Reproducible, high-yielding, biological caproate production from food waste using a single-phase anaerobic reactor system

Novel oxidising feed additives reduce in vitro methane emissions using the rumen simulation technique

Enteric methane (CH4) produced by ruminant livestock is a potent greenhouse gas and represents significant energy loss for the animal. The novel application of oxidising compounds as antimethanogenic agents with future potential to be included in ruminant feeds, was assessed across two separate experiments in this study. Low concentrations of oxidising agents, namely urea hydrogen peroxide (UHP) with and without potassium iodide (KI), and magnesium peroxide (MgO2), were investigated for their effects on CH4 production, total gas production (TGP), volatile fatty acid (VFA) profiles, and nutrient disappearance in vitro using the rumen simulation technique. In both experiments, the in vitro diet consisted of 50:50 grass silage:concentrate on a dry matter basis. Treatment concentrations were based on the amount of oxygen delivered and expressed in terms of fold concentration. In Experiment 1, four treatments were tested (Control, 1× UHP + KI, 1× UHP, and 0.5× UHP + KI), and six treatments were assessed in Experiment 2 (Control, 0.5× UHP + KI, 0.5× UHP, 0.25× UHP + KI, 0.25× UHP, and 0.12× MgO2). All treatments in this study had a reducing effect on CH4 parameters. A dose-dependent reduction of TGP and CH4 parameters was observed, where treatments delivering higher levels of oxygen resulted in greater CH4 suppression. 1× UHP + KI reduced TGP by 28 % (p = 0.611), CH4% by 64 % (p = 0.075) and CH4 mmol/g digestible organic matter by 71 % (p = 0.037). 0.12× MgO2 reduced CH4 volume by 25 % (p > 0.05) without affecting any other parameters. Acetate-to-propionate ratios were reduced by treatments in both experiments (p < 0.01). Molar proportions of acetate and butyrate were reduced, while propionate and valerate were increased in UHP treatments. High concentrations of UHP affected the degradation of neutral detergent fibre in the forage substrate. Future in vitro work should investigate alternative slow-release oxygen sources aimed at prolonging CH4 suppression.

Anaerobic acidification membrane bioreactor operating at acidic condition for treating concentrated municipal wastewater: Performance and implication

To address the low-carbon treatment requirements for municipal wastewater, a novel anaerobic acidification membrane bioreactor (AAMBR) was developed for recovering organic matter in terms of volatile fatty acids (VFAs). While the AAMBR successfully generated VFAs from municipal wastewater through forward osmosis (FO) membrane concentration, its operation was limited to a single pH value of 10.0. Here, performance of the AAMBR operating at acidic condition was evaluated and compared with that at alkaline condition. The findings revealed that the AAMBR with pH 5.0 efficiently transformed organic matter into acetic acid, propionic acid, and butyric acid, resulting in a VFAs yield of 0.48 g/g-CODfeed. In comparison with the AAMBR at pH 10.0, this study achieved a similar VFAs yield, a lower fouling tendency, a lower loss of nutrients and a lower controlling cost. In conclusion, this study demonstrated that a pH of 5.0 is optimal for the AAMBR treating municipal wastewater.

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.

Medium chain Fatty acids production from Food Waste via homolactic fermentation and lactate/ethanol elongation: Electron balance and thermodynamic assessment

This study explored the potential of Food Waste (FW) extract as a suitable substrate for Medium Chain Fatty Acids (MCFAs) production, in a single-phase reactor, where both fermentation and Chain Elongation (CE) processes occurred simultaneously. A continuous experiment was conducted with an Organic Loading Rate (OLR) = 20 gCOD L-1 d-1 and was fed in batch mode twice a week with pH = 6. In addition, four batch tests were performed, to assess the effects on the MCFAs production of caproate inhibition, hydrogen partial pressure (PH2) and different lactate/acetate ratios. Thermodynamics and electron flux were calculated to gain insights into the process pathways. Due to the presence of aminoacids, fermentation was mostly homolactic and both lactate and ethanol were produced as Electron Donors (EDs); the average MCFAs production efficiency was ∼ 12 %, although after 4 weeks the elongation process was halted, resulting in EDs accumulation. This occurred regardless of inoculum selection and the presence of caproate as a possible inhibitor, suggesting that EDs accumulation was due to the elongation process kinetics being slower than those of the fermentation step, thus calling for a longer Hydraulic Retention Time (HRT). It's worth noting that lactate was prevalently self-elongated to butyrate, whereas ethanol elongation only took place after lactate depletion, but was more efficient since it required other Electron Acceptors (EAs) such as butyrate, propionate or valerate. Moreover, the selected pH limited the acrylate pathway to a reasonable extent, whereas the high PH2 prevented both ethanol and lactate oxydation to acetate.

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.

Acclimation of microbial communities to low and moderate salinities in anaerobic digestion

In recent years anaerobic digestion (AD) has been investigated as suitable biotechnology to treat wastewater at elevated salinities. However, when starting up AD reactors with inocula that are not adapted to salinity, low concentrations of sodium (Na+) in the influent can already cause disintegration of microbial aggregates and wash-out. This study investigated biomass acclimation to 5 g Na+/L of two different non-adapted inocula in two lab-scale hybrid expanded granular sludge bed (EGSB)-anaerobic filter (AF) reactors fed with synthetic wastewater. After an initial biomass disintegration, new aggregates were formed relatively fast (i.e., after 95 days of operation), indicating microbial community adaptation. The newly formed microbial aggregates accumulated Na+ at the expense of calcium (Ca2+), but this did not hamper biomass retention or process performance. The hybrid reactor configuration, including a pumice stone filter in the upper section, and the low up-flow velocities applied, were key features for retaining the biomass within the system. This reactor configuration can be easily applied and represents a low-cost alternative for acclimating biomass to saline effluents, even in existing digesters. When the acclimated biomass was transferred from EGSB to an up-flow anaerobic sludge blanket (UASB) reactor configuration also fed with saline synthetic wastewater, more dense aggregates in the form of granules were obtained. The performances of the UASB inoculated with the acclimated biomass were comparable to another reactor seeded with saline-adapted granular sludge from a full-scale plant. Regardless of the inoculum origin, a defined core microbiome of Bacteria (Thermovirga, Bacteroidetes vadinHA17, Blvii28 wastewater-sludge group, Mesotoga, and Synergistaceae) and Archaea (Methanosaeta and Methanobacterium) was detected, highlighting the importance of these microbial groups in developing halotolerance and maintaining AD process stability.

