Metabolism of homocetogens

Metagenomic Exploration Uncovers Several Novel 'Candidatus' Species Involved in Acetate Metabolism in High-Ammonia Thermophilic Biogas Processes

Biogas reactors operating at elevated ammonia levels are commonly susceptible to process disturbances, further augmented at thermophilic temperatures. The major cause is assumed to be linked to inhibition followed by an imbalance between different functional microbial groups, centred around the last two steps of the anaerobic digestion, involving acetogens, syntrophic acetate oxidisers (SAOB) and methanogens. Acetogens are key contributors to reactor efficiency, acting as the crucial link between the hydrolysis and fermentation steps and the final methanogenesis step. Their major product is acetate, at high ammonia levels further converted by SAOB and hydrogenotrophic methanogens to biogas. Even though these functionally different processes are well recognised, less is known about the responsible organism at elevated temperature and ammonia conditions. The main aim of this study was to garner insights into the penultimate stages in three thermophilic reactors (52°C) operated under high ammonia levels (FAN 0.7-1.0 g/L; TAN 3.6-4.4 g/L). The primary objective was to identify potential acetogens and SAOBs. Metagenomic data from the three reactors were analysed for the reductive acetyl-CoA pathway (Wood-Ljungdahl Pathway) and glycine synthase reductase pathway. The results revealed a lack of true acetogens but uncovered three potential SAOB candidates that harbour the WLP, 'Candidatus Thermodarwinisyntropha acetovorans', 'Candidatus Thermosyntrophaceticus schinkii', 'Candidatus Thermotepidanaerobacter aceticum', and a potential lipid-degrader 'Candidatus Thermosyntrophomonas ammoiaca'.

Carbon monoxide inhibition on acidogenic glucose fermentation and aceticlastic methanogenesis

Syngas and CO-rich off-gases are key chemical platforms to produce biofuels and bioproducts. From the perspective of optimizing and up-scaling CO co-digestion with organic waste streams, this study aims at assessing and quantifying the inhibitory effects of CO on acidogenic glucose fermentation and aceticlastic methanogenesis. Mesophilic cultures were fed in two sets of batch assays, respectively, with glucose and acetate while being exposed to dissolved CO in equilibrium with partial pressures in the range of 0.25-1.00 atm. Cumulative methane production and microbial monitoring revealed that aceticlastic methanogenic archaea were significantly inhibited (2-20 % of the methane production of CO non-exposed cultures). The acidogenic glucose degrading community was also inhibited by CO, although, thanks to its functional redundancy, shifted its metabolism towards propionate production. Future work should assess the sensitivity of hereby estimated CO inhibition parameters, e.g., on the simulation output of a continuous syngas co-digestion process with organic substrates.

Anaerobic digestion of fruit and vegetable waste: a critical review of associated challenges

The depletion of fossil fuels coupled with stringent environmental laws has encouraged us to develop sustainable renewable energy. Due to its numerous benefits, anaerobic digestion (AD) has emerged as an environment-friendly technology. Biogas generated during AD is primarily a mixture of CH₄ (65-70%) and CO₂ (20-25%) and a potent energy source that can combat the energy crisis in today's world. Here, an attempt has been made to provide a broad understanding of AD and delineate the effect of various operational parameters influencing AD. The characteristics of fruit and vegetable waste (FVW) and its feasibility as a potent substrate for AD have been studied. This review also covers traditional challenges in managing FVW via AD, the implementation of various bioreactor systems to manage large amounts of organic waste and their operational boundaries, microbial consortia involved in each phase of digestion, and various strategies to increase biogas production.

Carbon credit reduction: A techno-economic analysis of "drop-in" fuel production

The current study elucidates the fundamentals of technical, financial, and environmental viability of the processes used for sustainable "drop-in" fuel generation. At present, the price of producing "drop-in" fuels is around two times as costly (5-6 USD/gallon) as the cost of fossil fuels (3 USD/gallon), especially when using second-generation feedstocks. Hence, this necessitates a comprehensive techno-economic understanding of the current technologies with respect to "drop-in"-fuel. This entitles technical-economic viability, and environmental sustainability to make the processes involved commercially viable. In this context, the present review addresses unique contrasts among the various processes involved in "drop-in" fuel production. Furthermore, principles and process flow of techno-economic analysis as well as environmental implications in terms of reduced carbon footprint and carbon credit are elucidated to discuss fundamentals of techno-economic analysis in terms of capital and operational expenditure, revenue, simulation, cash flow analysis, mass and energy balances with respect to evidence-based practices. Case specific techno-economic studies with current developments in this field of research with emphasis on software tools viz., Aspen Plus, Aspen HYSIS, Aspen Plus Economic Analyser (APEC) Aspen Icarus Process Evaluator (AIPE) are also highlighted. The study also emphasis on the carbon foot print of biofuels and its carbon credits (Carbon Offset Credits (COCs) and Carbon Reduction Credits (CRCs)) by leveraging a deep technical and robust business-oriented insights about the techno-economic analysis (TEA) exclusively for the biofuel production.

Co-cultivation is a powerful approach to produce a robust functionally designed synthetic consortium as a live biotherapeutic product (LBP)

The success of fecal microbiota transplants (FMT) has provided the necessary proof-of-concept for microbiome therapeutics. Yet, feces-based therapies have many associated risks and uncertainties, and hence defined microbial consortia that modify the microbiome in a targeted manner have emerged as a promising safer alternative to FMT. The development of such live biotherapeutic products has important challenges, including the selection of appropriate strains and the controlled production of the consortia at scale. Here, we report on an ecology- and biotechnology-based approach to microbial consortium construction that overcomes these issues. We selected nine strains that form a consortium to emulate the central metabolic pathways of carbohydrate fermentation in the healthy human gut microbiota. Continuous co-culturing of the bacteria produces a stable and reproducible consortium whose growth and metabolic activity are distinct from an equivalent mix of individually cultured strains. Further, we showed that our function-based consortium is as effective as FMT in counteracting dysbiosis in a dextran sodium sulfate mouse model of acute colitis, while an equivalent mix of strains failed to match FMT. Finally, we showed robustness and general applicability of our approach by designing and producing additional stable consortia of controlled composition. We propose that combining a bottom-up functional design with continuous co-cultivation is a powerful strategy to produce robust functionally designed synthetic consortia for therapeutic use.

Organic waste recycling for carbon smart circular bioeconomy and sustainable development: A review

The development of sustainable and low carbon impact processes for a suitable management of waste and by-products coming from different factors of the industrial value chain like agricultural, forestry and food processing industries. Implementing this will helps to avoid the negative environmental impact and global warming. The application of the circular bioeconomy (CB) and the circular economic models have been shown to be a great opportunity for facing the waste and by-products issues by bringing sustainable processing systems which allow to the value chains be more responsible and resilient. In addition, biorefinery approach coupled to CB context could offer different solution and insights to conquer the current challenges related to decrease the fossil fuel dependency as well as increase efficiency of resource recovery and processing cost of the industrial residues. It is worth to remark the important role that the biotechnological processes such as fermentative, digestive and enzymatic conversions play for an effective waste management and carbon neutrality.

