Anaerobic fermentation for n-caproic acid production: A review

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.

Improving swine wastewater anaerobic digestion via granular activated carbon, zero-valent iron, and nano-magnetite

Using electron-conducting materials (ECMs) is a strategy to boost anaerobic digestion (AD). However, research commonly investigates high doses of ECMs, often higher than the sludge concentration in the reactors, making AD more expensive and economically unviable. In contrast, this study evaluated the effect of granular activated carbon (GAC), zero-valent iron (ZVI), and nano-magnetite (Fe3O4) on the AD of swine wastewater (SW) at concentrations equal to or lower than the sludge content in the reactors. The biochemical methane potential (BMP) test was conducted in reactors with 100 mL of reaction volume inoculated with brewery sludge, operating in fed-batch mode in two treatment cycles, each with 35 days. The reactors were maintained at 37 ± 0.1 °C under orbital agitation of 150 rpm. ECMs were evaluated individually at concentrations of 0.4 g L-1 (0.1 rECM/VSS), 2.0 g L-1 (0.5 rECM/VSS), and 4.0 g L-1 (1.0 rECM/VSS). CH4 production kinetics were improved with 4.0 g L-1 of additive. GAC promoted the absence of lag phase and increased by 52.9 % and 63.6 % the speed and maximum rate of CH4 production, respectively. ZVI was the additive that most increased BMP (15.2 %) and CH4 content in biogas (13.5 %). Doses higher than 0.4 g L-1 of Fe3O4 did not produce additional positive impacts on BMP. ECMs enriched volatile fatty acid-oxidizing syntrophic bacteria and methanogenic archaea (Methanothrix soehngenii and Midas s 4938 - genus Methanolinea). ECMs (0.1, 0.5, and 1.0 rECM/VSS) improved the AD of SW from an inoculum not adapted to the evaluated substrate.

Fermentation of the organic fraction of municipal solid waste under different pH values and composition of microbial communities

The organic fraction of municipal solid waste (OFMSW) must be stored for hours or days before being fed to the anaerobic digestion reactors. This storage leads to spontaneous lactic acid fermentation, and volatile fatty acids (VFAs) and ethanol are produced by naturally occurring microorganisms. This research deals with fermentation and hydrolysis by controlling the OFMSW storage (silage) conditions. Using only naturally occurring microorganisms as inoculum, OFMSW fermentation in a semi-continuous reactor at pH values of 4, 5, and 6 was performed. During 6 days, samples were collected and analyzed daily for VFAs, ethanol, and lactic acid. At pH 4, the main products were ethanol, lactic acid, and acetic acid; at pH 5, lactic acid predominated, decreasing after day 4; at pH 6, acetic acid formed rapidly and after day 1, the concentration remained constant. At pH 6, butyric acid reached the highest concentration of all VFAs. The microbial diversity increased with pH. Metataxonomic analysis supports the possibility that the fungus of the Pichia genus is responsible for ethanol production and that various bacteria are responsible for VFAs, lactic acid production, and acetogenesis. Acetogenesis was the main pathway for the decrease in lactic acid and ethanol over time.

Advances and insights into modeling extracellular electron transfer in anaerobic bioprocesses

Extracellular electron transfer (EET) plays an important role in maintaining redox balance in both natural and engineered anaerobic microbial systems, driving key biochemical processes such as energy generation, bioremediation, and waste degradation. While EET has been characterized in a limited number of microbes and applied in anaerobic digestion and bioelectrochemical systems, further research is needed to explore its mechanism across a broader range of microbial species and anaerobic processes. This review highlights advanced modeling frameworks that provide deeper insights into EET mechanisms and dynamics, aiming to optimize research efforts and minimize time and resource expenditure. Mechanistic models, encompassing thermodynamics and kinetics, are discussed for their utility in calculating conduction rates of electroactive microbes and assessing the energetics of medium chain carboxylic acids production. Genome-scale metabolic models are highlighted for elucidating the roles of cytochromes and conductive pili in the EET pathway. Machine learning is presented as a tool to improve model accuracy and predict EET mechanisms. Furthermore, the integration of quantum mechanics/molecular mechanics methods offers molecular-level insights into electron transfer, while quantum computing addresses limitations of classical computers by simulating complex electron transfer processes in multi-heme cytochromes. Developing advanced modeling techniques will complement experimental techniques, enabling precise predictions and optimization strategies for developing innovative and sustainable anaerobic biotechnologies.

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.

Multi-omics analysis of microbial spatiotemporal succession and metabolite differences in pit mud of varying cellar ages and spatial positions

Microbes in pit mud (PM) are vital for the solid-state fermentation of strong-flavour baijiu (SFB), influencing the flavour and quality of SFB through metabolic activity. This study aimed to elucidate the differences in microbes and metabolites in PM at varying cellar ages and spatial positions. Microbes and metabolites in PM exhibited significant spatiotemporal variations, with temporal heterogeneity being more pronounced than spatial heterogeneity. Fourteen and 21 dominant genera were identified in 7-year and 50-year PM, respectively. Redundancy analysis suggested that pH, lactic acid, humus, and acetic acid drive microbial community succession. A total of 255 metabolites were identified, with acids, carbohydrates, and alcohols being the most prevalent. Significant positive correlations between the biomarkers and the main differential metabolites were revealed. Structural equation modeling demonstrated significant correlations between physicochemical factors, biomarkers, and the main differential metabolites. This study provides a foundation for future modifications of the quality and flavour of SFB.