How temperature shapes the biosynthesis of polyhydroxyalkanoates in mixed microbial cultures

Three sequential batch reactors were operated for the enrichment in microbial communities able to store polyhydroxyalkanoates (PHAs) using activated sludge as inoculum. They ran simultaneously under the same operational conditions (organic loading rate, hydraulic and solids retention time, cycle length, C/N ratio) just with the solely difference of the working temperature: psychrophilic (15°C), mesophilic (30°C), and thermophilic (48°C). The microbial communities enriched showed different behaviors in terms of consumption and production rates. In terms of PHA accumulation, the psychrophilic community was able to accumulate an average amount of 17.7 ± 5.7 wt% poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), the mesophilic 40.3 ± 7.0 wt% PHBV, and the thermophilic 14.8 ± 0.3 wt% PHBV in dry weight over total solids. The average PHBV production yields for each selected community were 0.41 ± 0.12 CmmolPHBV /CmmolVFA at 15°C, 0.64 ± 0.05 CmmolPHBV /CmmolVFA at 30°C, and 0.39 ± 0.14 CmmolPHBV /CmmolVFA at 48°C. The overall performance of the mesophilic reactor was better than the other two, and the copolymers obtained at this temperature contained a higher PHV fraction. The physico-chemical properties of the obtained biopolymers at each temperature were also measured, and major differences were found in the molecular weight, following an increasing trend with temperature. PRACTITIONER POINTS: PHBV molecular weight is influenced by the operational temperature increasing with it. Increasing temperatures promote the production of HB over HV. The best accumulation performance was found at 30°C for the tested operational conditions.

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.

Co-fermenting lactic acid and glucose towards caproic acid production

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

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

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

Psychrophilic and mesophilic anaerobic treatment of synthetic dairy wastewater with long chain fatty acids: Process performances and microbial community dynamics

Facilitating the anaerobic degradation of long chain fatty acids (LCFA) is the key to unlock the energy potential of lipids-rich wastewater. In this study, the feasibility of psychrophilic anaerobic treatment of LCFA-containing dairy wastewater was assessed and compared to mesophilic anaerobic treatment. The results showed that psychrophilic treatment at 15 ℃ was feasible for LCFA-containing dairy wastewater, with high removal rates of soluble COD (>90%) and LCFA (∼100%). However, efficient long-term treatment required prior acclimation of the biomass to psychrophilic temperatures. The microbial community analysis revealed that putative syntrophic fatty acid bacteria and Methanocorpusculum played a crucial role in LCFA degradation during both mesophilic and psychrophilic treatments. Additionally, a fungal-bacterial biofilm was found to be important during the psychrophilic treatment. Overall, these findings demonstrate the potential of psychrophilic anaerobic treatment for industrial wastewaters and highlight the importance of understanding the microbial communities involved in the process.

Performance and mechanisms of medium-chain fatty acid production by anaerobic fermentation of food waste without external electron donors

This study performed a long-term operation to achieve efficient medium-chain fatty acids (MCFAs) production by anaerobic fermentation of food waste without external electron donors. The results show that total MCFAs reached the highest concentration of 29,886.10 mg COD/L, and n-caproate was the primary product, reaching the current maximum concentration of 28,191.66 mg COD/L. Microbial composition analysis demonstrated Lactobacillus, Bifidobacterium, Sporanaerobacter, and Caproiciproducens constituted the core community throughout the process. Metagenomic analysis suggested that two pathways, reverse β-oxidization (RBO) and fatty acid biosynthesis (FAB), were observed, and the FAB pathway was the main CE pathway. Unclassified_f_Ruminococcaceae and Limosilactobacillus were the main participants in the FAB pathway. This study is expected to provide new insights into MCFAs production from organic waste.

Adding glucose delays the conversion of ethanol and acetic acid to caproic acid in Lacrimispora celerecrescens JSJ-1

Caproic acid is an important fatty acid with diverse applications. In this study, the biomass growth and metabolites of Lacrimispora celerecrescens JSJ-1 were investigated under different carbon sources (ethanol, starch, sucrose, and glucose), with a focus on the effect of the coexistence of glucose and ethanol on the synthesis of caproic acid. The results showed that starch, glucose, and sucrose all contributed to the biomass of L. celerecrescens JSJ-1. Under the three carbon sources, L. celerecrescens JSJ-1 produced acetic acid, butyric acid, lactic acid, ethanol, and butanol, but caproic acid was not produced. Ethanol was the optimal substrate for the production of caproic acid. When glucose and ethanol coexisted, the generation time of caproic acid was delayed compared with that in ethanol sodium acetate medium (ES medium). This was because glucose was preferentially consumed over ethanol. Lactic acid was generated as a result of glucose consumption, which led to a significant decrease in pH from 6.45 to 4.68. The low pH (< 5) inhibited the synthesis of caproic acid. Then, the strain's usage of lactic acid and the reaction between CaCO₃ and lactic acid caused the pH to increase. L. celerecrescens JSJ-1 did not start producing caproic acid using ethanol and acetic acid until the pH increased to 5.8. This research enriches the knowledge regarding the metabolism of L. celerecrescens JSJ-1 and provides guidelines for the industrial production of caproic acid by using L. celerecrescences JSJ-1. KEY POINTS: • Ethanol is the optimal substrate for the synthesis of caproic acid by Lacrimispora celerecrescens JSJ-1. • Lacrimispora celerecrescens JSJ-1 produced lactic acid rapidly when it used glucose, causing a sharp drop in pH. • pH is a crucial factor affecting the synthesis of caproic acid from ethanol by Lacrimispora celerecrescens JSJ-1.

Prediction and optimization of medium-chain carboxylic acids production from food waste using machine learning models

Machine learning models were developed in this study to predict and optimize the medium-chain carbolic acids (MCCAs) production from food waste. All three selected prediction algorithms achieved decent performance (accuracy > 0.85, R² > 0.707). Three optimization algorithms were applied for MCCA production optimization based on the prediction algorithms. The maximum MCCA production rate (0.68 g chemical oxygen demand per liter per day) was achieved by simulated annealing coupled with random forest under the optimal conditions of pH 8.3, temperature 50 °C, retention time 4 days, loading rate 15.8 g volatile solid per liter per day, and inoculum to food waste ratio 70:30 with semi-continuous mode. Further experiments validated (18 % error) that the MCCA production rate was 113 % higher than the highest production rate of current lab experiments and 60 % higher than the statistical optimization using response surface methodology. This study demonstrates the potential of using machine learning for MCCA production prediction and optimization.

Ecology of food waste chain-elongating microbiome

Microbial chain elongation has emerged as a valuable bioprocess for obtaining marketable products, such as medium chain fatty acids usable in several industrial applications, from organic waste. The understanding of the microbiology and microbial ecology in these systems is crucial to apply these microbiomes in reliable production processes controlling microbial pathways to promote favourable metabolic processes, which will in turn increase product specificity and yields. In this research, the dynamics, cooperation/competition and potentialities of bacterial communities involved in the long-term lactate-based chain elongation process from food waste extract were evaluated under different operating conditions by DNA/RNA amplicon sequencing and functional profile prediction. The feeding strategies and the applied organic loading rates strongly affected the microbial community composition. The use of food waste extract promoted the selection of primary fermenters (i.e., Olsenella, Lactobacillus) responsible for the in situ production of electron donors (i.e., lactate). The discontinuous feeding and the organic loading rate 15 gCOD L^-1 d^-1 selected the best performing microbiome in which microbes coexist and cooperate to complete the chain elongation process. Both at DNA and RNA level, this microbiome was composed by the lactate producer Olsenella, the short chain fatty acids producers Anaerostipes, Clostridium sensu stricto 7, C. sensu stricto 12, Corynebacterium, Erysipelotrichaceae UCG-004, F0332, Leuconostoc, and the chain elongator Caproiciproducens. This microbiome also showed the highest predicted abundance of short-chain acyl-CoA dehydrogenase, the functional enzyme responsible for the chain elongation process. The combined approach herein used allowed to study the microbial ecology of chain elongation process from food waste by identifying the main functional groups, establishing the presence of potential biotic interactions within the microbiomes, and predicting metabolic potentialities. This study provided pivotal indications for the selection of high-performance microbiome involved in caproate production from food waste that can serve as a basis for further improving system performance and engineering the process scale-up.

