Thermophilic caproic acid production from grass juice by sugar-based chain elongation

Medium chain carboxylic acids (MCCA) such as caproic acid have a plethora of applications, ranging from food additives to bioplastics. MCCA can be produced via microbial chain elongation using waste and side-streams as substrates, a process that can be more sustainable than conventional production routes. Most chain elongation studies have focused on mesophilic conditions, with only two recent studies hinting at the possibility of thermophilic chain elongation, but a systematic study of its mechanisms is lacking. Here, we investigated thermophilic chain elongation from grass juice, to understand the effect of key operational parameters (pH, temperature, substrate) on the process performance and to establish the key microbial genera and their role in the system. The genus Caproiciproducens was identified as responsible for thermophilic chain elongation, and caproic acid production was most favorable at pH 6.0 and 50 °C among the conditions tested, reaching an average concentration of 3.4 g/L. Batch experiments showed that the substrate for caproic acid production were glucose and xylose, while lactic acid led to the production of only butyric acid. Fed-batch experiments showed that substrate availability and the presence of caproic acid in the system play a major role in shaping the profile of thermophilic chain elongation. The increase of the total sugar concentration by glucose addition (without changing the organic load) during continuous operation led to a microbial community dominated (75 %) by Caproiciproducens and increased by 76 % the final average caproic acid concentration to 6.0 g/L (13 g_COD/L) which represented 32 % (g/g) of the total carboxylic acids. The highest concentration achieved was 7.2 g/L (day 197) which is the highest concentration reported under thermophilic conditions thus far. The results of this work pave the way to the potential development of thermophilic systems for upgrading various underexplored abundant and cheap sugar-rich side-streams to caproic acid.

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.

Model-aided targeted volatile fatty acid production from food waste using a defined co-culture microbial community

The production of volatile fatty acids (VFA) is gaining momentum due to their central role in the emerging carboxylate platform. Particularly, the production of the longest VFA (from butyrate to caproate) is desired due to their increased economic value and easier downstream processing. While the use of undefined microbial cultures is usually preferred with organic waste streams, the use of defined microbial co-culture processes could tackle some of their drawbacks such as poor control over the process outcome, which often leads to low selectivity for the desired products. However, the extensive experimentation needed to design a co-culture system hinders the use of this technology. In this work, a workflow based on the combined use of mathematical models and wet experimentation is proposed to accelerate the design of novel bioprocesses. In particular, a co-culture consisting of Pediococcus pentosaceus and Megaphaera cerevisiae is used to target the production of high-value odd- and even‑carbon VFA. An unstructured kinetic model was developed, calibrated and used to design experiments with the goal of increasing the selectivity for the desired VFA, which were experimentally validated. In the case of even‑carbon VFA, the experimental validation showed an increase of 38 % in caproate yield and, in the case of enhanced odd‑carbon VFA experiments, the yield of butyrate and caproate diminished by 62 % and 94 %, respectively, while propionate became one of the main end products and valerate yield value increased from 0.007 to 0.085 g_valearte per g_{consumed sugar.} The workflow followed in this work proved to be a sound tool for bioprocess design due to its capacity to explore and design new experiments in silico in a fast way and ability to quickly adapt to new scenarios.

The Impact of Preservation Techniques on Methane-Arrested Anaerobic Digestion of Nutrient-Rich Feedstocks

The carboxylate platform is a promising biomass-to-energy pathway that uses methane-arrested anaerobic digestion (MAAD) to convert biomass to carboxylic acids, which can be chemically converted to industrial chemicals and liquid fuels. Lignocellulose is an energy-rich carbon source, but lacks nutrients necessary for microbial growth. Chicken manure (rural waste) and sewage sludge (urban waste) are rich in nitrogen and useful macronutrients; therefore, co-digesting these wastes with lignocellulose improves MAAD performance. However, waste nutrients must be digested immediately, or preserved. This study investigated the effects of various preservation techniques - frozen (fresh), air-dried, and baked - on chicken manure and sewage sludge. Batch experiments were performed with office paper (carbon source) and chicken manure or sewage sludge (nutrient source) with different methods of preservation. Fresh substrates produced higher acid yields and biomass conversion (the amount of biomass consumed during digestion) than dried substrates. Baked chicken manure showed reduced conversion and total acid production, which suggests that oven-drying reduces digestibility. From the batch data, the Continuum Particle Distribution Model (CPDM) predicted results of a four-stage countercurrent digestion. The data are displayed on maps showing the impact of liquid residence time (LRT) and volatile solids loading rate (VSLR) on conversion and product concentration. Co-digesting office paper and wet chicken manure at a non-acid volatile solid (NAVS) concentration of 300 g/L_liq, the model predicted a high total acid concentration of 52.8 g/L and conversion of 0.89 g NAVS_digested/NAVS_fed at a volatile solid loading rate of 4 g/(L_liq·day) and liquid retention time of 35 days.