Roadmap from Microbial Communities to Individuality Modeling for Anaerobic Digestion of Sewage Sludge

Biological models describing anaerobic digestion (AD) of sewage sludge have been widely applied to test various control and operation strategies. Anaerobic digestion model 1 (ADM1) provides a generic platform that includes the main processes of AD, excluding homoacetogenesis and the microbial structure. Homoacetogenic bacteria have been identified as important competitors for hydrogen consumption and acetate production. Although recent advances in meta-omics techniques have improved our characterization of AD microbial communities, conventional models treat functional groups as homogeneous and overlook the physiology and behavior of microbial individuality, limiting insights into mechanisms governing process performance. A novel microbial individuality model (MIM) that integrates kinetics, energetics, and agent-based modeling to describe a microbiome's behavior as heterogenic populations, including homoacetogenesis, was developed. The MIM was validated with two datasets from previous studies through daily biogas production, methane content, compound concentrations, and microbial relative abundance changes. The MIM identified the emergence of Methanosaeta at low concentrations of acetate. Moreover, this simulation supports experimental studies confirming that the overlooked homoacetogenesis is an important hydrogen sink in AD. Validated MIMs are expected to provide insights into syntrophic and competitive interactions among microbiomes in AD systems while testing different operational parameters in a virtual environment. The MIM offers a methodological framework to other biological treatment systems and their microbial community dynamics.

Microbiological insights into anaerobic digestion for biogas, hydrogen or volatile fatty acids (VFAs): a review

In the past decades, considerable attention has been directed toward anaerobic digestion (AD), which is an effective biological process for converting diverse organic wastes into biogas, volatile fatty acids (VFAs), biohydrogen, etc. The microbial bioprocessing takes part during AD is of substantial significance, and one of the crucial approaches for the deep and adequate understanding and manipulating it toward different products is process microbiology. Due to highly complexity of AD microbiome, it is critically important to study the involved microorganisms in AD. In recent years, in addition to traditional methods, novel molecular techniques and meta-omics approaches have been developed which provide accurate details about microbial communities involved AD. Better understanding of process microbiomes could guide us in identifying and controlling various factors in both improving the AD process and diverting metabolic pathway toward production of selective bio-products. This review covers various platforms of AD process that results in different final products from microbiological point of view. The review also highlights distinctive interactions occurring among microbial communities. Furthermore, assessment of these communities existing in the anaerobic digesters is discussed to provide more insights into their structure, dynamics, and metabolic pathways. Moreover, the important factors affecting microbial communities in each platform of AD are highlighted. Finally, the review provides some recent applications of AD for the production of novel bio-products and deals with challenges and future perspectives of AD.

Effect of chewing betel nut on the gut microbiota of Hainanese

Betel nut chewing (BNC) is prevalent in South Asia and Southeast Asia. BNC can affect host health by modulating the gut microbiota. The aim of this study is to evaluate the effect of BNC on the gut microbiota of the host. Feces samples were obtained from 34 BNC individuals from Ledong and Lingshui, Hainan, China. The microbiota was analyzed by 16S rRNA gene sequencing. BNC decreased the microbial α-diversity. Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria were the predominant phyla, accounting for 99.35% of the BNC group. The Firmicutes-to-Bacteroidetes ratio was significantly increased in the BNC group compared to a control group. The abundances of the families Aerococcaceae, Neisseriaceae, Moraxellaceae, Porphyromonadaceae, and Planococcaceae were decreased in the BNC/BNC_Male/BNC_Female groups compared to the control group, whereas the abundances of Coriobacteriaceae, Streptococcaceae, Micrococcaceae, Xanthomonadaceae, Coxiellaceae, Nocardioidaceae, Rhodobacteraceae, and Succinivibrionaceae were increased. In general, the gut microbiome profiles suggest that BNC may have positive effects, such as an increase in the abundance of beneficial microbes and a reduction in the abundance of disease-related microbes. However, BNC may also produce an increase in the abundance of disease-related microbes. Therefore, extraction of prebiotic components could increase the beneficial value of betel nut.

Growth of the acetogenic bacterium Acetobacterium woodii on glycerol and dihydroxyacetone

More than 2 million tons of glycerol are produced during industrial processes each year and, therefore, glycerol is an inexpensive feedstock to produce biocommodities by bacterial fermentation. Acetogenic bacteria are interesting production platforms and there have been few reports in the literature on glycerol utilization by this ecophysiologically important group of strictly anaerobic bacteria. Here, we show that the model acetogen Acetobacterium woodii DSM1030 is able to grow on glycerol, but contrary to expectations, only for 2-3 transfers. Transcriptome analysis revealed the expression of the pdu operon encoding a propanediol dehydratase along with genes encoding bacterial microcompartments. Deletion of pduAB led to a stable growth of A. woodii on glycerol, consistent with the hypothesis that the propanediol dehydratase also acts on glycerol leading to a toxic end-product. Glycerol is oxidized to acetate and the reducing equivalents are reoxidized by reducing CO₂ in the Wood-Ljungdahl pathway, leading to an additional acetate. The possible oxidation product of glycerol, dihydroxyacetone (DHA), also served as carbon and energy source for A. woodii and growth was stably maintained on that compound. DHA oxidation was also coupled to CO₂ reduction. Based on transcriptome data and enzymatic analysis we present the first metabolic and bioenergetic schemes for glycerol and DHA utilization in A. woodii.

Deep insights into the network of acetate metabolism in anaerobic digestion: focusing on syntrophic acetate oxidation and homoacetogenesis

Acetate is a pivotal intermediate product during anaerobic decomposition of organic matter. Its generation and consumption network is quite complex, which almost covers the most steps in anaerobic digestion (AD) process. Besides acidogenesis, acetogenesis and methanogenesis, syntrophic acetate oxidation (SAO) replaced acetoclastic methanogenesis to release the inhibition of AD at some special conditions, and the importance of considering homoacetogenesis had also been proved when analysing anaerobic fermentations. Syntrophic acetate-oxidizing bacteria (SAOB), with function of SAO, can survive under high temperature and ammonia/ volatile fatty acids (VFAs) concentrations, while, homoacetogens, performed homoacetogenesis, are more active under acidic, alkaline and low temperature (10°C-20°C) conditions, This review summarized the roles of SAO and homoacetogenesis in AD process, which contains the biochemical reactions, metabolism pathways, physiological characteristics and energy conservation of functional bacteria. The specific roles of these two processes in the subprocess of AD (i.e., acidogenesis, acetogenesis and methanogenesis) were also analyzed in detail. A two phases anaerobic digester is proposed for protein-rich waste(water) treatment by enhancing the functions of homoacetogens and SAOB compared to the traditional two-phases anaerobic digesters, in which the first phase is fermentation phase including acidogens and homoacetogens for acetate production, and second phase is a mixed culture coupling syntrophic fatty acids bacteria, SAOB and hydrogenotrophic methanogens for methane production. This review provides a new insight into the network on production and consumption of acetate in AD process.