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

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

Revealing the Characteristics of Glucose- and Lactate-Based Chain Elongation for Caproate Production by Caproicibacterium lactatifermentans through Transcriptomic, Bioenergetic, and Regulatory Analyses

Caproate, an important medium-chain fatty acid, can only be synthesized by limited bacterial species by using ethanol, lactate, or certain saccharides. Caproicibacterium lactatifermentans is a promising caproate producer due to its glucose and lactate utilization capabilities. However, the global cellular responses of this bacterium to different carbon sources were not well understood. Here, C. lactatifermentans showed robust growth on glucose but more active caproate synthesis on lactate. Comparative transcriptome revealed that the genes involved in reverse β-oxidation for caproate synthesis and V-type ATPase-dependent ATP generation were upregulated under lactate condition, while several genes responsible for biomass synthesis were upregulated under glucose condition. Based on metabolic pathway reconstructions and bioenergetics analysis, the biomass accumulation on glucose condition may be supported by sufficient supplies of ATP and metabolite intermediates via glycolysis. In contrast, the ATP yield per glucose equivalent from lactate conversion into caproate was only 20% of that from glucose. Thus, the upregulation of the reverse β-oxidation genes may be essential for cell survival under lactate conditions. Furthermore, the remarkably decreased lactate utilization was observed after glucose acclimatization, indicating the negative modulation of lactate utilization by glucose metabolism. Based on the cotranscription of the lactate utilization repressor gene lldR with sugar-specific PTS genes and the opposite expression patterns of lldR and lactate utilization genes, a novel regulatory mechanism of glucose-repressed lactate utilization mediated via lldR was proposed. The results of this study suggested the molecular mechanism underlying differential physiologic and metabolic characteristics of C. lactatifermentans grown on glucose and lactate. IMPORTANCE Caproicibacterium lactatifermentans is a unique and robust caproate-producing bacterium in the family Oscillospiraceae due to its lactate utilization capability, whereas its close relatives such as Caproicibacterium amylolyticum, Caproiciproducens galactitolivorans, and Caproicibacter fermentans cannot utilize lactate but produce lactate as the main fermentation end product. Moreover, C. lactatifermentans can also utilize several saccharides such as glucose and maltose. Although the metabolic versatility of the bacterium makes it to be a promising industrial caproate producer, the cellular responses of C. lactatifermentans to different carbon sources were unknown. Here, the molecular mechanisms of biomass synthesis supported by glucose utilization and the cell survival supported by lactate utilization were revealed. A novel insight into the regulatory machinery in which glucose negatively regulates lactate utilization was proposed. This study provides a valuable basis to control and optimize caproate production, which will contribute to achieving a circular economy and environmental sustainability.

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

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

Emerging challenges for the agro-industrial food waste utilization: A review on food waste biorefinery

Modernization and industrialization has undoubtedly revolutionized the food and agro-industrial sector leading to the drastic increase in their productivity and marketing thereby accelerating the amount of agro-industrial food waste generated. In the past few decades the potential of these agro-industrial food waste to serve as bio refineries for the extraction of commercially viable products like organic acids, biochemical and biofuels was largely discussed and explored over the conventional method of disposing in landfills. The sustainable development of such strategies largely depends on understanding the techno economic challenges and planning for future strategies to overcome these hurdles. This review work presents a comprehensive outlook on the complex nature of agro-industrial food waste and pretreatment methods for their valorization into commercially viable products along with the challenges in the commercialization of food waste bio refineries that need critical attention to popularize the concept of circular bio economy.

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

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

Biorefining food waste through the anaerobic conversion of endogenous lactate into caproate: A fragile balance between microbial substrate utilization and product inhibition

New technologies development and renewable source exploitation are key tools to realize the European Green Deal and to boost the bio-based economy. In this context, fermentation of organic residues as food waste is an efficient method to obtain marketable products such as carboxylic acids widely applied in industrial production. Under favourable thermodynamic conditions, short chain fatty acids deriving from primary fermentation could be biologically converted into medium-chain fatty acids as caproate via chain elongation (CE) process, by using ethanol or lactate as electron donors. This study evaluates the effectivity of producing caproate from Food Waste extract rich in organics with in situ electron donor production. The test carried out at OLR 15 gCOD L^-1d^-1 showed high Volatile Fatty Acids (from acetic to caproic acid) yields (0.37 g g^-1COD_fed), with a maximum caproate concentration of 8 g L^-1. The associated microbiome was composed by lactate-producing bacteria (Corynebacterium, Lactobacillus, and Olsenella) and by chain elongators (Clostridiaceae and Caproiciproducens). By stressing the system with OLR increase up to 20 gCOD L^-1d^-1, the CE process was inhibited by the high concentration of caproate (low occurrence of Clostridiaceae and Caproiciproducens). Nevertheless, after few days of stop-feeding regime imposed to the system, the microbiome restored its capability to proceed with lactate-based CE pathways. Different batch tests carried out with the inhibited biomass at increasing initial caproate concentration confirmed its impact on lactate utilization kinetics.

Next-generation -omics approaches to drive carboxylate production by acidogenic fermentation of food waste: a review

Acidogenic fermentation of food waste using mixed microbial cultures can produce carboxylates [or volatile fatty acids (VFA)] as high-valued bioproducts via a complex interplay of microorganisms during different stages of this process. However, the present fermentation systems are incapable of reaching the industrially relevant VFA production yields of ≥50 g/L primarly due to the complex process operation, competitive metabolic pathways, and limited understanding of microbial interplays. Recent reports have demonstrated the significant roles played by microbial communities from different phyla, which work together to control the process kinetics of various stages underlying acidogenic fermentation. In order to fully delineate the abundance, structure, and functionality of these microbial communities, next-generation high-throughput meta-omics technologies are required. In this article, we review the potential of metagenomics and metatranscriptomics approaches to enable microbial community engineering. Specifically, a deeper analysis of taxonomic relationships, shifts in microbial communities, and differences in the genetic expression of key pathway enzymes under varying operational and environmental parameters of acidogenic fermentation could lead to the identification of species-level functionalities for both cultivable and non-cultivable microbial fractions. Furthermore, it could also be used for successful gene sequence-guided microbial isolation and consortium development for bioaugmentation to allow VFA production with high concentrations and purity. Such highly controlled and engineered microbial systems could pave the way for tailored and high-yielding VFA synthesis, thereby creating a petrochemically competitive waste-to-value chain and promoting the circular bioeconomy.Research HighlightsMixed microbial mediated acidogenic fermentation of food waste.Metagenomics and metatranscriptomics based microbial community analysis.Omics derived function-associated microbial isolation and consortium engineering.High-valued sustainable carboxylate bio-products, i.e. volatile fatty acids.

