CONCLUSIONS

Ensiled waste biomass is a suitable substrate for MCC production using CE. For an enhanced production of MCC from ensiled sugar beet leaves, the use of an enriched inoculum is recommended for a fast process start and high production performance.

Draft genome sequence of Magnusiomyces sp. LA-1 isolated from a C6-C8 acid-producing bioreactor

Here, we report the draft genome of Magnusiomyces sp. LA-1, which was isolated from a C6-C8 carboxylic acid-producing bioreactor. The draft genome of Magnusiomyces sp. LA-1 is 19,829,165 bp in length, is divided into six contigs that comprise 6,557 CDS regions, and has a GC content of 34.5%.

Comprehensive review of microbial production of medium-chain fatty acids from waste activated sludge and enhancement strategy

Microbial production of versatile applicability medium-chain fatty acids (MCFAs) (C6-C10) from waste activated sludge (WAS) provides a pioneering approach for wastewater treatment plants (WWTPs) to achieve carbon recovery. Mounting studies emerged endeavored to promote the MCFAs production from WAS while struggling with limited MCFAs production and selectivity. Herein, this review covers comprehensive introduction of the transformation process from WAS to MCFAs and elaborates the mechanisms for unsatisfactory MCFAs production. The enhancement strategies for biotransformation of WAS to MCFAs was presented. Especially, the robust performance of iron-based materials is highlighted. Furthermore, knowledge gaps are identified to outline future research directions. Recycling MCFAs from WAS presents a promising option for future WAS treatment, with iron-based materials emerging as a key regulatory strategy in advancing the application of WAS-to-MCFAs biotechnology. This review will advance the understanding of MCFAs recovery from WAS and promote sustainable resource management in WWTPs.

Altering operational conditions during protein fermentation to volatile fatty acids modifies the associated bacterial community

In recent years, the production of volatile fatty acids (VFA) through mixed culture fermentation (MCF) has been gaining attention. Most authors have focused on the fermentation of carbohydrates, while other possible substrates, such as proteins, have not been considered. Moreover, there is little information about how operational parameters affect the microbial communities involved in these processes, even though they are strongly related to reactor performance and VFA selectivity. Hence, this study aims to evaluate how microbial composition changes according to three different parameters (pH, type of protein and micronutrient addition) during anaerobic fermentation of protein-rich side streams. For this, two continuous stirred tank reactors (CSTR) were fed with two different proteins (casein and gelatine) and operated at different conditions: three pH values (5.0, 7.0 and 9.0) with only macronutrients supplementation and two pH values (5.0 and 7.0) with micronutrients' supplementation as well. Firmicutes, Proteobacteria and Bacteroidetes were the dominant phyla in the two reactors at all operational conditions, but their relative abundance varied with the parameters studied. At pH 7.0 and 9.0, the microbial composition was mainly affected by protein type, while at acidic conditions the driving force was the pH. The influence of micronutrients was dependent on the pH and the protein type, with a special effect on Clostridiales and Bacteroidales populations. Overall, this study shows that the acidogenic microbial community is affected by the three parameters studied and the changes in the microbial community can partially explain the macroscopic results, especially the process selectivity.

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.

Regulation on C2-C8 carboxylic acid biosynthesis from anaerobic CO2 fermentation

Bioconversion of CO2 into liquid fuels or chemicals, preferred medium chain carboxylic acids (caproic and caprylic acid), is an attractive CO2 utilization technology. The present study aims to investigate the effects of different ratios of H2/CO2 on regulating the distribution of C2-C8 carboxylic acid products, while the headspace pressure of 1.5 bar was set to amplify the effect of different ratios. The H2/CO2 ratio of 4:1 was more suitable for preparing acetic acid, where the highest acetic acid yield was 17.5 g/L. And the H2/CO2 ratio of 2:1 showed excellent chain elongation ability with the highest n-caprylic yield of 2.4 g/L. Additionally, the actual H2/CO2 ratios of 4:1 reactors were higher than that in 2:1 may be course chain elongation often accompanied by H2 production. The 16S rRNA genes analysis shows that the genus Terrisporobacter and Coriobacteriales may be related to acetic acid production enriched in H2/CO2 ratio 4:1 reactors, and the genus Clostridium and Paenibacillaceae may associate with the chain elongation pathway were enriched in H2/CO2 ratio 2:1 reactors.