Improving hydrogen and volatile fatty acids production through pretreatment of spent coffee grounds

Consumption of coffee produces large amounts of waste in the form of spent coffee grounds (SCG), a lignocellulosic material rich in carbohydrates, proteins, and polyphenols. This abundant feedstock is promising in terms of biofuels and value-added product generation. This study investigated the impact of pretreatments, such as alkaline (NaOH), ultrasound, and static magnetic field, on SCG bioconversion in terms of biomolecule release, H2 potential and volatile fatty acids production. Following treatment, the slurry (solid and liquid fraction mixture) was utilised in anaerobic fermentation tests at varying volatile solid (VS) concentrations (23.3 and 46.7 g VS/L). The highest H2 production range, 25 - 30 mL H2/g VS, was obtained using the alkaline-pretreated SCG slurry at 23.3 g VS/L. Nevertheless, inhibition of H2 production was observed when utilising the alkaline-pretreated slurry at 46.7 g VS/L owing to the excessive use of NaOH for pretreatment and chemicals to adjust the initial pH. In contrast, increasing the VS concentration had a positive impact on volatile fatty acids accumulation, with acetic (HAc) and caproic acid being dominant. Ultrasound-pretreated SCG achieved 3260.0 mg HAceq/L at a concentration of 46.7 g VS/L.

Caproiciproducens converts lactic acid into caproic acid during Chinese strong-flavor Baijiu brewing

Strong-flavor Baijiu, a type of Chinese liquor, is produced through anaerobic solid-state fermentation in a sealed mud pit. Ethyl caproate, the characteristic flavor compound of strong-flavor Baijiu, is influenced by caproic acid-producing bacteria in the pit mud. To better understand the formation of caproic acid, this study investigated the microbial composition and physicochemical parameters of pit mud from different layers (top, middle, and bottom) in Hubei and Sichuan provinces, China. The results revealed that Caproiciproducens plays a key role in caproic acid production by using lactic acid as a substrate, with its abundance increasing with the depth of the pit mud. A strain Caproiciproducens sp. R1, isolated from the pit mud, was shown to produce caproic acid from lactic acid within an initial pH range of 5.5-9.0 and lactic acid concentrations of 1 %-5 % (m/v). In addition, inoculation of strain R1 into Huangshui (a lactic acid-rich liquid from Baijiu production) resulted in 40.72 mM caproic acid production. This study demonstrates that Caproiciproducens plays a crucial role in caproic acid production from lactic acid during the fermentation process of strong-flavor Baijiu.

Microbial Electrochemical Technologies: Sustainable Solutions for Addressing Environmental Challenges

Addressing global challenges of waste management demands innovative approaches to turn biowaste into valuable resources. This chapter explores the potential of microbial electrochemical technologies (METs) as an alternative opportunity for biowaste valorisation and resource recovery due to their potential to address limitations associated with traditional methods. METs leverage microbial-driven oxidation and reduction reactions, enabling the conversion of different feedstocks into energy or value-added products. Their versatility spans across gas, food, water and soil streams, offering multiple solutions at different technological readiness levels to advance several sustainable development goals (SDGs) set out in the 2030 Agenda. By critically examining recent studies, this chapter uncovers challenges, optimisation strategies, and future research directions for real-world MET implementations. The integration of economic perspectives with technological developments provides a comprehensive understanding of the opportunities and demands associated with METs in advancing the circular economy agenda, emphasising their pivotal role in waste minimisation, resource efficiency promotion, and closed-loop system renovation.

Caproic acid production from short-chained carboxylic acids produced by arrested anaerobic digestion of green waste: Development and optimization of a mixed caproic acid producing culture

Caproic acid is an important compound for producing a variety of chemicals and a potential precursor for Sustainable Aviation Fuels (SAF). This study aimed at developing a stable mixed culture producing caproic acid using short-chain carboxylic acids (SCCAs) derived from arrested anaerobic digestion (AAD) of green waste and optimizing chain elongation (CE) conditions. The results showed that the mixed culture was dominated by Clostridium sensu stricto 12 (53.09 %), optimally producing caproic acid at 37 °C, pH 7.20, with a 15 % inoculum concentration. Under these conditions, 75.59 % of the consumed electrons contributed to caproic acid formation. Gradual ethanol feeding in bioreactor experiments resulted in caproic acid titer of 13.36 g/L, with carbon conversion efficiency (CCE) of 81.91 % and electron transfer efficiency (ETE) of 76.39 %, surpassing batch experiments without gradual ethanol addition. These findings demonstrate that the effectiveness of a mixed culture can be improved, resulting in increased caproic acid production.

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.

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.

Mechanisms for the Enhancement of Caproic Acid and H2 Production in Ruminococcaceae Bacterium CPB6 by Fe(II) and Mg(II): Growth and Gene Transcription Analyses

The production of caproic acid (CA) and hydrogen gas (H2) from organic wastewater is economically attractive. The Ruminococcaceae bacterium CPB6 has demonstrated potential for CA production from lactate-containing wastewater. However, our understanding of the effects of Fe2+ and Mg2+ on the growth and metabolism of strain CPB6 remains limited. Therefore, this study aims to investigate the impact of Fe2+ and Mg2+ on CA and H2 production, as well as on the expression of key genes involved in CA and H2 biosynthesis pathway. The results indicate that Fe2+ positively affects cell proliferation and H2 production while minimally impacting CA production. The highest levels of H2 production were achieved with the addition of 200 mg/L Fe2+. Conversely, Mg2+ significantly enhances CA and H2 production, with the optimal yield observed in a medium enriched with 300 mg/L Mg2+. Reverse transcription quantitative PCR (RT-qPCR) analysis reveals that Fe2+ promotes the expression of the hydrogenase gene, whereas Mg2+ has a negligible effect on hydrogenase expression. Notably, Fe2+ and Mg2+ inhibit the expression of key genes involved in CA synthesis. These findings suggest that Fe2+ enhances H2 production by boosting cell biomass and the expression of the hydrogenase gene, whereas Mg2+ improves CA and H2 production primarily by increasing cell biomass rather than influencing the expression of functional genes involved in CA biosynthesis.