Innovations in anaerobic digestion: a model-based study

BACKGROUND

Increasing the efficiency of the biogas production process is possible by modifying the technological installations of the biogas plant. In this study, specific solutions based on a mathematical model that lead to favorable results were proposed. Three configurations were considered: classical anaerobic digestion (AD) and its two modifications, two-phase AD (TPAD) and autogenerative high-pressure digestion (AHPD). The model has been validated based on measurements from a biogas plant located in Poland. Afterward, the TPAD and AHPD concepts were numerically tested for the same volume and feeding conditions.

RESULTS

The TPAD system increased the overall biogas production from 9.06 to 9.59%, depending on the feedstock composition, while the content of methane was slightly lower in the whole production chain. On the other hand, the AHPD provided the best purity of the produced fuel, in which a methane content value of 82.13% was reached. At the same time, the overpressure leads to a decrease of around 7.5% in the volumetric production efficiency. The study indicated that the dilution of maize silage with pig manure, instead of water, can have significant benefits in the selected configurations. The content of pig slurry strengthens the impact of the selected process modifications-in the first case, by increasing the production efficiency, and in the second, by improving the methane content in the biogas.

CONCLUSIONS

The proposed mathematical model of the AD process proved to be a valuable tool for the description and design of biogas plant. The analysis shows that the overall impact of the presented process modifications is mutually opposite. The feedstock composition has a moderate and unsteady impact on the production profile, in the tested modifications. The dilution with pig manure, instead of water, leads to a slightly better efficiency in the classical configuration. For the TPAD process, the trend is very similar, but the AHPD biogas plant indicates a reverse tendency. Overall, the recommendation from this article is to use the AHPD concept if the composition of the biogas is the most important. In the case in which the performance is the most important factor, it is favorable to use the TPAD configuration.

Formate-Dependent Acetogenic Utilization of Glucose by the Fecal Acetogen Clostridium bovifaecis

Acetogenic bacteria are a diverse group of anaerobes that use the reductive acetyl coenzyme A (acetyl-CoA) (Wood-Ljungdahl) pathway for CO₂ fixation and energy conservation. The conversion of 2 mol CO₂ into acetyl-CoA by using the Wood-Ljungdahl pathway as the terminal electron accepting process is the most prominent metabolic feature for these microorganisms. However, here, we describe that the fecal acetogen Clostridium bovifaecis strain BXX displayed poor metabolic capabilities of autotrophic acetogenesis, and acetogenic utilization of glucose occurred only with the supplementation of formate. Genome analysis of Clostridium bovifaecis revealed that it contains almost the complete genes of the Wood-Ljungdahl pathway but lacks the gene encoding formate dehydrogenase, which catalyzes the reduction of CO₂ to formate as the first step of the methyl branch of the Wood-Ljungdahl pathway. The lack of a gene encoding formate dehydrogenase was verified by PCR, reverse transcription-PCR analysis, enzyme activity assay, and its formate-dependent acetogenic utilization of glucose on DNA, RNA, protein, and phenotype level, respectively. The lack of a formate dehydrogenase gene may be associated with the adaption to a formate-rich intestinal environment, considering the isolating source of strain BXX. The formate-dependent acetogenic growth of Clostridium bovifaecis provides insight into a unique metabolic feature of fecal acetogens.IMPORTANCE The acetyl-CoA pathway is an ancient pathway of CO₂ fixation, which converts 2 mol of CO₂ into acetyl-CoA. Autotrophic growth with H₂ and CO₂ via the acetyl-CoA pathway as the terminal electron accepting process is the most unique feature of acetogenic bacteria. However, the fecal acetogen Clostridium bovifaecis strain BXX displayed poor metabolic capabilities of autotrophic acetogenesis, and acetogenic utilization of glucose occurred only with the supplementation of formate. The formate-dependent acetogenic growth of Clostridium bovifaecis was associated with its lack of a gene encoding formate dehydrogenase, which may result from adaption to a formate-rich intestinal environment. This study gave insight into a unique metabolic feature of fecal acetogens. Because of the requirement of formate for the acetogenic growth of certain acetogens, the ecological impact of acetogens could be more complex and important in the formate-rich environment due to their trophic interactions with other microbes.

Revealing formate production from carbon monoxide in wild type and mutants of Rnf- and Ech-containing acetogens, Acetobacterium woodii and Thermoanaerobacter kivui

Acetogenic bacteria have gained much attraction in recent years as they can produce different biofuels and biochemicals from H2 plus CO2 or even CO alone, therefore opening a promising alternative route for the production of biofuels from renewable sources compared to existing sugar-based routes. However, CO metabolism still raises questions concerning the biochemistry and bioenergetics in many acetogens. In this study, we focused on the two acetogenic bacteria Acetobacterium woodii and Thermoanaerobacter kivui which, so far, are the only identified acetogens harbouring a H2 -dependent CO2 reductase and furthermore belong to different classes of 'Rnf'- and 'Ech-acetogens'. Both strains catalysed the conversion of CO into the bulk chemical acetate and formate. Formate production was stimulated by uncoupling the energy metabolism from the Wood-Ljungdahl pathway, and specific rates of 1.44 and 1.34 mmol g-1  h-1 for A. woodii ∆rnf and T. kivui wild type were reached. The demonstrated CO-based formate production rates are, to the best of our knowledge, among the highest rates ever reported. Using mutants of ∆hdcr, ∆cooS, ∆hydBA, ∆rnf and ∆ech2 with deficiencies in key enzyme activities of the central metabolism enabled us to postulate two different CO utilization pathways in these two model organisms.

Homoacetogenic Conversion of Mannitol by the Thermophilic Acetogenic Bacterium Thermoanaerobacter kivui Requires External CO2

Acetogenic microorganisms utilize organic substrates such as sugars in addition to hydrogen (H₂) + carbon dioxide (CO₂). Recently, we reported that the thermophilic acetogenic microorganism Thermoanaerobacter kivui is among the few acetogens that utilize the sugar alcohol mannitol, dependent on a gene cluster encoding mannitol uptake, phosphorylation and oxidation of mannitol-1-phosphate to fructose-6-phosphate. Here, we studied mannitol metabolism with resting cells of T. kivui; and found that mannitol was "fermented" in a homoacetogenic manner, i.e., acetate was the sole product if HCO₃ ^- was present. We found an acetate:mannitol ratio higher than 3, indicating the requirement of external CO₂, and the involvement of the WLP as terminal electron accepting pathway. In the absence of CO₂ (or bicarbonate, HCO₃ ^-), however, the cells still converted mannitol to acetate, but slowly and with stoichiometric amounts of H₂ formed in addition, resulting in a "mixed" fermentation. This showed that-in addition to the WLP-the cells used an additional electron sink-protons, making up for the "missing" CO₂ as electron sink. Growth was 2.5-fold slower in the absence of external CO₂, while the addition of formate completely restored the growth rate. A model for mannitol metabolism is presented, involving the major three hydrogenases, to explain how [H] make their way from glycolysis into the products acetate or acetate + H₂.