Thermodynamic approach to foresee experimental CO2 reduction to organic compounds

Anaerobic gas fermentation is a promising approach to transform carbon dioxide (CO₂) into chemical building blocks. However, the main operational conditions to enhance the process and its selectivity are still unknown. The main objective of this study was to trigger chain elongation from a joint perspective of thermodynamic and experimental assessment. Thermodynamics revealed that acetic acid formation was the most spontaneous reaction, followed by n-caproic and n-butyric acids, while the doorway for alcohols production was bounded by the selected conditions. Best parameters combinations were applied in three 0.12 L fermenters. Experimentally, n-caproic acid formation was boosted at pH 7, 37 °C, Acetate:Ethanol mass ratio of 1:3 and low H₂ partial pressure. Though these conditions did not match with those required to produce their main substrates, the unification of both perspectives yielded the highest n-caproic acid concentration (>11 g L^-1) so far from simple substrates, accounting for 77 % of the total products.

Determining global trends in syngas fermentation research through a bibliometric analysis

Syngas fermentation, in which microorganisms convert H₂, CO, and CO₂ to acids and alcohols, is a promising alternative for carbon cycling and valorization. The intellectual landscape of the topic was characterized through a bibliometric analysis using a search query (SQ) that included all relevant documents on syngas fermentation available through the Web of Science database up to December 31st, 2021. The SQ was validated with a preliminary analysis in bibliometrix and a review of titles and abstracts of all sources. Although syngas fermentation began in the early 1980s, it grew rapidly beginning in 2008, with 92.5% of total publications and 87.3% of total citations from 2008 to 2021. The field has been steadily moving from fundamentals towards applications, suggesting that the field is maturing scientifically. The greatest number of publications and citations are from the USA, and researchers in China, Germany, and Spain also are highly active. Although collaborations have increased in the past few years, author-cluster analysis shows specialized research domains with little collaboration between groups. Based on topic trends, the main challenges to be address are related to mass-transfer limitations, and researchers are starting to explore mixed cultures, genetic engineering, microbial chain elongation, and biorefineries.

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.

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

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

Microbial communities for valorizing biomass using the carboxylate platform to produce volatile fatty acids: A review

The carboxylate platform employs a diverse microbial consortium of anaerobes in which the methanogens are inhibited. Nearly all biomass components are digested to a mixture of C1-C8 monocarboxylic acids and their corresponding salts. The methane-arrested anaerobic digestion proceeds readily without needing to sterilize biomass or equipment. It accepts a wide range of feedstocks (e.g., agricultural residues, municipal solid waste, sewage sludge, animal manure, food waste, algae, and energy crops), and produces high product yields. This review highlights several important aspects of the platform, including its thermodynamic underpinnings, influences of inoculum source and operating conditions on product formation, and downstream chemical processes that convert the carboxylates to hydrocarbon fuels and oxygenated chemicals. This review further establishes the carboxylate platform as a viable and economical route to industrial biomass utilization.

Integrating the Carboxylate Platform into a Red Seaweed Biorefinery

Macroalgae are an important source of food, fertilizer, hydrocolloids, and healthful bioactive components. Macroalgae are also being considered sources of biofuels, which require minimal demands for arable land, fresh water, or fertilizers. In this study, we explored the possibility of developing a red seaweed biorefinery process to extract carrageenan while producing chemical or biofuel co-products derived from the carrageenan extraction wastes. A common approach to processing organic wastes is to generate biogas; however, in this study, we targeted a potentially higher value option by applying acidogenic digestion to convert extraction wastes to carboxylic acids and derived compounds. Using an open culture of microorganisms, wastes from a carrageenan extraction plant were converted to mixed carboxylic acids, which were then neutralized and thermally decomposed to a variety of ketones. Batch digestions of the wastes were carried out at temperatures of 35 °C and 55 °C. Either calcium carbonate or ammonium bicarbonate was used as buffer. A solid-liquid counter-current percolation fermentation was operated in four stages at 35 °C. Digestion produced carboxylic acids ranging in chain length from one to seven carbons. The mesophilic temperature gave higher carboxylic acid yield and longer chain acids, with the highest acid titer reaching 18 g L^-1. Thermal decomposition of carboxylate salts produced a mixture of ketones which contained acetone, 3-pentanone, 2-hexanone, 2-heptanone, 3-heptanone, and 4-octanone as major products. These ketones could be sold as chemicals or hydrogenated to form corresponding chain length secondary alcohols which deliver higher energy density than ethanol.

Arrested methanogenesis digestion of high-strength cheese whey and brewery wastewater with carboxylic acid production

A new anaerobic digestion process based on arrested methanogenesis (AM) was developed to treat high-strength cheese whey and brewery wastewater with simultaneous carboxylic acid production. This study specifically determined the links between wastewater characteristics, microbial community structure, and the operation of AM digesters at the bench scale. The highest total carboxylic acids concentration (78 g/L) was achieved after 15 days under batch condition at 40 °C and near-neutral pH. Lactate conversion to chain-elongated volatile fatty acid was observed. Under fed-batch conditions, the highest total acid productivity was 16 g/(L_liq·d) with substrate conversion of 0.66 g COD_digested/g COD_fed at hydraulic residence time (HRT) of 4 days. Fed-batch digestion with biomass recycling resulted in a 2-fold increase in VFAs concentration (30 g/L) and a higher diversity in the microbial consortia. Experimental results show that highly efficient, robust, and productive community structure was established for sustainable carboxylate production from widely varying high-strength wastewaters.