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.

Lactobacillus inoculation mediated carboxylates and alcohols production from waste activated sludge fermentation system: Insight into process outcomes and metabolic network

Producing medium chain fatty acids (MCFAs) from waste activated sludge (WAS) is crucial for sustainable chemical industries. This study addressed the electron donor requirement for MCFAs production by inoculating Lactobacillus at varying concentrations (7.94 × 1010, 3.18 × 1011, and 6.35 × 1011 cell/L) to supply lactate internally. Interestingly, the highest MCFAs yield (∼2000 mg COD/L) occurred at the lowest Lactobacillus inoculation. Higher inoculation concentrations redirected more carbon from WAS towards alcohols production rather than MCFAs generation, with up to 2852 mg COD/L alcohols obtained under 6.35 × 1011 cell/L inoculation. Clostridium dominance and increased genes abundance for substrate hydrolysis, lactate conversion, and MCFAs/alcohol production collectively enhanced WAS-derived MCFAs and alcohols synthesis after Lactobacillus inoculation. Overall, the strategy of Lactobacillus inoculation regulated fermentation outcomes and subsequent carbon recovery in WAS, presenting a sustainable technology to achieve liquid bio-energy production from underutilized wet wastes.

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.

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.

A Novel Sustainable and Self-Sufficient Biotechnological Strategy for Directly Transforming Sewage Sludge into High-Value Liquid Biochemicals

Sewage sludge, as a carbon-rich byproduct of wastewater treatment, holds significant untapped potential as a renewable resource. Upcycling this troublesome waste stream represents great promise in addressing global escalating energy demands through its wide practice of biochemical recovery concurrently. Here, we propose a biotechnological concept to gain value-added liquid bioproducts from sewage sludge in a self-sufficient manner by directly transforming sludge into medium-chain fatty acids (MCFAs). Our findings suggest that yeast, a cheap and readily available commercial powder, would involve ethanol-type fermentation in chain elongation to achieve abundant MCFA production from sewage sludge using electron donors (i.e., ethanol) and acceptors (i.e., short-chain fatty acids) produced in situ. The enhanced abundance and transcriptional activity of genes related to key enzymes, such as butyryl-CoA dehydrogenase and alcohol dehydrogenase, affirm the robust capacity for the self-sustained production of MCFAs. This is indicative of an effective metabolic network established between yeast and anaerobic microorganisms within this innovative sludge fermentation framework. Furthermore, life cycle assessment and techno-economic analysis evidence the sustainability and economic competitiveness of this biotechnological strategy. Overall, this work provides insights into sewage sludge upgrading independent of additional carbon input, which can be applied in existing anaerobic sludge fermentation infrastructure as well as to develop new applications in a diverse range of industries.

Two-stage conversion of syngas and pyrolysis aqueous condensate into L-malate

Hybrid thermochemical-biological processes have the potential to enhance the carbon and energy recovery from organic waste. This work aimed to assess the carbon and energy recovery potential of multifunctional processes to simultaneously sequestrate syngas and detoxify pyrolysis aqueous condensate (PAC) for short-chain carboxylates production. To evaluate relevant process parameters for mixed culture co-fermentation of syngas and PAC, two identical reactors were run under mesophilic (37 °C) and thermophilic (55 °C) conditions at increasing PAC loading rates. Both the mesophilic and the thermophilic process recovered at least 50% of the energy in syngas and PAC into short-chain carboxylates. During the mesophilic syngas and PAC co-fermentation, methanogenesis was completely inhibited while acetate, ethanol and butyrate were the primary metabolites. Over 90% of the amplicon sequencing variants based on 16S rRNA were assigned to Clostridium sensu stricto 12. During the thermophilic process, on the other hand, Symbiobacteriales, Syntrophaceticus, Thermoanaerobacterium, Methanothermobacter and Methanosarcina likely played crucial roles in aromatics degradation and methanogenesis, respectively, while Moorella thermoacetica and Methanothermobacter marburgensis were the predominant carboxydotrophs in the thermophilic process. High biomass concentrations were necessary to maintain stable process operations at high PAC loads. In a second-stage reactor, Aspergillus oryzae converted acetate, propionate and butyrate from the first stage into L-malate, confirming the successful detoxification of PAC below inhibitory levels. The highest L-malate yield was 0.26 ± 2.2 molL-malate/molcarboxylates recorded for effluent from the mesophilic process at a PAC load of 4% v/v. The results highlight the potential of multifunctional reactors where anaerobic mixed cultures perform simultaneously diverse process roles, such as carbon fixation, wastewater detoxification and carboxylates intermediate production. The recovered energy in the form of intermediate carboxylates allows for their use as substrates in subsequent fermentative stages.

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.

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.