Microbial Resource Management for Ex Situ Biomethanation of Hydrogen at Alkaline pH

Biomethanation is a promising solution to convert H₂ (produced from surplus electricity) and CO₂ to CH₄ by using hydrogenotrophic methanogens. In ex situ biomethanation with mixed cultures, homoacetogens and methanogens compete for H₂/CO₂. We enriched a hydrogenotrophic microbiota on CO₂ and H₂ as sole carbon and energy sources, respectively, to investigate these competing reactions. The microbial community structure and dynamics of bacteria and methanogenic archaea were evaluated through 16S rRNA and mcrA gene amplicon sequencing, respectively. Hydrogenotrophic methanogens and homoacetogens were enriched, as acetate was concomitantly produced alongside CH₄. By controlling the media composition, especially changing the reducing agent, the formation of acetate was lowered and grid quality CH₄ (≥97%) was obtained. Formate was identified as an intermediate that was produced and consumed during the bioprocess. Stirring intensities ≥ 1000 rpm were detrimental, probably due to shear force stress. The predominating methanogens belonged to the genera Methanobacterium and Methanoculleus. The bacterial community was dominated by Lutispora. The methanogenic community was stable, whereas the bacterial community was more dynamic. Our results suggest that hydrogenotrophic communities can be steered towards the selective production of CH₄ from H₂/CO₂ by adapting the media composition, the reducing agent and the stirring intensity.

Utilizing bioaugmentation to improve performance of a two-phase AnMBR treating sewage sludge

Bioaugmentation in the acid-phase of a two-phase anaerobic membrane bioreactor (AnMBR) treating primary sludge was investigated as a means for targeting and improving hydrolysis and acetogenesis. Bioaugmentation was carried out using a proprietary bioculture blend containing a mixture of hydrolytic, acidogenic, and acetogenic microorganisms. This mixture was added on its own and in combination with recycled anaerobic sludge from the methane-phase of the AnMBR. Both bioaugmentation strategies had a positive effect on overall hydrolysis (25-38%), and acid-phase acetic acid generation (31-52%) compared to operation without bioaugmentation. This led to subsequent increases in average methane production (10-13%), and greater average solids reduction (25-55%). Microbial community analysis using 16S Illumina MiSeq generated sequences revealed increased relative abundance of Acetobacter and Syntrophomonas species in bioaugmented communities, suggesting these to be key players in improvements in process performance. However, in general the relative abundance of bioaugmented microorganisms within bioaugmented communities was relatively low, highlighting the need to optimize the bioculture composition and dosage. Overall, bioaugmentation was found to benefit the conversion of primary sludge to methane, when initial solubility was relatively low. Future work should optimize the bioculture composition and dosing strategy to improve its effectiveness and long-term stability, and minimize associated operating costs.

Formate Is Required for Growth of the Thermophilic Acetogenic Bacterium Thermoanaerobacter kivui Lacking Hydrogen-Dependent Carbon Dioxide Reductase (HDCR)

The hydrogen-dependent carbon dioxide reductase is a soluble enzyme complex that directly utilizes hydrogen (H₂) for the reduction of carbon dioxide (CO₂) to formate in the first step of the acetyl-coenzyme A- or Wood-Ljungdahl pathway (WLP). HDCR consists of 2 catalytic subunits, a hydrogenase and a formate dehydrogenase (FDH) and two small subunits carrying iron-sulfur clusters. The enzyme complex has been purified and characterized from two acetogenic bacteria, from the mesophile Acetobacterium woodii and, recently, from the thermophile Thermoanaerobacter kivui. Physiological studies toward the importance of the HDCR for growth and formate metabolism in acetogens have not been carried out yet, due to the lack of genetic tools. Here, we deleted the genes encoding HDCR in T. kivui taking advantage of the recently developed genetic system. As expected, the deletion mutant (strain TKV_MB013) did not grow with formate as single substrate or under autotrophic conditions with H₂ + CO₂. Surprisingly, the strain did also not grow on any other substrate (sugars, mannitol or pyruvate), except for when formate was added. Concentrated cell suspensions quickly consumed formate in the presence of glucose only. In conclusion, HDCR provides formate which was essential for growth of the T. kivui mutant. Alternatively, extracellularly added formate served as terminal electron acceptor in addition to CO₂, complementing the growth deficiency. The results show a tight coupling of multi-carbon substrate oxidation to the WLP. The metabolism in the mutant can be viewed as a coupled formate + CO₂ respiration, which may be an ancient metabolic trait.

Acetogenic Fermentation From Oxygen Containing Waste Gas

The microbial production of bulk chemicals from waste gas is becoming a pertinent alternative to industrial strategies that rely on fossil fuels as substrate. Acetogens can use waste gas substrates or syngas (CO, CO₂, H₂) to produce chemicals, such as acetate or ethanol, but as the feed gas often contains oxygen, which inhibits acetogen growth and product formation, a cost-prohibitive chemical oxygen removal step is necessary. Here, we have developed a two-phase microbial system to facilitate acetate production using a gas mixture containing CO and O₂. In the first phase the facultative anaerobic carboxydotroph Parageobacillus thermoglucosidasius was used to consume residual O₂ and produce H₂ and CO₂, which was subsequently utilized by the acetogen Clostridium ljungdahlii for the production of acetate. From a starting amount of 3.3 mmol of CO, 0.52 mmol acetate was produced in the second phase by C. ljungdahlii. In this set-up, the yield achieved was 0.16 mol acetate/mol CO, a 63% of the theoretical maximum. This system has the potential to be developed for the production of a broad range of bulk chemicals from oxygen-containing waste gas by using P. thermoglucosidasius as an oxygen scrubbing tool.