A hybrid dry-fermentation and membrane contactor system: Enhanced volatile fatty acid (VFA) production and recovery from organic solid wastes

Anaerobic dry-fermentation of food wastes can be utilized for the production of volatile fatty acids (VFA). However, especially for high load fermentation systems, accumulation of VFAs may result in inhibition of fermentation process. In this study, separation of VFAs from synthetic mixtures via a vapor permeation membrane contactor (VPMC) system with an air-filled polytetrafluoroethylene (PTFE) membrane was assessed at various temperatures and permeate solution concentrations. In addition, a pioneering integrated leach-bed fermentation and membrane separation system was operated with undefined mixed culture for the purpose of enhanced VFA production along with its recovery. Hybrid system resulted in 42% enhancement in total VFA production and 60% of total VFAs were recovered through the VPMC system. The results of this study revealed that integrated system can be exploited as a means of increasing organic loading to fermentation systems and increasing the value of VFA production.

Effects of preservation of rumen inoculum on volatile fatty acids production and the community dynamics during batch fermentation of fruit pomace

Rumen fluid (RF) as inocula is useful for evaluating biomass digestibility and has potential for producing volatile fatty acids (VFA) via the carboxylate platform. However, RF is not readily available, necessitating evaluation of potential preservation methods. Glycerol (50% v/v) and DMSO (5% v/v) were used to preserve rumen inocula for 3 months at -80 °C. Effects of cryo-preservation on digestibility, VFA production and community composition with β-diversity distance metrics were compared to fresh RF using apple, citrus and grape pomace as substrates. For all substrates, DMSO cryo-preserved rumen digestibility parameters, VFA yield and product distribution were more significantly comparable to fresh RF (P > 0.05) than was glycerol cryo-preserved RF. Similarly, β-diversity coefficient (unweighted unifrac) between DMSO cryo-preserved RF and fresh RF was 0.250 while the coefficient was 0.359 for the glycerol cryo-preserved RF compared to fresh RF. This showed that a DMSO cryo-preserved RF is less affected by preservation effects and is a more promising alternative to fresh RF.

Open microbiome dominated by Clostridium and Eubacterium converts methanol into i-butyrate and n-butyrate

Isobutyrate (i-butyrate) is a versatile platform chemical, whose acid form is used as a precursor of plastic and emulsifier. It can be produced microbially either using genetically engineered organisms or via microbiomes, in the latter case starting from methanol and short-chain carboxylates. This opens the opportunity to produce i-butyrate from non-sterile feedstocks. Little is known on the ecology and process conditions leading to i-butyrate production. In this study, we steered i-butyrate production in a bioreactor fed with methanol and acetate under various conditions, achieving maximum i-butyrate productivity of 5.0 mM day^-1, with a concurrent production of n-butyrate of 7.9 mM day^-1. The production of i-butyrate was reversibly inhibited by methanogenic inhibitor 2-bromoethanesulfonate. The microbial community data revealed the co-dominance of two major OTUs during co-production of i-butyrate and n-butyrate in two distinctive phases throughout a period of 54 days and 28 days, respectively. The cross-comparison of product profile with microbial community composition suggests that the relative abundance of Clostridium sp. over Eubacterium sp. is correlated with i-butyrate productivity over n-butyrate productivity.

Anaerobic bioprocessing of wastewater-derived duckweed: Maximizing product yields in a biorefinery value cascade

This study integrated the sugar and carboxylate platforms to enhance duckweed processing in biorefineries. Two or three bioprocesses (ethanol fermentation, acidogenic digestion, and methanogenic digestion) were sequentially integrated to maximize the carbon-to-carbon conversion of wastewater-derived duckweed into bioproducts, through a series of laboratory-scale experiments. Reactors were fed either raw (dried), liquid-hot-water-pretreated, or enzymatically-saccharified duckweed. Subsequently, the target bioproduct was separated from the reactor liquor and the residues further processed. The total bioproduct carbon yield of 0.69 ± 0.07 g per gram of duckweed-C was obtained by sequential acidogenic and methanogenic digestion. Three sequential bioprocesses revealed nearly as high yields (0.66 ± 0.08 g of bioproduct-C per duckweed-C), but caused more gaseous carbon (dioxide) loss. For this three-stage value cascade, yields of each process in conventional units were: 0.186 ± 0.001 g ethanol/g duckweed; 611 ± 64 mg volatile fatty acids as acetic acid/g VS; and 434 ± 0.2 ml methane/g VS.