Analysis of differences in microorganisms and aroma profiles between normal and off-flavor pit mud in Chinese strong-flavor Baijiu

The unique cellar fermentation process of Chinese strong-flavor Baijiu is the reason for its characteristic cellar aroma flavor. The types, abundance, community structure and metabolic activity of microorganisms in the pit mud directly affect the microbial balance in the white spirit production environment, promoting the formation of typical aromas and influencing the quality of CFSB. During the production process, the production of off-flavor in the cellar may occur. The aim of this study is to elucidate the differences in microbiota and flavor between normal pit mud and abnormal pit mud (pit mud with off-flavor). A total of 46 major volatile compounds were identified, and 24 bacterial genera and 21 fungal genera were screened. The esters, acids, and alcohols in the abnormal pit mud were lower than those in the normal pit mud, while the aldehydes were higher. 3-Methyl indole, which has been proven to be responsible for the muddy and musty flavors, was detected in both types of pit mud, and for the first time, high levels of 4-methylanisole was detected in the pit mud. The microbial composition of the two types of pit mud showed significant differences in the bacterial genera of Sporosarcina, Lactobacillus, Garciella, Anaerosalibacter, Lentimicrobium, HN-HF0106, Petrimonas, Clostridium_sensu_stricto_12 and Bacillus, and the fungal genera of Millerozyma, Penicillium, Mortierella, Monascus, Saccharomyces, Issatchenkia, Pithoascus, Pseudallescheria, and Wickerhamomyces. Additionally, we speculate that Sporosarcina is the predominant bacterial genus responsible for the imbalance of microbiota in pit mud.

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.

Understanding the acidification risk of cheese whey anaerobic digestion under psychrophilic and mesophilic conditions

Anaerobic digestion is a suitable technology to treat cheese whey (CW), a high-strength wastewater from cheesemaking. However, CW anaerobic digestion is limited by its high biodegradability, acidic pH, and lack of alkalinity. This publication evaluated the acidification risk of CW anaerobic digestion under psychrophilic and mesophilic conditions, aiming to improve digester design, operation, and decision-making when facing instability periods. To evaluate the acidification risk of CW anaerobic digestion, biochemical methane potential (BMP) tests were carried out at four different organic loads, each under psychrophilic (20 °C) and mesophilic (35 °C) conditions. Besides methane production, pH, soluble chemical oxygen demand, volatile fatty acid and alcohols were also monitored. Experimental results showed that CW can be successfully degraded under both temperature conditions, with methane yields of 389-436 mLCH4/gVS. The organic load had a greater impact on the accumulation of intermediate products than temperature, indicating that process inhibition by overloading is plausible under psychrophilic and mesophilic conditions. However, the degradation rate under mesophilic conditions was faster than under psychrophilic conditions. Experimental results also revealed a higher imbalance between fermentation and methanogenesis rate under psychrophilic conditions, which resulted in higher concentrations of intermediate products (volatile fatty acids and alcohols) and prolonged lower pHs. These results indicate that the degradation of intermediate products is less favourable under psychrophilic conditions compared to mesophilic conditions. This implies that psychrophilic digesters have a lower capacity to recover from process disturbances, increasing the risk of process underperformance or even failure under psychrophilic conditions.

Enhancement of medium-chain fatty acids production from sludge anaerobic fermentation liquid under moderate sulfate reduction

In recent years, there has been a marked increase in the production of excess sludge. Chain-elongation (CE) fermentation presents a promising approach for carbon resource recovery from sludge, enabling the transformation of carbon into medium-chain fatty acids (MCFAs). However, the impact of sulfate, commonly presents in sludge, on the CE process remains largely unexplored. In this study, batch tests for CE process of sludge anaerobic fermentation liquid (SAFL) under different SCOD/SO42- ratios were performed. The moderate sulfate reduction under the optimum SCOD/SO42- of 20:1 enhanced the n-caproate production, giving the maximum n-caproate concentration, selectivity and production rate of 5.49 g COD/L, 21.4% and 4.87 g COD/L/d, respectively. The excessive sulfate reduction under SCOD/SO42- ≤ 5 completely inhibited the CE process, resulting in almost no n-caproate generation. The variations in n-caproate production under different conditions of SCOD/SO42- were all well fitted with the modified Gompertz kinetic model. Alcaligenes and Ruminococcaceae_UCG-014 were the dominant genus-level biomarkers under moderate sulfate reduction (SCOD/SO42- = 20), which enhanced the n-caproate production by increasing the generation of acetyl-CoA and the hydrolysis of difficult biodegradable substances in SAFL. The findings presented in this work elucidate a strategy and provide a theoretical framework for the further enhancement of MCFAs production from excess sludge.

Electrochemical promotion of organic waste fermentation: Research advances and prospects

The current methods of treating organic waste suffer from limited resource usage and low product value. Research and development of value-added products emerges as an unavoidable trend for future growth. Electro-fermentation (EF) is a technique employed to stimulate cell proliferation, expedite microbial metabolism, and enhance the production of value-added products by administering minute voltages or currents in the fermentation system. This method represents a novel research direction lying at the crossroads of electrochemistry and biology. This article documents the current progress of EF for a range of value-added products, including gaseous fuels, organic acids, and other organics. It also presents novel value-added products, such as 1,3-propanediol, 3-hydroxypropionic acid, succinic acid, acrylic acid, and lysine. The latest research trends suggest a focus on EF for cogeneration of value-added products, studying microbial community structure and electroactive bacteria, exploring electron transfer mechanisms in EF systems, developing effective methods for nutrient recovery of nitrogen and phosphorus, optimizing EF conditions, and utilizing biosensors and artificial neural networks in this area. In this paper, an analysis is conducted on the challenges that currently exist regarding the selection of conductive materials, optimization of electrode materials, and development of bioelectrochemical system (BES) coupling processes in EF systems. The aim is to provide a reference for the development of more efficient, advanced, and value-added EF technologies. Overall, this paper aims to provide references and ideas for the development of more efficient and advanced EF technology.

Why do lactic acid bacteria thrive in chain elongation microbiomes?