Enrichment of Marinobacter sp. and Halophilic Homoacetogens at the Biocathode of Microbial Electrosynthesis System Inoculated With Red Sea Brine Pool

Homoacetogens are efficient CO₂ fixing bacteria using H₂ as electron donor to produce acetate. These organisms can be enriched at the biocathode of microbial electrosynthesis (MES) for electricity-driven CO₂ reduction to acetate. Studies exploring homoacetogens in MES are mainly conducted using pure or mix-culture anaerobic inocula from samples with standard environmental conditions. Extreme marine environments host unique microbial communities including homoacetogens that may have unique capabilities due to their adaptation to harsh environmental conditions. Anaerobic deep-sea brine pools are hypersaline and metalliferous environments and homoacetogens can be expected to live in these environments due to their remarkable metabolic flexibility and energy-efficient biosynthesis. However, brine pools have never been explored as inocula for the enrichment of homacetogens in MES. Here we used the saline water from a Red Sea brine pool as inoculum for the enrichment of halophilic homoacetogens at the biocathode (-1 V vs. Ag/AgCl) of MES. Volatile fatty acids, especially acetate, along with hydrogen gas were produced in MES systems operated at 25 and 10% salinity. Acetate concentration increased when MES was operated at a lower salinity ∼3.5%, representing typical seawater salinity. Amplicon sequencing and genome-centric metagenomics of matured cathodic biofilm showed dominance of the genus Marinobacter and phylum Firmicutes at all tested salinities. Seventeen high-quality draft metagenome-assembled genomes (MAGs) were extracted from the biocathode samples. The recovered MAGs accounted for 87 ± 4% of the quality filtered sequence reads. Genome analysis of the MAGs suggested CO₂ fixation via Wood-Ljundahl pathway by members of the phylum Firmicutes and the fixed CO₂ was possibly utilized by Marinobacter sp. for growth by consuming O₂ escaping from the anode to the cathode for respiration. The enrichment of Marinobacter sp. with homoacetogens was only possible because of the specific cathodic environment in MES. These findings suggest that in organic carbon-limited saline environments, Marinobacter spp. can live in consortia with CO₂ fixing bacteria such as homoacetogens, which can provide them with fixed carbon as a source of carbon and energy.

Anaerobic Dechlorination by a Humin-Dependent Pentachlorophenol-Dechlorinating Consortium under Autotrophic Conditions Induced by Homoacetogenesis

Anoxic aquifers suffer from energy limitations due to the unavailability of organic substrates, as dictated by hydrogen (H2) for various electron-accepting processes. This deficiency often results in the accumulation of persistent organic pollutants, where bioremediation using organic compounds often leads to secondary contamination. This study involves the reductive dechlorination of pentachlorophenol (PCP) by dechlorinators that do not use H2 directly, but rather through a reduced state of humin-a solid-phase humic substance-as the extracellular electron donor, which requires an organic donor such as formate, lactate, etc. This shortcoming was addressed by the development of an anaerobic mixed culture that was capable of reductively dechlorinating PCP using humin under autotrophic conditions induced by homoacetogenesis. Here, H2 was used for carbon-dioxide fixation to acetate; the acetate produced was used for the reduction of humin; and consequently used for dechlorination through reduced humin. The 16SrRNA gene sequencing analysis showed Dehalobacter and Dehalobacterium as the possible dechlorinators, while Clostridium and Oxobacter were identified as the homoacetogens. Thus, this work contributes to the development of an anaerobic consortium that balanced H2 dependency, where efficiency of humin reduction extends the applicability of anaerobic microbial remediation in aquifers through autotrophy, syntrophy, and reductive dechlorination.

Redox potential changes during ATP-dependent corrinoid reduction determined by redox titrations with europium(II)-DTPA

Corrinoids are essential cofactors of enzymes involved in the C₁ metabolism of anaerobes. The active, super-reduced [Co^I ] state of the corrinoid cofactor is highly sensitive to autoxidation. In O-demethylases, the oxidation to inactive [Co^II ] is reversed by an ATP-dependent electron transfer catalyzed by the activating enzyme (AE). The redox potential changes of the corrinoid cofactor, which occur during this reaction, were studied by potentiometric titration coupled to UV/visible spectroscopy. By applying europium(II)-diethylenetriaminepentaacetic acid (DTPA) as a reductant, we were able to determine the midpoint potential of the [Co^II ]/[Co^I ] couple of the protein-bound corrinoid cofactor in the absence and presence of AE and/or ATP. The data revealed that the transfer of electrons from a physiological donor to the corrinoid as the electron-accepting site is achieved by increasing the potential of the corrinoid cofactor from -530 ± 15 mV to -250 ± 10 mV (E_SHE , pH 7.5). The first 50 to 100 mV of the shift of the redox potential seem to be caused by the interaction of nucleotide-bound AE with the corrinoid protein or its cofactor. The remaining 150-200 mV had to be overcome by the chemical energy of ATP hydrolysis. The experiments revealed that Eu(II)-DTPA, which was already known as a powerful reducing agent, is a suitable electron donor for titration experiments of low-potential redox centers. Furthermore, the results of this study will contribute to the understanding of thermodynamically unfavorable electron transfer processes driven by the power of ATP hydrolysis.

Improvement of continuous hydrogen production using individual and binary enzymatic hydrolysates of agave bagasse in suspended-culture and biofilm reactors

Continuous hydrogen (H₂) production from individual (Stonezyme, IH) and binary (Celluclast-Viscozyme, BH) enzymatic hydrolysates of agave bagasse was evaluated in continuous stirred-tank reactors (CSTR) and trickling bed reactors (TBR). The volumetric H₂ production rates (VHPR) in CSTR were 13 and 2.25 L H₂/L-d with BH and IH, respectively. Meanwhile, VHPR of 5.76 and 2.0 L H₂/L-d were obtained in the TBR configuration using BH and IH, respectively. Differences on VHPR between reactors could be explained by substrate availability, which is intrinsic to the growth mode of each reactor configuration; while differences of VHPR between hydrolysates were possibly related to the composition of enzymatic hydrolysates. Furthermore, homoacetogenesis was strongly influenced by H₂ and substrate transfer conditions. Considering VHPR, H₂ yields, and costs of hydrolysis, hydrogen production from binary hydrolysates of agave bagasse was identified as the most promising alternative evaluated with scale-up potential for the production of energy biofuels.

Reductive metabolism of the important atmospheric gas isoprene by homoacetogens

Isoprene is the most abundant biogenic volatile organic compound (BVOC) in the Earth's atmosphere and plays important roles in atmospheric chemistry. Despite this, little is known about microbiological processes serving as a terrestrial sink for isoprene. While aerobic isoprene degrading bacteria have been identified, there are no known anaerobic, isoprene-metabolizing organisms. In this study an H₂-consuming homoacetogenic enrichment was shown to utilize 1.6 μmoles isoprene h^-1 as an electron acceptor in addition to HCO₃^-. The isoprene-reducing community was dominated by Acetobacterium spp. and isoprene was shown to be stoichiometrically reduced to three methylbutene isomers (2-methyl-1-butene (>97%), 3-methyl-1-butene (≤2%), 2-methyl-2-butene (≤1%). In the presence of isoprene, 40% less acetate was formed suggesting that isoprene reduction is coupled to energy conservation in Acetobacterium spp. This study improves our understanding of linkages and feedbacks between biogeochemistry and terrestrial microbial activity.