Valorisation of the invasive species, Prosopis juliflora, using the carboxylate platform to produce volatile fatty acids

Biomass derived from low-value, high-volume invasive plant species is an attractive, alternative feedstock to produce biofuels and biochemicals. This study aimed to use the carboxylate platform to valorize the invasive leguminous shrub, Prosopis juliflora (Mesquite), by utilizing in vitro rumen fermentations without chemical pretreatment to produce volatile fatty acids. The three fractions of the mesquite: leaves (ProL), stems (ProS) and branches (ProB) were compared regarding chemical composition, neutral detergent fiber (NDF) digestibility at 7 time points and VFA production after 72 h with sugarcane bagasse (SCB) as a reference. NDF digestibility was significantly (P < 0.05) higher in ProL (35.8%) than ProS (30.4%) and ProB (20.9%) compared to SCB (21.9%). VFA concentrations from 20 g biomass L^-1 showed significant differences with 8.07, 6.71 and 6.51 g L^-1 for ProL, ProS and ProB respectively, while SCB yielded 4.02 g L^-1. These concentrations were comparable with other platforms that employ chemically pretreated biomass for VFA production.

Shifts in fermentation end products and bacterial community composition in long-term, sequentially transferred in vitro ruminal enrichment cultures fed switchgrass with and without ethanol as a co-substrate

In vitro ruminal fermentations resemble in vivo fermentations with respect to substrate consumption and distribution of fermentation products in short term (1-5 d) incubations. However, little is known regarding changes in in vitro fermentations over prolonged incubation or multiple transfers. Gas production, pH, fermentation products, and bacterial community composition were examined in duplicate in vitro fermentations of switchgrass plus distillers grains that were transferred at 3-4 d intervals over 900 d. Additionally, duplicate fermentations inoculated from 160 d-old enrichments into the same medium but supplemented with ethanol, and transferred at 3-4 d over a 730 d period were characterized. SWG and SWG + E fermentation showed marked differences in community composition, pH, total product concentrations and ratios, relative to each other and to the original inoculum. The results have implications for the use of ruminal inocula for industrial production of short- and medium-chain fatty acids via the carboxylate platform.

Volatile fatty acids impacting phototrophic growth kinetics of purple bacteria: Paving the way for protein production on fermented wastewater

Nutrient losses in our food chain severely surpass our planetary boundaries. Resource recovery can contribute to mitigation, for instance through converting wastewater resources to microbial protein for animal feed. Wastewater typically holds a complex mixture of organics, posing a challenge to selectively produce heterotrophic biomass. Ensuring the product's quality could be achieved by anaerobic generation of volatile fatty acids (VFAs) followed by photoheterotrophic production of purple non-sulfur bacteria (PNSB) with infrared light. This study aimed to determine the most suitable PNSB culture for VFA conversion and map the effect of acetate, propionate, butyrate and a VFA mixture on growth and biomass yield. Six cultures were screened in batch: (i) Rhodopseudomonas palustris, (ii) Rhodobacter sphaeroides, (iii) Rhodospirillum rubrum, (iv) a 3-species synthetic community (i+ii+iii), (v) a community enriched on VFA holding Rb. capsulatus, and (vi) Rb. capsulatus (isolate 'v'). The VFA mixture elevated growth rates with a factor 1.3-2.5 compared to individual VFA. Rb. capsulatus showed the highest growth rates: 1.8-2.2 d^-1 (enriched) and 2.3-3.8 d^-1 (isolated). In a photobioreactor (PBR) inoculated with the Rb. capsulatus enrichment, decreasing sludge retention time (SRT) yielded lower biomass concentrations, yet increased productivities, reaching 1.7 g dry weight (DW) L^-1 d^-1, the highest phototrophic rate reported thus far, and a growth rate of up to 5 d^-1. PNSB represented 26-57% of the community and the diversity index was low (3-7), with a dominance of Rhodopseudomonas at long SRT and Rhodobacter at short SRT. The biomass yield for all cultures, in batch and reactor cultivation, approached 1 g COD_Biomass g^-1 COD_Removed. An economic estimation for a two-stage approach on brewery wastewater (load 2427 kg COD d^-1) showed that 0.5 d SRT allowed for the lowest production cost (€ 10 kg^-1 DW; equal shares for capex and opex). The findings strengthen the potential for a novel two-stage approach for resource recovery from industrial wastewater, enabling high-rate PNSB production.

Effect of pH and temperature on microbial community structure and carboxylic acid yield during the acidogenic digestion of duckweed

BACKGROUND

Duckweeds (Lemnaceae) are efficient aquatic plants for wastewater treatment due to their high nutrient-uptake capabilities and resilience to severe environmental conditions. Combined with their rapid growth rates, high starch, and low lignin contents, duckweeds have also gained popularity as a biofuel feedstock for thermochemical conversion and alcohol fermentation. However, studies on the acidogenic anaerobic digestion of duckweed into carboxylic acids, another group of chemicals which are precursors of higher-value chemicals and biofuels, are lacking. In this study, a series of laboratory batch experiments were performed to determine the favorable operating conditions (i.e., temperature and pH) to maximize carboxylic acid production from wastewater-derived duckweed during acidogenic digestion. Batch reactors with 25 g/l solid loading were operated anaerobically for 21 days under mesophilic (35 °C) or thermophilic (55 °C) conditions at an acidic (5.3) or basic (9.2) pH. At the conclusion of the experiment, the dominant microbial communities under various operating conditions were assessed using high-throughput sequencing.