Efficient waste management is necessary to transition towards a more sustainable society. An emerging trend is to use mixed culture biotechnology to produce chemicals from organic waste. Insights into the metabolic interactions between community members and their growth characterization are needed to mediate knowledge-driven bioprocess development and optimization. Here, a granular sludge bioprocess for the production of caproic acid through sugar-based chain elongation metabolism was established. Lactic acid and chain-elongating bacteria were identified as the two main functional guilds in the granular community. The growth features of the main community representatives (isolate Limosilactobacillus musocae G03 for lactic acid bacteria and type strain Caproiciproducens lactatifermentans for chain-elongating bacteria) were characterized. The measured growth rates of lactic acid bacteria (0.051 ± 0.005 h-1) were two times higher than those of chain-elongating bacteria (0.026 ± 0.004 h-1), while the biomass yields of lactic acid bacteria (0.120 ± 0.005 g biomass/g glucose) were two times lower than that of chain-elongating bacteria (0.239 ± 0.007 g biomass/g glucose). This points towards differential growth strategies, with lactic acid bacteria resembling that of a r-strategist and chain-elongating bacteria resembling that of a K-strategist. Furthermore, the half-saturation constant of glucose for L. mucosae was determined to be 0.35 ± 0.05 g/L of glucose. A linear trend of caproic acid inhibition on the growth of L. mucosae was observed, and the growth inhibitory caproic acid concentration was predicted to be 13.6 ± 0.5 g/L, which is the highest reported so far. The pre-adjustment of L. mucosae to 4 g/L of caproic acid did not improve the overall resistance to it, but did restore the growth rates at low caproic acid concentrations (1-4 g/L) to the baseline values (i.e., growth rate at 0 g/L of caproic acid). High resistance to caproic acid enables lactic acid bacteria to persist and thrive in the systems intended for caproic acid production. Here, insights into the growth of two main functional guilds of sugar-based chain elongation systems are provided which allows for a better understanding of their interactions and promotes future bioprocess design and optimization.

Wetlands harbor lactic acid-driven chain elongators

IMPORTANCE

Wetlands are globally significant carbon cycling hotspots that both sequester large amounts of CO2 as soil carbon as well as emit a third of all CH4 globally. Their outsized role in the global carbon cycle makes it critical to understand microbial processes contributing to carbon breakdown and storage in these ecosystems. Here, we confirm the presence of chain-elongating organisms in freshwater wetland soils. These organisms take small carbon compounds formed during the breakdown of biomass and turn them into larger compounds (six to eight carbon organic acids) that may potentially contribute to the formation of soil organic matter and long-term carbon storage. Moreover, we find that these chain-elongating organisms may be widely distributed in wetlands globally. Future work should identify these organisms' contribution to carbon cycling in wetlands and the potential role of the products they form in carbon sequestration in wetlands.

Selective butyric acid production from CO2 and its upgrade to butanol in microbial electrosynthesis cells

Microbial electrosynthesis (MES) is a promising carbon utilization technology, but the low-value products (i.e., acetate or methane) and the high electric power demand hinder its industrial adoption. In this study, electrically efficient MES cells with a low ohmic resistance of 15.7 mΩ m2 were operated galvanostatically in fed-batch mode, alternating periods of high CO2 and H2 availability. This promoted acetic acid and ethanol production, ultimately triggering selective (78% on a carbon basis) butyric acid production via chain elongation. An average production rate of 14.5 g m-2 d-1 was obtained at an applied current of 1.0 or 1.5 mA cm-2, being Megasphaera sp. the key chain elongating player. Inoculating a second cell with the catholyte containing the enriched community resulted in butyric acid production at the same rate as the previous cell, but the lag phase was reduced by 82%. Furthermore, interrupting the CO2 feeding and setting a constant pH2 of 1.7-1.8 atm in the cathode compartment triggered solventogenic butanol production at a pH below 4.8. The efficient cell design resulted in average cell voltages of 2.6-2.8 V and a remarkably low electric energy requirement of 34.6 kWhel kg-1 of butyric acid produced, despite coulombic efficiencies being restricted to 45% due to the cross-over of O2 and H2 through the membrane. In conclusion, this study revealed the optimal operating conditions to achieve energy-efficient butyric acid production from CO2 and suggested a strategy to further upgrade it to valuable butanol.

Redox mediators stimulated chain elongation process in fluidized cathode electro-fermentation systems for caproate production

Medium chain fatty acids (MCFAs), the secondary products of traditional anaerobic fermentation, can be produced via chain elongation (CE), a process often retarded due to the difficulty during interspecies electron transfer (IET). This study employed redox mediators, neutral red (NR), methyl viologen (MV), and methylene blue (MB) as electron shuttles to expedite the electro-fermentation for caproate production by improving IET. Results showed that MV increased the MCFAs production by promoting acetate to ethanol conversion, leading to the highest MCFAs selectivity of 68.73%. While NR was indicated to improve CE by encouraging H2 production, and the biocathode had the highest electrical activity due to the smallest internal resistance and largest capacitance increase of 96% than the control. A higher proportion of Sutterella, Prevotella, and Hydrogenophaga, linked with the H2 mediated interspecies electron transfer (MIET) during CE process, was observed across redox mediators supplied groups compared to the control. The presence of mediators led to an elevated abundance of key enzymes for enhanced CE process and electron transfer. This study provided the perspective of the stimulated electron transfer for improved MCFAs production in electro-fermentation systems.