Alanine, a Novel Growth Substrate for the Acetogenic Bacterium Acetobacterium woodii

Acetogenic bacteria are an ecophysiologically important group of strictly anaerobic bacteria that grow lithotrophically on H2 plus CO2 or on CO or heterotrophically on different substrates such as sugars, alcohols, aldehydes, or acids. Amino acids are rarely used. Here, we describe that the model acetogen Acetobacterium woodii can use alanine as the sole carbon and energy source, which is in contrast to the description of the type strain. The alanine degradation genes have been identified and characterized. A key to alanine degradation is an alanine dehydrogenase which has been characterized biochemically. The resulting pyruvate is further degraded to acetate by the known pathways involving the Wood-Ljungdahl pathway. Our studies culminate in a metabolic and bioenergetic scheme for alanine-dependent acetogenesis in A. woodiiIMPORTANCE Peptides and amino acids are widespread in nature, but there are only a few reports that demonstrated use of amino acids as carbon and energy sources by acetogenic bacteria, a central and important group in the anaerobic food web. Our finding that A. woodii can perform alanine oxidation coupled to reduction of carbon dioxide not only increases the number of substrates that can be used by this model acetogen but also raises the possibility that other acetogens may also be able to use alanine. Indeed, the alanine genes are also present in at least two more acetogens, for which growth on alanine has not been reported so far. Alanine may be a promising substrate for industrial fermentations, since acid formation goes along with the production of a base (NH3) and pH regulation is a minor issue.

The Rnf Complex Is an Energy-Coupled Transhydrogenase Essential To Reversibly Link Cellular NADH and Ferredoxin Pools in the Acetogen Acetobacterium woodii

The Rnf complex is a respiratory enzyme that catalyzes the oxidation of reduced ferredoxin to the reduction of NAD⁺, and the negative free energy change of this reaction is used to generate a transmembrane ion gradient. In one class of anaerobic acetogenic bacteria, the Rnf complex is believed to be essential for energy conservation and autotrophic growth. We describe here a methodology for markerless mutagenesis in the model bacterium of this class, Acetobacterium woodii, which enabled us to delete the rnf genes and to test their in vivo role. The rnf mutant did not grow on H₂ plus CO₂, nor did it produce acetate or ATP from H₂ plus CO₂, and ferredoxin:NAD⁺ oxidoreductase activity and Na⁺ translocation were also completely lost, supporting the hypothesis that the Rnf complex is the only respiratory enzyme in this metabolism. Unexpectedly, the mutant also did not grow on low-energy substrates, such as ethanol or lactate. Oxidation of these substrates is not coupled to the reduction of ferredoxin but only of NAD⁺, and we speculated that the growth phenotype is caused by a loss of reduced ferredoxin, indispensable for biosynthesis and CO₂ reduction. The electron-bifurcating hydrogenase of A. woodii reduces ferredoxin, and indeed, the addition of H₂ to the cultures restored growth on ethanol and lactate. This is consistent with the hypothesis that endergonic reduction of ferredoxin with NADH is driven by reverse electron transport catalyzed by the Rnf complex, which renders the Rnf complex essential also for growth on low-energy substrates.IMPORTANCE Ferredoxin and NAD⁺ are key electron carriers in anaerobic bacteria, but energetically, they are not equivalent, since the redox potential of ferredoxin is lower than that of the NADH/NAD⁺ couple. We describe by mutant studies in Acetobacterium woodii that the main function of Rnf is to energetically link cellular pools of ferredoxin and NAD⁺ When ferredoxin is greater than NADH, exergonic electron flow from ferredoxin to NAD⁺ generates a chemiosmotic potential. This is essential for energy conservation during autotrophic growth. When NADH is greater than ferredoxin, Rnf works in reverse. This reaction is essential for growth on low-energy substrates to provide reduced ferredoxin, indispensable for biosynthesis and CO₂ reduction. Our studies put a new perspective on the cellular function of the membrane-bound ion-translocating Rnf complex widespread in bacteria.

Novel Solutions in Modeling of Anaerobic Digestion Process - Two-Phase AD Models Development and Comparison

This work focuses on Two Phase Anaerobic Digestion – a novel approach to split reactions’ chain in two separated tanks, each with specialized microbe community to achieve the best possible efficiency for each part of conversion. More specifically, the article tackle the topic of evaluation the possibility to adapt a mathematical model, previously dedicated for Autogenerative High Pressure Digestion (AHPD), to make use of it in simulation of Temperature Phased Anaerobic Digestion (TPAD) process. A comprehensive study of available solutions for biogas production simulation in conventional and TPAD configuration is additionally performed. Basing on its results, a reference model from literature, for comparison purpose is selected. Revisions and modifications, necessary to adjust previously developed model, to TPAD process, are described. Also, additional improvements like redesigned pH calculating algorithm is presented. Finally, the comparison between model predictions, a reference model and pilot-scale data is carried out. The results show that our current model needs further optimization, however even at this stage it provides acceptable results in short-range simulations (not longer than 42 days). Further works should focus on process stability improvement, especially in the thermophilic stage of biogas production. There's an opportunity for innovation as the research shows that requirement of accurate, large-scale optimized TPAD process model, is still not fulfilled.

Enhanced Methane Production from Food Waste Using Cysteine To Increase Biotransformation of l-Monosaccharide, Volatile Fatty Acids, and Biohydrogen

The enhancement of two-stage anaerobic digestion of polysaccharide-enriched food waste by the addition of cysteine-an oxygen scavenger, electron mediator, and nitrogen source-to the acidification stage was reported. It was found that in the acidification stage the accumulation of volatile fatty acids (VFA), which mainly consisted of acetate, butyrate, and propionate, was increased by 49.3% at a cysteine dosage of 50 mg/L. Although some cysteine was biodegraded in the acidification stage, the VFA derived from cysteine was negligible. In the methanogenesis stage, the biotransformations of both VFA and biohydrogen to methane were enhanced, and the methane yield was improved by 43.9%. The mechanisms study showed that both d-glucose and l-glucose (the model monosaccharides) were detectable in the hydrolysis product, and the addition of cysteine remarkably increased the acidification of l-glucose, especially acetic acid and hydrogen generation, due to key enzymes involved in l-glucose metabolism being enhanced. Cysteine also improved the activity of homoacetogens by 34.8% and hydrogenotrophic methanogens by 54%, which might be due to the electron transfer process being accelerated. This study provided an alternative method to improve anaerobic digestion performance and energy recovery from food waste.