RESULTS

The highest duckweed-carboxylic acid conversion of 388 ± 28 mg acetic acid equivalent per gram volatile solids was observed under mesophilic and basic conditions, with an average production rate of 0.59 g/l/day. This result is comparable to those reported for acidogenic digestion of other organics such as food waste. The superior performance observed under these conditions was attributed to both chemical treatment and microbial bioconversion. Hydrogen recovery was only observed under acidic thermophilic conditions, as 23.5 ± 0.5 ml/g of duckweed volatile solids added. More than temperature, pH controlled the overall structure of the microbial communities. For instance, differentially abundant enrichments of Veillonellaceae acidaminococcus were observed in acidic samples, whereas enrichments of Clostridiaceae alkaliphilus were found in the basic samples. Acidic mesophilic conditions were found to enrich acetoclastic methanogenic populations over processing times longer than 10 days.

CONCLUSIONS

Operating conditions have a significant effect on the yield and composition of the end products resulting from acidogenic digestion of duckweed. Wastewater-derived duckweed is a technically feasible alternative feedstock for the production of advanced biofuel precursors; however, techno-economic analysis is needed to determine integrated full-scale system feasibility and economic viability.

Increasing the economic value of lignocellulosic stillage through medium-chain fatty acid production

BACKGROUND

Lignocellulosic biomass is seen as an abundant renewable source of liquid fuels and chemicals that are currently derived from petroleum. When lignocellulosic biomass is used for ethanol production, the resulting liquid residue (stillage) contains large amounts of organic material that could be further transformed into recoverable bioproducts, thus enhancing the economics of the biorefinery.

RESULTS

Here we test the hypothesis that a bacterial community could transform the organics in stillage into valuable bioproducts. We demonstrate the ability of this microbiome to convert stillage organics into medium-chain fatty acids (MCFAs), identify the predominant community members, and perform a technoeconomic analysis of recovering MCFAs as co-products of ethanol production. Steady-state operation of a stillage-fed bioreactor showed that 18% of the organic matter in stillage was converted to MCFAs. Xylose and complex carbohydrates were the primary substrates transformed. During the MCFA production period, the five major genera represented more than 95% of the community, including Lactobacillus, Roseburia, Atopobium, Olsenella, and Pseudoramibacter. To assess the potential benefits of producing MCFAs from stillage, we modeled the economics of ethanol and MCFA co-production, at MCFA productivities observed during reactor operation.

CONCLUSIONS

The analysis predicts that production of MCFAs, ethanol, and electricity could reduce the minimum ethanol selling price from $2.15 to $1.76 gal-1 ($2.68 gal-1 gasoline equivalents) when compared to a lignocellulosic biorefinery that produces only ethanol and electricity.

Recovery of mixed volatile fatty acids from anaerobically fermented organic wastes by vapor permeation membrane contactors

Volatile fatty acids (VFAs) are attractive compounds in renewable based bio-refinery industries and can be produced through anaerobic digestion of organic wastes. Nevertheless, the recovery of VFAs from anaerobically digested organic wastes is the bottleneck of the resource recovery. In this study, VFA recovery from synthetic VFA solutions and fermented organic wastes via air-filled and tertiary amine extractant-filled PTFE membranes through vapor pressure membrane contactors (VPMC) was investigated. Acetic acid was recovered with greater than 45% efficiency in all the fermented wastes. Recovery of propionic, butyric, valeric, and caproic acids through trioctylamine-filled PTFE membrane was greater than 86% and 95% from landfill leachate and fermentation broth of anaerobically digested organic waste, respectively. This study reveals that VFA separation can be effectively achieved via economic and environmental friendly VPMC system and the process is implementable as it can be coupled to a fermentation process to prevent inhibition and to recover VFAs.

Acidogenic fermentation of the organic fraction of municipal solid waste and cheese whey for bio-plastic precursors recovery - Effects of process conditions during batch tests

The problem of fossil fuels dependency is being addressed through sustainable bio-fuels and bio-products production worldwide. At the base of this bio-based economy there is the efficient use of biomass as non-virgin feedstock. Through acidogenic fermentation, organic waste can be valorised in order to obtain several precursors to be used for bio-plastic production. Some investigations have been done but there is still a lack of knowledge that must be filled before moving to effective full scale plants. Acidogenic fermentation batch tests were performed using food waste (FW) and cheese whey (CW) as substrates. Effects of nine different combinations of substrate to inoculum (S/I) ratio (2, 4, and 6) and initial pH (5, 7, and 9) were investigated for metabolites (acetate, butyrate, propionate, valerate, lactate, and ethanol) productions. Results showed that the most abundant metabolites deriving from FW fermentation were butyrate and acetate, mainly influenced by the S/I ratio (acetate and butyrate maximum productions of 21.4 and 34.5g/L, respectively, at S/I=6). Instead, when dealing with CW, lactate was the dominant metabolite significantly correlated with pH (lactate maximum production of 15.7g/L at pH = 9).