Acidogenic fermentation of food waste to generate electron acceptors and donors towards medium-chain carboxylic acids production

This study investigated the optimum pH, temperature, and food-to-microorganisms (F/M) ratio for regulating the formation of electron acceptors and donors during acidogenic fermentation to facilitate medium-chain carboxylic acids (MCCAs) production from food waste. Mesophilic fermentation at pH 6 was optimal for producing mixed volatile fatty acids (719 ± 94 mg COD/g VS) as electron acceptors. Under mesophilic conditions, the F/M ratio (g VS/g VS) could be increased to 6 to generate 22 ± 2 g COD/L of electron acceptors alongside 2 ± 0 g COD/L of caproic acid. Thermophilic fermentation at pH 6 was the best condition for producing lactic acid as an electron donor. However, operating at F/M ratios above 3 g VS/g VS under thermophilic settings significantly reduced lactic acid yield. A preliminary techno-economic evaluation revealed that converting lactic acid and butyric acid generated during acidogenic fermentation to caproic acid was the most profitable food waste valorization scenario and could generate 442-468 €/t VS/y. The results presented in this study provide insights into how to tailor acidogenic fermentation reactions to desired intermediates and will help maximize MCCAs synthesis.

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.

Sustainable circular biorefinery approach for novel building blocks and bioenergy production from algae using microbial fuel cell

The imminent need for transition to a circular biorefinery using microbial fuel cells (MFC), based on the valorization of renewable resources, will ameliorate the carbon footprint induced by industrialization. MFC catalyzed by bioelectrochemical process drew significant attention initially for its exceptional potential for integrated production of biochemicals and bioenergy. Nonetheless, the associated costly bioproduct production and slow microbial kinetics have constrained its commercialization. This review encompasses the potential and development of macroalgal biomass as a substrate in the MFC system for L-lactic acid (L-LA) and bioelectricity generation. Besides, an insight into the state-of-the-art technological advancement in the MFC system is also deliberated in detail. Investigations in recent years have shown that MFC developed with different anolyte enhances power density from several µW/m2 up to 8160 mW/m2. Further, this review provides a plausible picture of macroalgal-based L-LA and bioelectricity circular biorefinery in the MFC system for future research directions.

Effect of decoupling hydraulic and solid retention times on carbohydrate-rich residue valorization into carboxylic acids

This research assessed the effect of decoupling hydraulic retention time (HRT) and solid retention time (SRT) on the production of volatile fatty acids (VFAs) via anaerobic fermentation of beet molasses. The performance of a continuous stirred tank reactor (CSTR, STR = HTR = 30 days) and two anaerobic sequencing batch reactors (AnSBR) with decoupled STR (30 days) and HRT (20 and 10 days) was compared. Previously, a temperature study in batch reactors (25, 35, and 55 °C) revealed 25 °C as the optimal temperature to maximize the VFAs yield and the long-chain VFAs (> C4) production, being selected for the continuous reactors operation. An HRT of 20 days in AnSBR led to an enhancement in bioconversion efficiency into VFAs (55.5% chemical oxygen demand basis) compared to the CSTR (34.9%). In contrast, the CSTR allowed the production of valuable caproic acid (25.4% vs 4.1% w/w of total VFAs in AnSBR). Decreasing further the HRT to 10 days in AnSBR was detrimental in terms of bioconversion efficiency (21.7%) due to primary intermediates (lactate) accumulation. By decoupling HRT and SRT, VFAs were maximized, revealing HRT as an effective tool to drive specific conversion routes (butyrate- or lactate-fermentation).

Effect of pH and medium composition on chain elongation with Megasphaera hexanoica producing C4-C8 fatty acids

Introduction

Chain elongation technology, which involves fermentation with anaerobic bacteria, has gained attention for converting short and medium chain substrates into valuable and longer-chain products like medium chain fatty acids (MCFAs). In the recent past, the focus of studies with pure chain elongating cultures was on species of other genera, mainly Clostridium kluyveri. Recently, other chain elongators have been isolated that deserve further research, such as Megasphaera hexanoica.

Methods

In this study, batch studies were performed in bottles with two different media to establish the optimal conditions for growth of M. hexanoica: (a) a medium rich in different sources of nitrogen and (b) a medium whose only source of nitrogen is yeast extract. Also, batch bioreactor studies at pH values of 5.8, 6.5 and 7.2 were set up to study the fermentation of lactate (i.e., electron donor) and acetate (i.e., electron acceptor) by M. hexanoica.

Results and discussion

Batch bottle studies revealed the yeast extract (YE) containing medium as the most promising in terms of production/cost ratio, producing n-caproate rapidly up to 2.62 ± 0.24 g/L. Subsequent bioreactor experiments at pH 5.8, 6.5, and 7.2 confirmed consistent production profiles, yielding C4-C8 fatty acids. A fourth bioreactor experiment at pH 6.5 and doubling both lactate and acetate concentrations enhanced MCFA production, resulting in 3.7 g/L n-caproate and 1.5 g/L n-caprylate. H2 and CO2 production was observed in all fermentations, being especially high under the increased substrate conditions. Overall, this study provides insights into M. hexanoica's behavior in lactate-based chain elongation and highlights optimization potential for improved productivity.