Machine learning-assisted identification of bioindicators predicts medium-chain carboxylate production performance of an anaerobic mixed culture

BACKGROUND

The ability to quantitatively predict ecophysiological functions of microbial communities provides an important step to engineer microbiota for desired functions related to specific biochemical conversions. Here, we present the quantitative prediction of medium-chain carboxylate production in two continuous anaerobic bioreactors from 16S rRNA gene dynamics in enriched communities.

RESULTS

By progressively shortening the hydraulic retention time (HRT) from 8 to 2 days with different temporal schemes in two bioreactors operated for 211 days, we achieved higher productivities and yields of the target products n-caproate and n-caprylate. The datasets generated from each bioreactor were applied independently for training and testing machine learning algorithms using 16S rRNA genes to predict n-caproate and n-caprylate productivities. Our dataset consisted of 14 and 40 samples from HRT of 8 and 2 days, respectively. Because of the size and balance of our dataset, we compared linear regression, support vector machine and random forest regression algorithms using the original and balanced datasets generated using synthetic minority oversampling. Further, we performed cross-validation to estimate model stability. The random forest regression was the best algorithm producing more consistent results with median of error rates below 8%. More than 90% accuracy in the prediction of n-caproate and n-caprylate productivities was achieved. Four inferred bioindicators belonging to the genera Olsenella, Lactobacillus, Syntrophococcus and Clostridium IV suggest their relevance to the higher carboxylate productivity at shorter HRT. The recovery of metagenome-assembled genomes of these bioindicators confirmed their genetic potential to perform key steps of medium-chain carboxylate production.

CONCLUSIONS

Shortening the hydraulic retention time of the continuous bioreactor systems allows to shape the communities with desired chain elongation functions. Using machine learning, we demonstrated that 16S rRNA amplicon sequencing data can be used to predict bioreactor process performance quantitatively and accurately. Characterizing and harnessing bioindicators holds promise to manage reactor microbiota towards selection of the target processes. Our mathematical framework is transferrable to other ecosystem processes and microbial systems where community dynamics is linked to key functions. The general methodology used here can be adapted to data types of other functional categories such as genes, transcripts, proteins or metabolites. Video Abstract.

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

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

Production of n-caproate using food waste through thermophilic fermentation without addition of external electron donors

The effectiveness of producing n-caproate from food waste without external electron donors (EDs) was investigated through batch and semi-continuous fermentation. The maximum concentration of n-caproate reached 10,226.28 mg COD/L during semi-continuous fermentation. The specificity for n-caproate was the highest at 40.19 ± 3.95%, and the soluble COD conversion rate of n-caproate reached up to 22.50 ± 1.09% at the end of batch fermentation. The production of n-caproate was coupled with the generation of lactate as an ED to facilitate chain elongation reactions. When lactate was used as the only substrate, n-butyrate (64.12 ± 20.11%) markedly dominated the products, instead of n-caproate (0.63 ± 0.07%). Microbial community analysis revealed that Caproiciproducens, Rummeliibacillus, and Clostridium_sensu_stricto_12 were the key genera related to n-caproate production. In addition to n-caproate, n-butyrate dominated the products in batch and semi-continuous fermentation with a maximum specificity of 47.59 ± 3.39%. Clostridium_sensu_stricto_7 was committed to producing n-butyrate from lactate.

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

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

First proof of concept for full-scale, direct, low-temperature anaerobic treatment of municipal wastewater

Municipal wastewater constitutes the largest fraction of wastewater, and yet treatment processes are largely removal-based. High-rate anaerobic digestion (AD) has revolutionised the sustainability of industrial wastewater treatment and could additionally provide an alternative for municipal wastewater. While AD of dilute municipal wastewater is common in tropical regions, the low temperatures of temperate climates has resulted in slow uptake. Here, we demonstrate for the first time, direct, high-rate, low-temperature AD of low-strength municipal wastewater at full-scale. An 88 m³ hybrid reactor was installed at the municipal wastewater treatment plant in Builth Wells, UK and operated for 290 days. Ambient temperatures ranged from 2 to 18 °C, but remained below 15 °C for > 100 days. Influent BOD fluctuated between 2 and 200 mg L^-1. However, BOD removal often reached > 85%. 16S rRNA amplicon sequencing of DNA from the biomass revealed a highly adaptable core microbiome. These findings could provide the basis for the next-generation of municipal wastewater treatment.

Genome-Wide Transcriptomic Analysis of n-Caproic Acid Production in Ruminococcaceae Bacterium CPB6 with Lactate Supplementation

n-Caproic acid (CA) is gaining increased attention due to its high value as a chemical feedstock. Ruminococcaceae bacterium strain CPB6 is an anaerobic mesophilic bacterium that is highly prolific in its ability to perform chain elongation of lactate to CA. However, little is known about the genome-wide transcriptional analysis of strain CPB6 for CA production triggered by the supplementation of exogenous lactate. In this study, cultivation of strain CPB6 was carried out in the absence and presence of lactate. Transcriptional profiles were analyzed using RNA-seq, and differentially expressed genes (DEGs) between the lactate-supplemented cells and control cells without lactate were analyzed. The results showed that lactate supplementation led to earlier CA p,roduction, and higher final CA titer and productivity. 295 genes were substrate and/or growth dependent, and these genes cover crucial functional categories. Specifically, 5 genes responsible for the reverse β-oxidation pathway, 11 genes encoding ATP-binding cassette (ABC) transporters, 6 genes encoding substrate-binding protein (SBP), and 4 genes encoding phosphotransferase system (PTS) transporters were strikingly upregulated in response to the addition of lactate. These genes would be candidates for future studies aiming at understanding the regulatory mechanism of lactate conversion into CA, as well as for the improvement of CA production in strain CPB6. The findings presented herein reveal unique insights into the biomolecular effect of lactate on CA production at the transcriptional level.