Reaction engineering analysis of the autotrophic energy metabolism of Clostridium aceticum

Acetogenesis with CO2:H2 or CO via the reductive acetyl-CoA pathway does not provide any net ATP formation in homoacetogenic bacteria. Autotrophic energy conservation is coupled to the generation of chemiosmotic H+ or Na+ gradients across the cytoplasm membrane using either a ferredoxin:NAD+ oxidoreductase (Rnf), a ferredoxin:H+ oxidoreductase (Ech) or substrate-level phosphorylation via cytochromes. The first isolated acetogenic bacterium Clostridium aceticum shows both cytochromes and Rnf complex, putting it into an outstanding position. Autotrophic batch processes with continuous gas supply were performed in fully controlled stirred-tank bioreactors to elucidate energy metabolism of C. aceticum. Varying the initial Na+ concentration in the medium showed sodium-dependent growth of C. aceticum with a growth optimum between 60 and 90 mM Na+. The addition of the Na+-selective ionophore ETH2120 or the protonophore CCCP or the H+/cation-antiporter monensin revealed that an H+ gradient is used as primary energy conservation mechanism, which strengthens the exceptional position of C. aceticum as acetogenic bacterium showing an H+-dependent energy conservation mechanism as well as Na+-dependent growth.

Formic Acid Formation by Clostridium ljungdahlii at Elevated Pressures of Carbon Dioxide and Hydrogen

Low productivities of bioprocesses using gaseous carbon and energy sources are usually caused by the low solubility of those gases (e.g., H₂ and CO). It has been suggested that increasing the partial pressure of those gases will result in higher dissolved concentrations and should, therefore, be helpful to overcome this obstacle. Investigations of the late 1980s with mixtures of hydrogen and carbon monoxide showed inhibitory effects of carbon monoxide partial pressures above 0.8 bar. Avoiding any effects of carbon monoxide, we investigate growth and product formation of Clostridium ljungdahlii at absolute process pressures of 1, 4, and 7 bar in batch stirred tank reactor cultivations with carbon dioxide and hydrogen as sole gaseous carbon and energy source. With increasing process pressure, the product spectrum shifts from mainly acetic acid and ethanol to almost only formic acid at a total system pressure of 7 bar. On the other hand, no significant changes in overall product yield can be observed. By keeping the amount of substance flow rate constant instead of the volumetric gas feed rate when increasing the process pressure, we increased the overall product yield of 7.5 times of what has been previously reported in the literature. After 90 h of cultivation at a total pressure of 7 bar a total of 4 g L⁻¹ of products is produced consisting of 82.7 % formic acid, 15.6 % acetic acid, and 1.7 % ethanol.

Natural attenuation of chlorinated ethenes in hyporheic zones: A review of key biogeochemical processes and in-situ transformation potential

Chlorinated ethenes (CEs) are legacy contaminants whose chemical footprint is expected to persist in aquifers around the world for many decades to come. These organohalides have been reported in river systems with concerning prevalence and are thought to be significant chemical stressors in urban water ecosystems. The aquifer-river interface (known as the hyporheic zone) is a critical pathway for CE discharge to surface water bodies in groundwater baseflow. This pore water system may represent a natural bioreactor where anoxic and oxic biotransformation process act in synergy to reduce or even eliminate contaminant fluxes to surface water. Here, we critically review current process understanding of anaerobic CE respiration in the competitive framework of hyporheic zone biogeochemical cycling fuelled by in-situ fermentation of natural organic matter. We conceptualise anoxic-oxic interface development for metabolic and co-metabolic mineralisation by a range of aerobic bacteria with a focus on vinyl chloride degradation pathways. The superimposition of microbial metabolic processes occurring in sediment biofilms and bulk solute transport delivering reactants produces a scale dependence in contaminant transformation rates. Process interpretation is often confounded by the natural geological heterogeneity typical of most riverbed environments. We discuss insights from recent field experience of CE plumes discharging to surface water and present a range of practical monitoring technologies which address this inherent complexity at different spatial scales. Future research must address key dynamics which link supply of limiting reactants, residence times and microbial ecophysiology to better understand the natural attenuation capacity of hyporheic systems.

Biological hydrogen methanation - A review

Surplus energy out of fluctuating energy sources like wind and solar energy is strongly increasing. Biological hydrogen (H₂) methanation (BHM) is a highly promising approach to move the type of energy from electricity to natural gas via electrolysis and the subsequent step of the Sabatier-reaction. This review provides an overview of the numerous studies concerning the topic of BHM. The technical and biological parameters regarding the research results of these studies are compared and analyzed hereafter. A holistic view on how to overcome physical limitations of the fermentation process, such as gas-liquid mass transfer or a rise of the pH value, and on the enhancement of environmental circumstances for the bacterial biomass are delivered within. With regards to ex-situ methanation, the evaluated studies show a distinct connection between methane production and the methane percentage in the off-gas.

Major phylum-level differences between porefluid and host rock bacterial communities in the terrestrial deep subsurface

Earth's deep subsurface biosphere (DSB) is home to a vast number and wide variety of microorganisms. Although difficult to access and sample, deep subsurface environments have been probed through drilling programs, exploration of mines and sampling of deeply sourced vents and springs. In an effort to understand the ecology of deep terrestrial habitats, we examined bacterial diversity in the Sanford Underground Research Facility (SURF), the former Homestake gold mine, in South Dakota, USA. Whole genomic DNA was extracted from deeply circulating groundwater and corresponding host rock (at a depth of 1.45 km below ground surface). Pyrotag DNA sequencing of the 16S rRNA gene revealed diverse communities of putative chemolithoautotrophs, aerobic and anaerobic heterotrophs, numerous candidate phyla and unique rock-associated microbial assemblage. There was a clear and near-total separation of communities between SURF deeply circulating fracture fluids and SURF host-rocks. Sequencing data from SURF compared against five similarly sequenced terrestrial subsurface sites in Europe and North America revealed classes Clostridia and Betaproteobacteria were dominant in terrestrial fluids. This study presents a unique analysis showing differences in terrestrial subsurface microbial communities between fracture fluids and host rock through which those fluids permeate.

Insights into the Genome of the Anaerobic Acetogen Sporomusa silvacetica DSM 10669

Sporomusa silvacetica is a spore-forming, anaerobic acetogen isolated from soil derived from east central Germany. The genome contains genes of the Wood-Ljungdahl pathway required for carbon fixation and genes involved in the biosynthesis of the amino acid pyrrolysine. The genome (5.92 Mb) harbors 4,355 predicted protein-encoding genes.

Genome Sequence of the Homoacetogenic, Gram-Negative, Endospore-Forming Bacterium Sporomusa acidovorans DSM 3132

Sporomusa acidovorans DSM 3132 is a strictly anaerobic, spore-forming and acetogenic bacterium, which was isolated from effluent of an alcohol distillation fermenter. The genome harbors genes involved in the Wood-Ljungdahl pathway for carbon fixation and several genes for glycerol metabolism. The genome (6.06 Mb) contains 4,506 predicted protein-encoding genes.