In-situ production of lactate driving the biotransformation of waste activated sludge to medium-chain fatty acid

Medium-chain fatty acids (MCFAs) have drawn great attention due to their high energy density and superior hydrophobicity. Waste activated sludge (WAS) has been documented as a renewable feedstock for MCFAs production via anaerobic fermentation. However, MCFAs production from WAS depends on exogenous addition of electron donor (ED, e.g., lactate) for chain elongation (CE) bioprocess, which results in increased economic cost and limited practical application. In this study, a novel biotechnology was proposed to produce MCFAs from WAS with in-situ self-formed lactate by inoculating Yoghurt starter powder containing with Lactobacillales cultures. The batch experimental results revealed that the lactate was in-situ generated from WAS and the maximum production of MCFAs increased from 1.17 to 3.99 g COD/L with the increased addition of Lactobacillales cultures from 6✕107 to 2.3✕108 CFU/mL WAS. In continuous long-term test over 97 days, average MCFA production reached up to 3.94 g COD/L with a caproate yield of 82.74% at sludge retention time (SRT) 12 days, and the average MCFA production increased to 5.87 g COD/L with 69.28% caproate and 25.18% caprylate at SRT 15 days. A comprehensive analysis of the metagenome and metatranscriptome demonstrated that the genus of Lactobacillus and Streptococcus were capable of producing lactate from WAS and upgrading to MCFAs. Moreover, another genus, i.e., Candidatus Promineofilum, was firstly revealed that it might be responsible for lactate and MCFAs production. Further investigation of related microbial pathways and enzyme expression suggested that D-lactate dehydrogenase and pyruvate ferredoxin oxidoreductase contributed to lactate and acetyl-CoA production, which were the crucial steps for MCFAs generation and were most actively expressed. This study provides a conceptual framework of MCFAs from WAS with endogenous ED, potentially enhancing the energy recovery from WAS treatment.

Genome Analysis and Optimization of Caproic Acid Production of Clostridium butyricum GD1-1 Isolated from the Pit Mud of Nongxiangxing Baijiu

Caproic acid is a precursor substance for the synthesis of ethyl caproate, the main flavor substance of nongxiangxing baijiu liquor. In this study, Clostridium butyricum GD1-1, a strain with high caproic acid concentration (3.86 g/l), was isolated from the storage pit mud of nongxiangxing baijiu for sequencing and analysis. The strain's genome was 3,840,048 bp in length with 4,050 open reading frames. In addition, virulence factor annotation analysis showed C. butyricum GD1-1 to be safe at the genetic level. However, the annotation results using the Kyoto Encyclopedia of Genes and Genomes Automatic Annotation Server predicted a deficiency in the strain's synthesis of alanine, methionine, and biotin. These results were confirmed by essential nutrient factor validation experiments. Furthermore, the optimized medium conditions for caproic acid concentration by strain GD1-1 were (g/l): glucose 30, NaCl 5, yeast extract 10, peptone 10, beef paste 10, sodium acetate 11, L-cysteine 0.6, biotin 0.004, starch 2, and 2.0% ethanol. The optimized fermentation conditions for caproic acid production by C. butyricum GD1-1 on a single-factor basis were: 5% inoculum volume, 35°C, pH 7, and 90% loading volume. Under optimal conditions, the caproic acid concentration of strain GD1-1 reached 5.42 g/l, which was 1.40 times higher than the initial concentration. C. butyricum GD1-1 could be further used in caproic acid production, NXXB pit mud strengthening and maintenance, and artificial pit mud preparation.

Caproate production from Enset fiber in one-pot two-step fermentation using anaerobic fungi (Neocallimastix cameroonii strain G341) and Clostridium kluyveri DSM 555

BACKGROUND

Lignocellulosic biomass plays a crucial role in creating a circular bioeconomy and minimizing environmental impact. Enset biomass is a byproduct of traditional Ethiopian Enset food processing that is thrown away in huge quantities. This study aimed to produce caproate from Enset fiber using Neocallimastix cameroonii strain G341 and Clostridium kluyveri DSM 555 in one-pot two-step fermentation.

RESULTS

The process started by growing N. cameroonii on Enset fiber as a carbon source for 7 days. Subsequently, the fungal culture was inoculated with active C. kluyveri preculture and further incubated. The results showed that N. cameroonii grew on 0.25 g untreated Enset fiber as the sole carbon source and produced 1.16 mmol acetate, 0.51 mmol hydrogen, and 1.34 mmol formate. In addition, lactate, succinate, and ethanol were detected in small amounts, 0.17 mmol, 0.08 mmol, and 0.7 mmol, respectively. After inoculating with C. kluyveri, 0.3 mmol of caproate and 0.48 mmol of butyrate were produced, and hydrogen production also increased to 0.95 mmol compared to sole N. cameroonii fermentation. Moreover, after the culture was supplemented with 2.18 mmol of ethanol during C. kluyveri inoculation, caproate, and hydrogen production was further increased to 1.2 and 1.36 mmol, respectively, and the consumption of acetate also increased.

CONCLUSION

A novel microbial cell factory was developed to convert untreated lignocellulosic Enset fiber into the medium chain carboxylic acid caproate and H2 by a co-culture of the anaerobic fungi N. cameroonii and C. kluyveri. This opens a new value chain for Enset farmers, as the process requires only locally available raw materials and low-price fermenters. As the caproate production was mainly limited by the available ethanol, the addition of locally produced ethanol-containing fermentation broth ("beer") would further increase the titer.

Physiological and stoichiometric characterization of ethanol-based chain elongation in the absence of short-chain carboxylic acids

Hexanoate is a valuable chemical that can be produced by microorganisms that convert short-chain- to medium-chain carboxylic acids through a process called chain elongation. These microorganisms usually produce mixtures of butyrate and hexanoate from ethanol and acetate, but direct conversion of ethanol to hexanoate is theoretically possible. Steering microbial communities to ethanol-only elongation to hexanoate circumvents the need for acetate addition and simplifies product separation. The biological feasibility of ethanol elongation to hexanoate was validated in batch bioreactor experiments with a Clostridium kluyveri-dominated enrichment culture incubated with ethanol, acetate and butyrate in different ratios. Frequent liquid sampling combined with high-resolution off-gas measurements allowed to monitor metabolic behavior. In experiments with an initial ethanol-to-acetate ratio of 6:1, acetate depletion occurred after ± 35 h of fermentation, which triggered a metabolic shift to direct conversion of ethanol to hexanoate despite the availability of butyrate (± 40 mCmol L-1). When only ethanol and no external electron acceptor was supplied, stable ethanol to hexanoate conversion could be maintained until 60-90 mCmol L-1 of hexanoate was produced. After this, transient production of either acetate and butyrate or butyrate and hexanoate was observed, requiring a putative reversal of the Rnf complex. This was not observed before acetate depletion or in presence of low concentrations (40-60 mCmol L-1) of butyrate, suggesting a stabilizing or regulatory role of butyrate or butyrate-related catabolic intermediates. This study sheds light on previously unknown versatility of chain elongating microbes and provides new avenues for optimizing (waste) bioconversion for hexanoate production.