Influence of initial pH on the production of volatile fatty acids and hydrogen during dark fermentation of kitchen waste

The purpose of this work was to determine the effect of initial pH on the production of volatile fatty acids (VFA) and hydrogen (H₂) in the dark fermentation processes of kitchen waste. The study was conducted in batch bioreactors of working volume 1 L for different initial pH in the range from 5.5 to 9.0. The dark fermentation processes were carried out for 4 days at 37°C. Initial organic load of the kitchen waste in all bioreactors amounted to 25.5 gVS/L. Buffering of pH during the fermentation process was carried out with the use of ammonia contained mainly in digested sludge. The optimal conditions for the production of VFA and H₂ were achieved at the initial pH of 8. Production of VFA and H₂ in these conditions was, respectively, 13.9 g/L and 72.4 mL/gVS. The main produced components of VFA were acetic and butyric acids. The production of ethanol and lactic acid was at very low levels due to the high ratio of the volatile fatty acids to total organic content of 0.86. With the optimal initial pH of 8 the yield of CO₂ production was 0.30 gC/gC. High initial pH value (above 8) extended the lag phase duration in the course of H₂ production. The dominant groups of micro-organisms at the most favourable initial pH of 8 for the production of VFA and H₂ were Bacteroidetes, Firmicutes, Spirochaetes and Waste Water of Evry 1 (WWE1) at the phylum level.

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

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

The survival of blown pack spoilage associated Clostridium estertheticum and Clostridium gasigenes spores during the ensiling of grass

Blown pack spoilage (BPS) of vacuum packaged primals, caused by Clostridium estertheticum and Clostridium gasigenes, is a serious issue for the beef industry. There are multiple sources of these bacteria on beef farms, including grass and associated feed preparations. The aim of this study was to investigate the survival of C. estertheticum and C. gasigenes spores during the ensiling of grass and the subsequent opening of the silos. Grass, harvested from fields, with and without cattle slurry amendment, was inoculated with approximately 100 spores/g and ensiled using a laboratory (silo) model system at 20°C in the dark. Adding formic acid or sucrose resulted in six treatment combination as follows: no slurry (NS), no slurry plus formic acid (NSFA), no slurry plus sucrose (NSS), slurry (S), slurry plus formic acid (SFA) and slurry plus sucrose (SS). During the silage fermentation, samples were removed periodically and tested for C. estertheticum, C. gasigenes, total viable, Escherichia coli, Enterobacteriaceae and lactic acid bacteria (LAB) counts. The pH, ethanol, volatile fatty acids (VFA), lactic acid and ammonia concentrations were also monitored throughout the experiment. C. estertheticum did not survive the ensiling process, regardless of treatment. In contrast, C. gasigenes grew in the early stages and was detected during the entirety of the fermentation for all treatments. Based on these observations, it was concluded that the silage fermentation process described would not remove C. gasigenes and contaminated grass may result in contaminated feed for animals.

Adaptability of a caproate-producing bacterium contributes to its dominance in an anaerobic fermentation system

The transformation of diverse feedstocks into medium-chain fatty acids (MCFAs) by mixed cultures is a promising biorefinery route because of the high value of MCFAs. A particular concern is how to maintain the microbial consortia in mixed cultures to achieve stable MCFA production. Chinese strong aroma-type liquor (Baijiu) fermentation system continually produces caproic acid for decades through a spontaneous inoculation of anaerobes from pit mud into fermented grains. Therefore, illuminating the dominant caproate-producing bacterium (CPB) in pit mud and how the CPB sustains in the spontaneous fermentation system will benefit to reveal the microbiological mechanisms of the stable caproate production. Here, we examined pit mud samples across four Chinese strong aroma-type Baijiu producing areas and found that a caproate-producing Caproicibacterium sp. was widely distributed in these distilleries with relative abundance ranging from 1.4% to 35.5% and an average abundance of 11.4%. Through controlling carbon source availability, we achieved different simplified caproate-producing consortia and found that the growth advantage of Caproicibacterium sp. was highly dependent on glucose. Then two strains, named Caproicibacterium sp. LBM19010 and Caproicibacterium sp. JNU-WLY1368, were isolated from pit mud of two regions. The metabolic versatility of this bacterium utilizing starch, maltose, glucose and lactate reflected its adaptability to the fermentation environment where these carbon sources coexist. The simultaneous utilization of glucose and lactate contributed to the balance between cell growth and pH homeostasis. This study reveals that multiple adaptation strategies employed by the predominant CPB promotes its stability and dominance in a saccharide- and lactate-rich anaerobic habitat. IMPORTANCE Chinese strong aroma-type liquor (Baijiu) fermentation environment is a typical medium-chain fatty acid producing system with complex nutrients. Although several studies have revealed the correlation between microbial community composition and abiotic factors, the adaptation mechanisms of dominant species to abiotic environment are still unknown in this special anaerobic habitat. This study identified the predominant CPB in Chinese strong aroma-type Baijiu fermentation system. Metabolic versatility and flexibility of the dominant CPB with a small-size genome indicated that this bacterium can effectively exploit available carbon and nitrogen sources, which could be a key factor to promote its ecological success in a multi-species environment. The understanding of growth and metabolic features of CPB responsible for its dominance in microbial community will not only contribute to the improvement of Chinese strong aroma-type Baijiu production but also expand its potential industrial applications in caproate production.

Anaerobic digestion of organic waste: Recovery of value-added and inhibitory compounds from liquid fraction of digestate

Anaerobic digestion, as an eco-friendly waste treatment technology, is facing the problem of low stability and low product value. Harvesting value-added products beyond methane and removing the inhibitory compounds will unleash new vitality of anaerobic digestion, which need to be achieved by selective separation of certain compounds. Various methods are reviewed in this study for separating valuable products (volatile fatty acids, medium-chain carboxylic acids, lactic acid) and inhibitory substance (ammonia) from the liquid fraction of digestate, including their performance, applicability, corresponding limitations and roadmaps for improvement. In-situ extraction that allows simultaneous production and extraction is seen as promising approach which carries good potential to overcome the barriers for continuous production. The prospects and challenges of the future development are further analyzed based on in-situ extraction and economics.

nZVI Impacts Substrate Conversion and Microbiome Composition in Chain Elongation From D- and L-Lactate Substrates

Medium-chain carboxylates (MCC) derived from biomass biorefining are attractive biochemicals to uncouple the production of a wide array of products from the use of non-renewable sources. Biological conversion of biomass-derived lactate during secondary fermentation can be steered to produce a variety of MCC through chain elongation. We explored the effects of zero-valent iron nanoparticles (nZVI) and lactate enantiomers on substrate consumption, product formation and microbiome composition in batch lactate-based chain elongation. In abiotic tests, nZVI supported chemical hydrolysis of lactate oligomers present in concentrated lactic acid. In fermentation experiments, nZVI created favorable conditions for either chain-elongating or propionate-producing microbiomes in a dose-dependent manner. Improved lactate conversion rates and n-caproate production were promoted at 0.5-2 g nZVI⋅L^-1 while propionate formation became relevant at ≥ 3.5 g nZVI⋅L^-1. Even-chain carboxylates (n-butyrate) were produced when using enantiopure and racemic lactate with lactate conversion rates increased in nZVI presence (1 g⋅L^-1). Consumption of hydrogen and carbon dioxide was observed late in the incubations and correlated with acetate formation or substrate conversion to elongated products in the presence of nZVI. Lactate racemization was observed during chain elongation while isomerization to D-lactate was detected during propionate formation. Clostridium luticellarii, Caproiciproducens, and Ruminococcaceae related species were associated with n-valerate and n-caproate production while propionate was likely produced through the acrylate pathway by Clostridium novyi. The enrichment of different potential n-butyrate producers (Clostridium tyrobutyricum, Lachnospiraceae, Oscillibacter, Sedimentibacter) was affected by nZVI presence and concentrations. Possible theories and mechanisms underlying the effects of nZVI on substrate conversion and microbiome composition are discussed. An outlook is provided to integrate (bio)electrochemical systems to recycle (n)ZVI and provide an alternative reducing power agent as durable control method.