Acidogenic Digestion of Pre-pulping Extracts for Production of Fuels and Bioproducts Via Carboxylate Platform Processing

Hemicellulose extracted from wood prior to processing the wood into paper or composite materials can be a resource for the production of biofuels or bioproducts. Mixed microbial cultures are capable of converting biomass into mixed carboxylic acids, which can be purified as products or converted to biofuels or other biochemicals. Mixed cultures are robust conversion systems and do not require added enzymes to hydrolyze biomass to sugars. We produced mixed carboxylic acids using mesophilic and thermophilic fermentation of raw, unconditioned green liquor and hot water hardwood extracts, as well as baseline sugar solutions. Daily samples were taken from the fermentations and analyzed for composition, pH, and gas volume. The extract digestions were capable of hydrolyzing oligomeric hemicellulose without supplemental enzymes and converting all types of released sugars. Lactic acid was prominent in lower pH systems and acetic acid, the main product at more neutral pH. Compared to thermophilic systems, mesophilic fermentations had higher hydrolysis conversion, carbohydrate conversion, acid yields, and selectivity for C₃-C₇ acids. Carbon balances on the wood extracts closed to within ±9%. Methane production in all cases was essentially zero.

Upgrading syngas fermentation effluent using Clostridium kluyveri in a continuous fermentation

BACKGROUND

The product of current syngas fermentation systems is an ethanol/acetic acid mixture and the goal is to maximize ethanol recovery. However, ethanol currently has a relatively low market value and its separation from the fermentation broth is energy intensive. We can circumvent these disadvantages of ethanol production by converting the dilute ethanol/acetic acid mixture into products with longer carbon backbones, which are of higher value and are more easily extracted than ethanol. Chain elongation, which is the bioprocess in which ethanol is used to elongate short-chain carboxylic acids to medium-chain carboxylic acids (MCCAs), has been studied with pure cultures and open cultures of microbial consortia (microbiomes) with several different substrates. While upgrading syngas fermentation effluent has been studied with open cultures, to our knowledge, no study exists that has performed this with pure cultures.

RESULTS

Here, pure cultures of Clostridium kluyveri were used in continuous bioreactors to convert ethanol/acetic acid mixtures into MCCAs. Besides changing the operating conditions in regards to substrate loading rates and composition, the effect of in-line product extraction, pH, and the use of real syngas fermentation effluent on production rates were tested. Increasing the organic loading rates resulted in proportionally higher production rates of n-caproic acid, which were up to 40 mM day^-1 (4.64 g L^-1 day^-1) at carbon conversion efficiencies of 90% or higher. The production rates were similar for bioreactors with and without in-line product extraction. Furthermore, a lower ethanol/acetic acid ratio (3:1 instead of 10:1) enabled faster and more efficient n-caproic acid production. In addition, n-caprylic acid production was observed for the first time with C. kluyveri (up to 2.19 ± 0.34 mM in batch). Finally, the use of real effluent from syngas fermentation, without added yeast extract, but with added defined growth factors, did maintain similar production rates. Throughout the operating period, we observed that the metabolism of C. kluyveri was inhibited at a mildly acidic pH value of 5.5 compared to a pH value of 7.0, while reactor microbiomes perform successfully at mildly acidic conditions.

CONCLUSIONS

Clostridium kluyveri can be used as a biocatalyst to upgrade syngas fermentation effluent into MCCAs at pH values above 5.5.

Carbon recovery by fermentation of CO-rich off gases - Turning steel mills into biorefineries

Technological solutions to reduce greenhouse gas (GHG) emissions from anthropogenic sources are required. Heavy industrial processes, such as steel making, contribute considerably to GHG emissions. Fermentation of carbon monoxide (CO)-rich off gases with wild-type acetogenic bacteria can be used to produce ethanol, acetate, and 2,3-butanediol, thereby, reducing the carbon footprint of heavy industries. Here, the processes for the production of ethanol from CO-rich off gases are discussed and a perspective on further routes towards an integrated biorefinery at a steel mill is given. Recent achievements in genetic engineering as well as integration of other biotechnology platforms to increase the product portfolio are summarized. Already, yields have been increased and the portfolio of products broadened. To develop a commercially viable process, however, the extraction from dilute product streams is a critical step and alternatives to distillation are discussed. Finally, another critical step is waste(water) treatment with the possibility to recover resources.