Elevated caproic acid production from one-stage anaerobic fermentation of organic waste and its selective recovery by electro-membrane process

A constructed microbial consortia-based strategy to enhance caproic acid production from one-stage mixed-fermentation of glucose was developed, which incubated with acidogens (Clostridium sensu stricto 1, 11 dominated) and chain elongators (including Clostridium sensu stricto 12, Sporanaerobacter, and Caproiciproducens) acclimated from anaerobic sludge. Significant product upgrading toward caproic acid (8.31 g‧L-1) and improved substrate degradation was achieved, which can be greatly attributed to the lactic acid platform. Whereas, a small amount of caproic acid was observed in the control incubating with acidogens, with an average concentration of 2.09 g‧L-1. The strategy accelerated the shape and cooperation of the specific microbial community dominated by Clostridium sensu stricto and Caproiciproducens, which thereby contributed to caproic acid production via the fatty acid biosynthesis pathway. Moreover, the tailored electrodialysis with bipolar membrane enabled progressive up-concentration and acidification, allowing selective separation of caproic acid as an immiscible product with a purity of 82.58 % from the mixture.

Tandem Acidic CO2 Electrolysis Coupled with Syngas Fermentation: A Two-Stage Process for Producing Medium-Chain Fatty Acids

The tandem application of CO2 electrolysis with syngas fermentation holds promise for achieving heightened production rates and improved product quality. However, the significant impact of syngas composition on mixed culture-based microbial chain elongation remains unclear. Additionally, effective methods for generating syngas with an adjustable composition from acidic CO2 electrolysis are currently lacking. This study successfully demonstrated the production of medium-chain fatty acids from CO2 through tandem acidic electrolysis with syngas fermentation. CO could serve as the sole energy source or as the electron donor (when cofed with acetate) for caproate generation. Furthermore, the results of gas diffusion electrode structure engineering highlighted that the use of carbon black, either alone or in combination with graphite, enabled consistent syngas generation with an adjustable composition from acidic CO2 electrolysis (pH 1). The carbon black layer significantly improved the CO selectivity, increasing from 0% to 43.5% (0.05 M K+) and further to 92.4% (0.5 M K+). This enhancement in performance was attributed to the promotion of K+ accumulation, stabilizing catalytically active sites, rather than creating a localized alkaline environment for CO2-to-CO conversion. This research contributes to the advancement of hybrid technology for sustainable CO2 reduction and chemical production.

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.

Effects of yeast inoculation methods on caproic acid production and microbial community during anaerobic fermentation of Chinese cabbage waste

To provide a sufficient supply of electron donors for the synthesis of caproic acid, yeast fermentation was employed to increase ethanol production in the anaerobic fermentation of Chinese cabbage waste (CCW). The results showed that the caproic acid yield of CCW with ethanol pre-fermentation was 7750.3 mg COD/L, accounting for 50.2% of the total volatile fatty acids (TVFAs), which was 32.5% higher than that of the CCW without yeast inoculation. The synchronous fermentation of yeast and seed sludge significantly promoted the growth of butyric acid consuming bacterium Bacteroides, resulting in low yields of butyric acid and caproic acid. With yeast inoculation, substrate competition for the efficient ethanol conversion in the early stage of acidogenic fermentation inhibited the hydrolysis and acidfication. Without yeast inoculation, the rapid accumulation of TVFAs severely inhibited the growth of Bacteroidetes. In the reactor with ethanol pre-fermentation, the key microorganism for caproic acid production, Clostridium_sensu_stricto_12, was selectively enriched.

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.