Effect of high concentration of ammonium on production of n-caproate: Recovery of a high-value biochemical from food waste via lactate-driven chain elongation

Ammonium accumulation is inevitable during the fermentation of food waste (FW), challenging the application of chain elongation process upgrading FW into the high-value biochemical n-caproate, which is a medium chain carboxylate. This study is the first to investigate ammonium inhibition of lactate-driven chain elongation process. The short-term exposure of a Clostridium IV-dominated chain elongating reactor microbiome at an ammonium concentration of 1-4 g L^-1 linearly decreased n-caproate production by 25-80%. High levels of ammonium (≥5 g L^-1) could cause failure of chain elongation, shifting the product from n-caproate to propionate. The involved mechanisms revealed that ammonium reshaped the microbial community from Clostridium IV domination to Clostridium IV and Propionibacterium co-domination (based on 16S rRNA sequencing) and reduced the activities of key enzymes involved in the reversed β-oxidization pathway. We propose an effective strategy from our study, which is the first one to do in our knowledge, to upgrade raw FW without dilution to n-caproate: lowering the ammonium accumulation to 1.0 g L^-1 at the setup phase for adaptation and prolonging the hydraulic retention time (10 days) during the operation phase for the colonization of chain-elongation bacteria. These findings lay a foundation for the implementation of the LCE process on FW, providing an alternative way to alleviate the global FW crisis.

Microbial Ecological Mechanism for Long-Term Production of High Concentrations of n-Caproate via Lactate-Driven Chain Elongation

Lactate-driven chain elongation (LCE) has emerged as a new biotechnology to upgrade organic waste streams into a valuable biochemical and fuel precursor, medium-chain carboxylate, n-caproate. Considering that a low cost of downstream extraction is critical for biorefinery technology, a high concentration of n-caproate production is very important to improve the scale-up of the LCE process. We report here that in a nonsterile open environment, the n-caproate concentration was increased from the previous record of 25.7 g·liter^-1 to a new high level of 33.7 g·liter^-1 (76.8 g chemical oxygen demand [COD]·liter ^{- 1}), with the highest production rate being 11.5 g·liter^-1·day^-1 (26.2 g COD·liter ^{- 1}·day^-1). In addition, the LCE process remained stable, with an average concentration of n-caproate production of 20.2 ± 5.62 g·liter^-1 (46.1 ± 12.8 g COD·liter ^{- 1}) for 780 days. Dynamic changes in taxonomic composition integrated with metagenomic data reveal the microbial ecology for long-term production of high concentrations of n-caproate: (i) the core microbiome is related to efficient functional groups, such as Ruminococcaceae (with functional strain CPB6); (ii) the core bacteria can maintain stability for long-term operation; (iii) the microbial network has relatively low microbe-microbe interaction strength; and (iv) low relative abundance and variety of competitors. The network structure could be shaped by hydraulic retention time (HRT) over time, and long-term operation at an HRT of 8 days displayed higher efficacy.IMPORTANCE Our research revealed the microbial network of the LCE reactor microbiome for n-caproate production at high concentrations, which will provide a foundation for designing or engineering the LCE reactor microbiome to recover n-caproate from organic waste streams in the future. In addition, the hypothetical model of the reactor microbiome that we proposed may offer guidance for researchers to find the underlying microbial mechanism when they encounter low-efficiency n-caproate production from the LCE process. We anticipate that our research will rapidly advance LCE biotechnology with the goal of promoting the sustainable development of human society.

Novel strategy for enhancing acetic and formic acids generation in acidogenesis of anaerobic digestion via targeted adjusting environmental niches

Optimization of acetic acid and formic acid production efficient methanogenesis is always the research hot spot in anaerobic digestion. It is a promising approach to adjust the operation parameters to influence the functional microorganisms for better acetic acid and formic acid production in acidogenesis. Herein, the effects of pH, oxidation-reduction potential (ORP) and carbon-nitrogen (C/N) ratio were determined in batch experiments to probe acetic and formic acids production, and were further verified in continuous stirred tank reactor (CSTR). The results revealed that the content of volatile fatty acids (VFAs) reached to maximum at pH 6.0 or ORP -350 mV, while the production of acetic and formic acids was the highest at pH 7.0 or ORP -450 mV in 9 h fermentation. Also, fermentation products dominated by acetic and formic acids were adjusted in the CSTR under the operating conditions of pH 7.0 and ORP -450 mV. Microbiological analysis from batch test showed that fermentation at pH value of 7.0 enriched the diversity of microorganism, and provided a niche for microbes (Petrimonas, norank_f__Synergistaceae, vadinBC27_wastewater-sludge_group, and Trichococcus) to produce acetic and formic acids. Correspondingly, 78.70% of the carbon was converted to acetic and formic acids in pH 7.0. This study provides a promising strategy for the targeted regulation of acetic and formic acids production in acidogenesis of anaerobic digestion.

Hydrogen as a Co-electron Donor for Chain Elongation With Complex Communities

Electron donor scarcity is seen as one of the major issues limiting economic production of medium-chain carboxylates from waste streams. Previous studies suggest that co-fermentation of hydrogen in microbial communities that realize chain elongation relieves this limitation. To better understand how hydrogen co-feeding can support chain elongation, we enriched three different microbial communities from anaerobic reactors (A, B, and C with ascending levels of diversity) for their ability to produce medium-chain carboxylates from conventional electron donors (lactate or ethanol) or from hydrogen. In the presence of abundant acetate and CO₂, the effects of different abiotic parameters (pH values in acidic to neutral range, initial acetate concentration, and presence of chemical methanogenesis inhibitors) were tested along with the enrichment. The presence of hydrogen facilitated production of butyrate by all communities and improved production of i-butyrate and caproate by the two most diverse communities (B and C), accompanied by consumption of acetate, hydrogen, and lactate/ethanol (when available). Under optimal conditions, hydrogen increased the selectivity of conventional electron donors to caproate from 0.23 ± 0.01 mol e^-/mol e^- to 0.67 ± 0.15 mol e^-/mol e^- with a peak caproate concentration of 4.0 g L^-1. As a trade-off, the best-performing communities also showed hydrogenotrophic methanogenesis activity by Methanobacterium even at high concentrations of undissociated acetic acid of 2.9 g L^-1 and at low pH of 4.8. According to 16S rRNA amplicon sequencing, the suspected caproate producers were assigned to the family Anaerovoracaceae (Peptostreptococcales) and the genera Megasphaera (99.8% similarity to M. elsdenii), Caproiciproducens, and Clostridium sensu stricto 12 (97-100% similarity to C. luticellarii). Non-methanogenic hydrogen consumption correlated to the abundance of Clostridium sensu stricto 12 taxa (p < 0.01). If a robust methanogenesis inhibition strategy can be found, hydrogen co-feeding along with conventional electron donors can greatly improve selectivity to caproate in complex communities. The lessons learned can help design continuous hydrogen-aided chain elongation bioprocesses.