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.

Advances in understanding entire process of medium chain carboxylic acid production from organic wastes via chain elongation

Chain elongation is an environmentally friendly biological technology capable of converting organic wastes into medium chain carboxylic acids (MCCAs). This review aims to offer a comprehensive analysis of MCCA production from organic wastes via chain elongation. Seven kinds of organic wastes are introduced and classified as easily degradable and hardly degradable. Among them, food waste, fruit and vegetable waste are the most potential organic wastes for MCCA production. Combined pretreatment technologies should be encouraged for the pretreatment of hardly degradable organic wastes. Furthermore, the mechanisms during MCCA production are analyzed, and the key influencing factors are evaluated, which affect the MCCA production and chain elongation efficiency indirectly. Extracting MCCA simultaneously is the most important way to improve MCCA production efficiency, and technologies for sequentially extracting different kinds of MCCAs are recommended. Finally, some perspectives for future chain elongation researches are proposed to promote the large-scale application of chain elongation.

Hydraulic Retention Time as an Operational Tool for the Production of Short-Chain Carboxylates via Anaerobic Fermentation of Carbohydrate-Rich Waste

The carboxylate platform is a sustainable and cost-effective way to valorize wastes into biochemicals that replace those of fossil origin. Short-chain fatty acids (SCFAs) are intermediates generated during anaerobic fermentation (AF) and are considered high-value-added biochemicals among carboxylates. This investigation aimed to produce SCFAs through the AF of sugar beet molasses at 25 °C and semi-continuous feeding mode in completely stirred tank reactors. A particular focus was devoted to the role of hydraulic retention time (HRT) variation in SCFAs production and distribution profile. The highest SCFAs concentration (44.1 ± 2.3 gCOD/L) was reached at the HRT of 30 days. Caproic acid accounted for 32.5-35.5% (COD-concentration basis) at the long HRTs of 20 and 30 days due to the carbon chain elongation of shorter carboxylic acids. The findings of this study proved that HRT could be used to steer the anaerobic process toward the targeted SCFAs for specific uses. Furthermore, the successful operation at low-temperature conditions (i.e., 25 °C) makes the process economically promising.

The role of hydrochloric acid pretreated activated carbon in chain elongation of D-lactate to caproate: Adsorption and facilitation

Medium chain fatty acids (MCFA) generation is attracting growing interest due to fossil fuel depletion. To promote the production of MCFA, especially caproate, hydrochloric acid pretreated activated carbon (AC) was introduced into chain elongation fermentation. In this study, the role of pretreated AC on caproate production was investigated using lactate and butyrate as electron donor and electron acceptor, respectively. The results showed that AC did not improve the chain elongation reaction at beginning but promoted the caproate production at later stage. The addition of 15 g/L AC facilitated reactor reaching the peak of caproate concentration (78.92 mM), caproate electron efficiency (63.13%), and butyrate utilization rate (51.88%). The adsorption experiment revealed a positive correlation between the adsorption capacity of pretreated AC and the concentration as well as the carbon chain length of carboxylic acids. Moreover, the adsorption of undissociated caproate by pretreated AC contributed to a mitigated toxicity towards microorganisms, thereby facilitating the production of MCFA. Microbial community analysis revealed an increasing enrichment of key functional chain elongation bacteria, including Eubacterium, Megasphaera, Caproiciproducens, and Pseudoramibacter, but a suppression on acrylate pathway microorganism Veillonella, as the dosage of pretreated AC increasing. The findings of this study demonstrated the substantial impact of the adsorption effect of acid-pretreated AC on promoting caproate production, which would aid to the development of more efficient caproate production process.

Predicting the performance of chain elongating microbiomes through flow cytometric fingerprinting

As part of the circular bio-economy paradigm shift, waste management and valorisation practices have moved away from sanitation and towards the production of added-value compounds. Recently, the development of mixed culture bioprocess for the conversion of waste(water) to platform chemicals, such as medium chain carboxylic acids, has attracted significant interest. Often, the microbiology of these novel bioprocesses is less diverse and more prone to disturbances, which can lead to process failure. This issue can be tackled by implementing an advanced monitoring strategy based on the microbiology of the process. In this study, flow cytometry was used to monitor the microbiology of lactic acid chain elongation for the production of caproic acid, and assess its performance both qualitatively and quantitatively. Two continuous stirred tank reactors for chain elongation were monitored flow cytometrically for over 336 days. Through community typing, four specific community types could be identified and correlated to both a specific functionality and genotypic diversity. Additionally, the machine-learning algorithms trained in this study demonstrated the ability to predict production rates of, amongst others, caproic acid with high accuracy in the present (R² > 0.87) and intermediate accuracy in the near future (R² > 0.63). The identification of specific community types and the development of predictive algorithms form the basis of advanced bioprocess monitoring based on flow cytometry, and have the potential to improve bioprocess control and optimization, leading to better product quality and yields.