Microbial synthesis of a branched-chain ester platform from organic waste carboxylates

Processing of lignocellulosic biomass or organic wastes produces a plethora of chemicals such as short, linear carboxylic acids, known as carboxylates, derived from anaerobic digestion. While these carboxylates have low values and are inhibitory to microbes during fermentation, they can be biologically upgraded to high-value products. In this study, we expanded our general framework for biological upgrading of carboxylates to branched-chain esters by using three highly active alcohol acyltransferases (AATs) for alcohol and acyl CoA condensation and modulating the alcohol moiety from ethanol to isobutanol in the modular chassis cell. With this framework, we demonstrated the production of an ester library comprised of 16 out of all 18 potential esters, including acetate, propionate, butanoate, pentanoate, and hexanoate esters, from the 5 linear, saturated C₂-C₆ carboxylic acids. Among these esters, 5 new branched-chain esters, including isobutyl acetate, isobutyl propionate, isobutyl butyrate, isobutyl pentanoate, and isobutyl hexanoate were synthesized in vivo. During 24 h in situ fermentation and extraction, one of the engineered strains, EcDL208 harnessing the SAAT of Fragaria ananassa produced ~63 mg/L of a mixture of butyl and isobutyl butyrates from glucose and butyrate co-fermentation and ~127 mg/L of a mixture of isobutyl and pentyl pentanoates from glucose and pentanoate co-fermentation, with high specificity. These butyrate and pentanoate esters are potential drop-in liquid fuels. This study provides better understanding of functional roles of AATs for microbial biosynthesis of branched-chain esters and expands the potential use of these esters as drop-in biofuels beyond their conventional flavor, fragrance, and solvent applications.

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Expand the general framework for microbial biosynthesis of branched-chain ester platforms.

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Biologically upgrade 5 carboxylates to 16 out of a total of 18 potential esters.

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Characterize in vivo three alcohol acyltransferases for branched-chain ester biosynthesis.

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Discuss targeted esters as potential fuels beyond flavor, fragrant, and solvent applications.

Conversion of L-lactate into n-caproate by a continuously fed reactor microbiome

Conversion of lactate to n-caproate had been described for the type strain Megasphaera elsdenii in batch systems. Recently, investigators have also described production of n-caproate from endogenous or exogenous lactate with batch-fed reactor microbiome systems. However, no reports exist of lactate to n-caproate conversion within a continuously fed bioreactor. Since continuously fed systems are advantageous for biotechnology production platforms, our objective was to develop such a system. Here, we demonstrated continuous lactate to n-caproate conversion for more than 165 days. The volumetric n-caproate production rate (productivity) was improved when we decreased the operating pH from 5.5 to 5.0, and was again improved when we utilized in-line product recovery via pertraction (membrane-based liquid-liquid extraction). We observed a maximum n-caproate productivity of 6.9 g COD/L-d for a period of 17 days at an L-lactate loading rate of 9.1 g COD/L-d, representing the highest sustained lactate to n-caproate conversion rate ever reported. We had to manage two competing lactate conversion pathways: 1) the reverse β-oxidation pathway to n-caproate; and 2) the acrylate pathway to propionate. We found that maintaining a low residual lactate concentration in the bioreactor broth was necessary to direct lactate conversion towards n-caproate instead of propionate. These findings provide a foundation for the development of new resource recovery processes to produce higher-value liquid products (e.g., n-caproate) from carbon-rich wastewaters containing lactate or lactate precursors (e.g., carbohydrates).

Piezo-tolerant natural gas-producing microbes under accumulating pCO2

BACKGROUND

It is known that a part of natural gas is produced by biogenic degradation of organic matter, but the microbial pathways resulting in the formation of pressurized gas fields remain unknown. Autogeneration of biogas pressure of up to 20 bar has been shown to improve the quality of biogas to the level of biogenic natural gas as the fraction of CO₂ decreased. Still, the pCO₂ is higher compared to atmospheric digestion and this may affect the process in several ways. In this work, we investigated the effect of elevated pCO₂ of up to 0.5 MPa on Gibbs free energy, microbial community composition and substrate utilization kinetics in autogenerative high-pressure digestion.

RESULTS

In this study, biogas pressure (up to 2.0 MPa) was batch-wise autogenerated for 268 days at 303 K in an 8-L bioreactor, resulting in a population dominated by archaeal Methanosaeta concilii, Methanobacterium formicicum and Mtb. beijingense and bacterial Kosmotoga-like (31% of total bacterial species), Propioniferax-like (25%) and Treponema-like (12%) species. Related microorganisms have also been detected in gas, oil and abandoned coal-bed reservoirs, where elevated pressure prevails. After 107 days autogeneration of biogas pressure up to 0.50 MPa of pCO₂, propionate accumulated whilst CH₄ formation declined. Alongside the Propioniferax-like organism, a putative propionate producer, increased in relative abundance in the period of propionate accumulation. Complementary experiments showed that specific propionate conversion rates decreased linearly from 30.3 mg g^-1 VS_added day^-1 by more than 90% to 2.2 mg g^-1 VS_added day^-1 after elevating pCO₂ from 0.10 to 0.50 MPa. Neither thermodynamic limitations, especially due to elevated pH₂, nor pH inhibition could sufficiently explain this phenomenon. The reduced propionate conversion could therefore be attributed to reversible CO₂-toxicity.