Lactate Metabolism and Microbiome Composition Are Affected by Nitrogen Gas Supply in Continuous Lactate-Based Chain Elongation

Chain elongation reactor microbiomes produce valuable medium-chain carboxylates (MCC) from non-sterile residual substrates where lactate is a relevant intermediate. Gas supply has been shown to impact chain elongation performance. In the present study, the effect of nitrogen gas (N2) supply on lactate metabolism, conversion rates, biomass growth, and microbiome composition was evaluated in a lactate-fed upflow anaerobic reactor with continuous or intermittent N2 gas supply. Successful MCC production was achieved with continuous N2 gas supply at low superficial gas velocities (SGV) of 0.22 m∙h−1. Supplying N2 at high SGV (>2 m∙h−1) either continuously (2.2 m∙h−1) or intermittently (3.6 m∙h−1) disrupted chain elongation, resulting in production of short-chain carboxylates (SCC), i.e., acetate, propionate, and n-butyrate. Caproiciproducens-dominated chain-elongating microbiomes enriched at low SGV were washed out at high SGV where Clostridium tyrobutyricum-dominated microbiomes thrived, by displaying higher lactate consumption rates. Suspended growth seemed to be dominant regardless of SGV and gas supply regime applied with no measurable sludge bed formed. The highest MCC production from lactate of 10 g COD∙L−1∙d−1 with electron selectivities of 72 ± 5%was obtained without N2 gas supply at a hydraulic retention time (HRT) of 1 day. The addition of 5 g∙L−1 of propionate did not inhibit chain elongation, but rather boosted lactate conversion rates towards MCC with n-heptylate reaching 1.8 g COD∙L−1∙d−1. N2 gas supply can be used for mixing purposes and to steer lactate metabolism to MCC or SCC production.

Direct Conversion of Food Waste Extract into Caproate: Metagenomics Assessment of Chain Elongation Process

In a circular economy strategy, waste resources can be used for the biological production of high added-value substances, such as medium chain fatty acids (MCFAs), thus minimising waste and favouring a sustainable process. This study investigates single-stage fermentation processes for the production of MCFAs in a semi-continuous reactor treating the extract of real food waste (FW), without the addition of external electron donors. Two sequential acidogenic fermentation tests were carried out at an organic loading rate (OLR) of 5 and 15 gCOD L⁻¹d⁻¹ with a hydraulic retention time of 4 days and pH controlled at 6 ± 0.2. The highest level of caproate (4.8 g L⁻¹) was observed at OLR of 15 gCOD L⁻¹d⁻¹ with a microbiome mainly composed by lactate-producing Actinomyces, Atopobium, and Olsenella species and caproate-producing Pseudoramibacter. Metagenomic analysis revealed the presence of key enzymes for the production of lactate, such as lactate dehydrogenase and pyruvate ferredoxin oxidoreductase, as well as several enzymes involved in the reverse β-oxidation pathway, thus suggesting the occurrence of a lactate-based chain elongation process.

Potential of medium chain fatty acids production from municipal solid waste leachate: Effect of age and external electron donors

A large quantity of leachate is generated during municipal solid waste collection operation and in landfills due to the large amount of organic waste and high humidity. The content of medium chain fatty acids (MCFAs) in the leachate is a low cost feedstock for bio-based chemical and fuel production processes. The aim of this study is to investigate the MCFA production potential of three leachate ages through chain elongation process under uncontrolled pH batch test. Moreover, the effect of using different external electron donors (ethanol, methanol and a mixture of both) is studied. The experiment consists of characterizing the samples then adding external electron donors with a specific ratio to leachate samples under mesophilic temperature. For this investigation, also a statistical analysis is done, which shows the production of MCFAs is highly influenced by leachate age. The results indicate that the production of even-numbered acids increase from 600 to 1,000 mg/L by the end of the ethanol chain elongation experiment for young leachate. However, a higher MCFA production of more than 1,000 mg/L is achieved by using the mixture of methanol and ethanol as electron donor. Furthermore, all methanol chain elongation experiments lead to an odd-numbered production of MCFAs, such as pentanoic and heptanoic acids. These results confirm the potential improvement of MCFA production from leachate through choosing the optimal leachate age and electron donor. Overall, producing MCFAs from leachate is a good example of circular bio-economy because waste is used to produce biochemicals, which closes the material cycle.

Chain elongators, friends, and foes

Bioproduction of medium chain carboxylic acids has recently emerged as an alternative strategy to valorize low-value organic waste and side-streams. Key to this route is chain elongation, an anaerobic microbial process driven by ethanol, lactic acid, or carbohydrates. Because these technologies use wastes as feedstocks, mixed microbial communities are often considered as biocatalysts. Understanding and steering these microbiomes is key to optimize bioprocess performance. From a meta-analysis of publicly available sequencing data, we (i) explore how the current collection of isolated chain elongators compares to microbiome members, (ii) discuss the main beneficial and antagonistic interactions with community partners, and (iii) identify the key research gaps and needs to help understand chain elongation microbiomes, and design/steer these novel bioproduction processes.

Continuous medium chain carboxylic acids production from excess sludge by granular chain-elongation process

Short chain carboxylic acids (SCCAs) production is one of the primary ways to recycle excess sludge (ES). However, the high cost for the SCCAs separation/extraction due to its complete miscibility in water hinders the practical application of SCCAs and the popularization of this recycling way. To overcome this barrier, this study performed an emerging chain elongation (CE) technology to upgrade the SCCAs-rich sludge fermentation broth into the highly hydrophobic medium chain carboxylic acids (MCCAs). In a continuous expanded granule sludge bed (EGSB) reactor, a maximal MCCAs yield of 67.39 % and the corresponding concentration of 9.80 g COD/L (224.97 mM C/L) were achieved. By supplying CO₂ at a loading rate of 2 [Formula: see text] to lower the hydrogen partial pressure, the ethanol utilization rate and the resulting MCCAs yield were further improved. In addition, three branched-MCCAs including iso-caproate, iso-heptylate, and iso-caprylate were obtained the first time from waste biomass with the average proportions of 6.17 %, 3.65 %, and 0.8 %, respectively. The branched-MCCAs came from the CE of branched-SCCAs. The granule sludges performing CE were mainly consisted of rod-shaped cells, and dominated by Clostridium sensu stricto and Clostridium IV. This study is expected to lay a foundation for recycling ES to MCCAs.