Electro-stimulated microbial factory for value added product synthesis

Interplay of charge between bacteria and electrode has led to emergence of bioelectrochemical systems which leads to applications such as production of electricity, wastewater treatment, bioremediation and production of value added products. Many electroactive bacteria have been identified that have unique external electron transport systems. Coupling of electron transport with carbon metabolism has opened a new approach of carbon dioxide sequestration. The electron transport mechanism involves various cellular and sub cellular molecules. The outer membrane cytochromes, Mtr-complex and Ech-complex are few key molecules involved in electron transport in many electrogenic bacteria. Few cytochrome independent acetogenic electroactive bacteria were also discovered using Rnf complex to transport electrons. For improved productivity, an efficient bioreactor design is mandatory. It should encompass all certain critical issues such as microbial cell retention, charge dissipation, separators and simultaneous product recovery.

Sequential Mixed Cultures: From Syngas to Malic Acid

Synthesis gas (syngas) fermentation using acetogenic bacteria is an approach for production of bulk chemicals like acetate, ethanol, butanol, or 2,3-butandiol avoiding the fuel vs. food debate by using carbon monoxide, carbon dioxide, and hydrogen from gasification of biomass or industrial waste gases. Suffering from energetic limitations, yields of C₄-molecules produced by syngas fermentation are quite low compared with ABE fermentation using sugars as a substrate. On the other hand, fungal production of malic acid has high yields of product per gram metabolized substrate but is currently limited to sugar containing substrates. In this study, it was possible to show that Aspergilus oryzae is able to produce malic acid using acetate as sole carbon source which is a main product of acetogenic syngas fermentation. Bioreactor cultivations were conducted in 2.5 L stirred tank reactors. During the syngas fermentation part of the sequential mixed culture, Clostridium ljungdahlii was grown in modified Tanner medium and sparged with 20 mL/min of artificial syngas mimicking a composition of clean syngas from entrained bed gasification of straw (32.5 vol-% CO, 32.5 vol-% H₂, 16 vol-% CO₂, and 19 vol-% N₂) using a microsparger. Syngas consumption was monitored via automated gas chromatographic measurement of the off-gas. For the fungal fermentation part gas sparging was switched to 0.6 L/min of air and a standard sparger. Ammonia content of medium for syngas fermentation was reduced to 0.33 g/L NH₄Cl to meet the requirements for fungal production of dicarboxylic acids. Malic acid production performance of A. oryzae in organic acid production medium and syngas medium with acetate as sole carbon source was verified and gave Y_P∕S values of 0.28 g/g and 0.37 g/g respectively. Growth and acetate formation of C. ljungdahlii during syngas fermentation were not affected by the reduced ammonia content and 66 % of the consumed syngas was converted to acetate. The overall conversion of CO and H₂ into malic acid was calculated to be 3.5 g malic acid per mol of consumed syngas or 0.22 g malic acid per gram of syngas.

Investigating Moorella thermoacetica metabolism with a genome-scale constraint-based metabolic model

Moorella thermoacetica is a strictly anaerobic, endospore-forming, and metabolically versatile acetogenic bacterium capable of conserving energy by both autotrophic (acetogenesis) and heterotrophic (homoacetogenesis) modes of metabolism. Its metabolic diversity and the ability to efficiently convert a wide range of compounds, including syngas (CO + H2) into acetyl-CoA have made this thermophilic bacterium a promising host for industrial biotechnology applications. However, lack of detailed information on M. thermoacetica's metabolism is a major impediment to its use as a microbial cell factory. In order to overcome this issue, a genome-scale constraint-based metabolic model of Moorella thermoacetica, iAI558, has been developed using its genome sequence and physiological data from published literature. The reconstructed metabolic network of M. thermoacetica comprises 558 metabolic genes, 705 biochemical reactions, and 698 metabolites. Of the total 705 model reactions, 680 are gene-associated while the rest are non-gene associated reactions. The model, in addition to simulating both autotrophic and heterotrophic growth of M. thermoacetica, revealed degeneracy in its TCA-cycle, a common characteristic of anaerobic metabolism. Furthermore, the model helped elucidate the poorly understood energy conservation mechanism of M. thermoacetica during autotrophy. Thus, in addition to generating experimentally testable hypotheses regarding its physiology, such a detailed model will facilitate rapid strain designing and metabolic engineering of M. thermoacetica for industrial applications.

Effects of hydrogen partial pressure on autotrophic growth and product formation of Acetobacterium woodii

Low aqueous solubility of the gases for autotrophic fermentations (e.g., hydrogen gas) results in low productivities in bioreactors. A frequently suggested approach to overcome mass transfer limitation is to increase the solubility of the limiting gas in the reaction medium by increasing the partial pressure in the gas phase. An increased inlet hydrogen partial pressure of up to 2.1 bar (total pressure of 3.5 bar) was applied for the autotrophic conversion of hydrogen and carbon dioxide with Acetobacterium woodii in a batch-operated stirred-tank bioreactor with continuous gas supply. Compared to the autotrophic batch process with an inlet hydrogen partial pressure of 0.4 bar (total pressure of 1.0 bar) the final acetate concentration after 3.1 days was reduced to 50 % (29.2 g L(-1) compared to 59.3 g L(-1)), but the final formate concentration was increased by a factor of 18 (7.3 g L(-1) compared to 0.4 g L(-1)). Applying recombinant A. woodii strains overexpressing either genes for enzymes in the methyl branch of the Wood-Ljungdahl pathway or the genes phosphotransacetylase and acetate kinase at an inlet hydrogen partial pressure of 1.4 bar reduced the final formate concentration by up to 40 % and increased the final dry cell mass and acetate concentrations compared to the wild type strain. Solely the overexpression of the two genes for ATP regeneration at the end of the Wood-Ljungdahl pathway resulted in an initial switch off of formate production at increased hydrogen partial pressure until the maximum of the hydrogen uptake rate was reached.

Biomass hydrolysis inhibition at high hydrogen partial pressure in solid-state anaerobic digestion

In solid-state anaerobic digestion, so-called ss-AD, biogas production is inhibited at high total solids contents. Such inhibition is likely caused by a slow diffusion of dissolved reaction intermediates that locally accumulate. In this study, we investigated the effect of H2 and CO2 partial pressure on ss-AD. Partial pressure of H2 and/or CO2 was artificially fixed, from 0 to 1 557mbars for H2 and from 0 to 427mbars for CO2. High partial pressure of H2 showed a significant effect on methanogenesis, while CO2 had no impact. At high [Formula: see text] , the overall substrate degradation decreased with no accumulation of metabolites from acidogenic bacteria, indicating that the hydrolytic activity was specifically impacted. Interestingly, such inhibition did not occur when CO2 was added with H2. This result suggests that CO2 gas transfer is probably a key factor in ss-AD from biomass.