CONCLUSIONS

The results of this study suggest a generic role of the detected bacterial and archaeal species in biogenic methane formation at elevated pressure. The propionate conversion rate and subsequent methane production rate were inhibited by up to 90% by the accumulating pCO₂ up to 0.5 MPa in the pressure reactor, which opens opportunities for steering carboxylate production using reversible CO₂-toxicity in mixed-culture microbial electrosynthesis and fermentation.Graphical abstractThe role of pCO₂ in steering product formation in autogenerative high pressure digestion.

Electrolytic extraction drives volatile fatty acid chain elongation through lactic acid and replaces chemical pH control in thin stillage fermentation

BACKGROUND

Volatile fatty acids (VFA) are building blocks for the chemical industry. Sustainable, biological production is constrained by production and recovery costs, including the need for intensive pH correction. Membrane electrolysis has been developed as an in situ extraction technology tailored to the direct recovery of VFA from fermentation while stabilizing acidogenesis without caustic addition. A current applied across an anion exchange membrane reduces the fermentation broth (catholyte, water reduction: H2O + e(-) → ½ H2 + OH(-)) and drives carboxylate ions into a clean, concentrated VFA stream (anolyte, water oxidation: H2O → 2e(-) + 2 H(+) + O2).

RESULTS

In this study, we fermented thin stillage to generate a mixed VFA extract without chemical pH control. Membrane electrolysis (0.1 A, 3.22 ± 0.60 V) extracted 28 ± 6 % of carboxylates generated per day (on a carbon basis) and completely replaced caustic control of pH, with no impact on the total carboxylate production amount or rate. Hydrogen generated from the applied current shifted the fermentation outcome from predominantly C2 and C3 VFA (64 ± 3 % of the total VFA present in the control) to majority of C4 to C6 (70 ± 12 % in the experiment), with identical proportions in the VFA acid extract. A strain related to Megasphaera elsdenii (maximum abundance of 57 %), a bacteria capable of producing mid-chain VFA at a high rate, was enriched by the applied current, alongside a stable community of Lactobacillus spp. (10 %), enabling chain elongation of VFA through lactic acid. A conversion of 30 ± 5 % VFA produced per sCOD fed (60 ± 10 % of the reactive fraction) was achieved, with a 50 ± 6 % reduction in suspended solids likely by electro-coagulation.

CONCLUSIONS

VFA can be extracted directly from a fermentation broth by membrane electrolysis. The electrolytic water reduction products are utilized in the fermentation: OH(-) is used for pH control without added chemicals, and H2 is metabolized by species such as Megasphaera elsdenii to produce greater value, more reduced VFA. Electro-fermentation displays promise for generating added value chemical co-products from biorefinery sidestreams and wastes.

Production of medium-chain volatile fatty acids by mixed ruminal microorganisms is enhanced by ethanol in co-culture with Clostridium kluyveri

Mixed bacterial communities from the rumen ferment cellulosic biomass primarily to C2-C4 volatile fatty acids, and perform only limited chain extension to produce C5 (valeric) and C6 (caproic) acids. The aim of this study was to increase production of caproate and valerate in short-term in vitro incubations. Co-culture of mixed ruminal microbes with a rumen-derived strain of the bacterium Clostridium kluyveri converted cellulosic biomass (alfalfa stems or switchgrass herbage) plus ethanol to VFA mixtures that include valeric and caproic acids as the major fermentation products over a 48-72h run time. Concentrations of caproate reached 6.1gL(-1), similar to or greater than those reported in most conventional carboxylate fermentations that employ substantially longer run times.

Evaluating the performance of carboxylate platform fermentations across diverse inocula originating as sediments from extreme environments

To test the hypothesis that microbial communities from saline and thermal sediment environments are pre-adapted to exhibit superior fermentation performances, 501 saline and thermal samples were collected from a wide geographic range. Each sediment sample was screened as inoculum in a 30-day batch fermentation. Using multivariate statistics, the capacity of each community was assessed to determine its ability to degrade a cellulosic substrate and produce carboxylic acids in the context of the inoculum sediment chemistry. Conductance of soils was positively associated with production of particular acids, but negatively associated with conversion efficiency. In situ sediment temperature and conversion efficiency were consistently positively related. Because inoculum characteristics influence carboxylate platform productivity, optimization of the inoculum is an important and realistic goal.

Upgrading dilute ethanol from syngas fermentation to n-caproate with reactor microbiomes

Fermentation of syngas from renewable biomass, which is part of the syngas platform, is gaining momentum. Here, the objective was to evaluate a proof-of-concept bioprocessing system with diluted ethanol and acetic acid in actual syngas fermentation effluent as the substrate for chain elongation into the product n-caproic acid, which can be separated with less energy input than ethanol. Chain elongation is performed with open cultures of microbial populations (reactor microbiomes) as part of the carboxylate platform. The highest concentration of n-caproic acid of ~1 g L(-1) was produced at a pH of 5.44 and a production rate of 1.7 g L(-1) day(-1). A higher n-butyrate production rate of 20 g L(-1) day(-1) indicated that product toxicity was limiting the chain elongation step from n-butyric acid to n-caproic acid. This result shows that the syngas and carboxylate platforms can be integrated within a biorefinery, but that product separation is necessary.