Propionate Production by Bioelectrochemically-Assisted Lactate Fermentation and Simultaneous CO2 Recycling

Production of volatile fatty acids (VFAs), fundamental building blocks for the chemical industry, depends on fossil fuels but organic waste is an emerging alternative substrate. Lactate produced from sugar-containing waste streams can be further processed to VFAs. In this study, electrofermentation (EF) in a two-chamber cell is proposed to enhance propionate production via lactate fermentation. At an initial pH of 5, an applied potential of −1 V vs. Ag/AgCl favored propionate production over butyrate from 20 mM lactate (with respect to non-electrochemical control incubations), due to the pH buffering effect of the cathode electrode, with production rates up to 5.9 mM d^–1 (0.44 g L^–1 d^–1). Microbial community analysis confirmed the enrichment of propionate-producing microorganisms, such as Tyzzerella sp. and Propionibacterium sp. Organisms commonly found in microbial electrosynthesis reactors, such as Desulfovibrio sp. and Acetobacterium sp., were also abundant at the cathode, indicating their involvement in recycling CO₂ produced by lactate fermentation into acetate, as confirmed by stoichiometric calculations. Propionate was the main product of lactate fermentation at substrate concentrations up to 150 mM, with a highest production rate of 12.9 mM d^–1 (0.96 g L^–1 d^–1) and a yield of 0.48 mol mol^–1 lactate consumed. Furthermore, as high as 81% of the lactate consumed (in terms of carbon) was recovered as soluble product, highlighting the potential for EF application with high-carbon waste streams, such as cheese whey or other food wastes. In summary, EF can be applied to control lactate fermentation toward propionate production and to recycle the resulting CO₂ into acetate, increasing the VFA yield and avoiding carbon emissions and addition of chemicals for pH control.

Effect of ultrasonic pretreatment on chain elongation of saccharified residue from food waste by anaerobic fermentation

Converting biowaste into value-added products has raised the researchers' interests. In this study, bioconversion was applied to produce chain acids from food waste by anaerobic fermentation. To improve the caproic acid production, different pretreatments (i.e., ultrasonic, hydrothermal, and alkaline-thermal) were used for investigating their effects on the acidogenic production and microbial communities. The results showed that ultrasonic and hydrothermal pretreatments (207.8 and 210.1 mg COD/g VS, respectively) were very efficient for enhancing the caproic acid production, compared to the alkaline-thermal pretreated samples and control samples (72.6 and 97.5 mg COD/g VS, respectively). The ultrasonic pretreatment was beneficial for reducing volatile fatty acids (VFAs) during the caproic acid production, resulting in converting more lactic acid to caproic acid by adding the hydrothermal pretreatment. The microbial community analysis showed that the acidogenic bacteria Caproiciproducens dominated the fermentation in this bioconversion process of food waste into chain acids. The Caproiciproducens mainly degraded the proteins and carbohydrates from the saccharified residues of food waste to produce caproic acids through chain elongation procedure. The investigation and optimized method may help develop the bioconversion technology for producing VFAs products from food wastes.

Pre-treatment and temperature effects on the use of slow release electron donor for biological sulfate reduction

Lignocellulosic materials can be used as slow release electron donor (SRED) for biological sulfate reduction, potentially enhancing the subsequent metal sulfide precipitation. Lignocellulosic materials require a pre-treatment step in other biotechnological applications, but pre-treatment strategies for its use as a SRED for biological sulfate reduction have not yet been tested. Three pre-treatments strategies (mechanical, acid, and mechanical followed by acid pre-treatment) were tested to enhance electron donor release from brewery spent grain (BSG), and compared to a non-pre-treated control. Mechanical pre-treatment provided the highest sulfate removal rate (82.8 ± 8.8 mg SO₄^2-.(g TVS.day)^-1), as well as the highest final sulfide concentration (441.0 ± 34.4 mg.L^-1) at mesophilic conditions. BSG submitted to mechanical pre-treatment was also assessed under psychrophilic and thermophilic conditions. Under mesophilic and psychrophilic conditions, both sulfate reduction and methane production occurred. Under psychrophilic conditions, the sulfate reduction rate was lower (25 ± 2.0 mg SO₄^2-.(g TVS.day)^-1), and the sulfide formation depended on lactate addition. A metal precipitation assay was conducted to assess whether the use of SRED enhances metal recovery. Zinc precipitation and recovery with chemical or biogenic sulfide from the BSG batches were tested. Sulfide was provided in a single spike or slowly added, mimicking the effect of SRED. ZnS was formed in all conditions, but better settling particles were obtained when sulfide was slowly added, regardless of the sulfide source.

Fermentation of Organic Residues to Beneficial Chemicals: A Review of Medium-Chain Fatty Acid Production

Medium-chain fatty acids (MCFAs) have a variety of uses in the production of industrial chemicals, food, and personal care products. These compounds are often produced through palm refining, but recent work has demonstrated that MCFAs can also be produced through the fermentation of complex organic substrates, including organic waste streams. While “chain elongation” offers a renewable platform for producing MCFAs, there are several limitations that need to be addressed before full-scale implementation becomes widespread. Here, we review the history of work on MCFA production by both pure and mixed cultures of fermenting organisms, and the unique metabolic features that lead to MCFA production. We also offer approaches to address the remaining challenges and increase MCFA production from renewable feedstocks.

Adjusting Organic Load as a Strategy to Direct Single-Stage Food Waste Fermentation from Anaerobic Digestion to Chain Elongation

Production of medium chain carboxylic acids (MCCA) as renewable feedstock bio-chemicals, from food waste (FW), requires complicated reactor configurations and supplementation of chemicals to achieve product selectivity. This study evaluated the manipulation of organic loading rate in an un-supplemented, single stage stirred tank reactor to steer an anaerobic digestion (AD) microbiome towards acidogenic fermentation (AF), and thence to chain elongation. Increasing substrate availability by switching to a FW feedstock with a higher COD stimulated chain elongation. The MCCA species n-caproic (10.1 ± 1.7 g L−1) and n-caprylic (2.9 ± 0.8 g L−1) acid were produced at concentrations comparable to more complex reactor set-ups. As a result, of the adjusted operating strategy, a more specialised microbiome developed containing several MCCA-producing bacteria, lactic acid-producing Olsenella spp. and hydrogenotrophic methanogens. By contrast, in an AD reactor that was operated in parallel to produce biogas, the retention times had to be doubled when fed with the high-COD FW to maintain biogas production. The AD microbiome comprised a diverse mixture of hydrolytic and acidogenic bacteria, and acetoclastic methanogens. The results suggest that manipulation of organic loading rate and food-to-microorganism ratio may be used as an operating strategy to direct an AD microbiome towards AF, and to stimulate chain elongation in FW fermentation, using a simple, un-supplemented stirred tank set-up. This outcome provides the opportunity to repurpose existing AD assets operating on food waste for biogas production, to produce potentially higher value MCCA products, via simple manipulation of the feeding strategy.