Acetogenic Research Articles

Microbiomes Associated With the Surfaces of Northern Argentinian Fruits Show a Wide Species Diversity.

The fiber, vitamin, and antioxidant contents of fruits contribute to a balanced human diet. In countries such as Argentina, several tropical fruits are witnessing a high yield in the harvest season, with a resulting surplus. Fruit fermentation using autochthonous starter cultures can provide a solution for food waste. However, limited knowledge exists about the microbiota present on the surfaces of fruits and the preceding flowers. In the present exploratory study, the microbiomes associated with the surfaces of tropical fruits from Northern Argentina, such as white guava, passion fruit and papaya were investigated using a shotgun metagenomic sequencing approach. Hereto, one sample composed of 14 white guava fruits, two samples of passion fruits with each two to three fruits representing the almost ripe and ripe stage of maturity, four samples of papaya with each two to three fruits representing the unripe, almost ripe, and ripe stage of maturity were processed, as well as a sample of closed and a sample of open Japanese medlar flowers. A considerable heterogeneity was found in the composition of the fruits’ surface microbiota at the genus and species level. While bacteria dominated the microbiota of the fruits and flowers, a small number of the metagenomic sequence reads corresponded with yeasts and filamentous fungi. A minimal abundance of bacterial species critical in lactic acid and acetic acid fermentations was found. A considerable fraction of the metagenomic sequence reads from the fruits’ surface microbiomes remained unidentified, which suggested that intrinsic species are to be sequenced or discovered.

Methane Production Potential and Methanogenic Pathways in Paddy Soils Under Different Rice-based Cropping Systems

Differences inmethane (CH4) production potential in paddy soils under different rice-based cropping systems and especially in the methanogenic pathways (mainly acetate fermentation and CO2/H2 reduction) remain unclear. Anaerobic incubations of soil with or without fluoromethane (CH3F) inhibitor (2% and 0%) were conducted. With the soils from three typical paddy ecosystems (rice-wheat rotation, RW; rice-fallow, RF; double-rice, DR) in China, the cumulative concentration of CH4 production, CH4 production potential, dissolved organic carbon (DOC) content, and acetic acid content were determined. Meanwhile, the relative contribution of acetate-dependent methanogenesis (fac) was quantified using the stable carbon isotope method. The results showed that the CH4 production potential was 7.18 μg·(g·d)-1 in RF, which was significantly lower than that in RW[10.33 μg·(g·d)-1]and DR[13.42 μg·(g·d)-1] (P<0.05). Correlation analysis showed that CH4 production potential was significantly negatively correlated with soil cation exchange capacity and pH (P<0.01); the addition of CH3F significantly inhibited CH4 production (P<0.05). The content of DOC and acetic acid in DR were 255 mg·kg-1 and 7.34 mg·kg-1, respectively, which were 17%-51% and 22%-23% higher than those in RW and RF, respectively. The δ13CH4 and δ13CO2 values were affected greatly by different rice-based cropping systems, and the highest δ13CH4 value was -43.89‰ in RF, which was more positive than that of RW and DR by 11.06‰ and 8.33‰, respectively (P<0.05). By contrast, the lowest value of δ13CO2 was observed in RF, which was more negative than that of RW (7.63‰) and DR (5.14‰) (P<0.05). The α(CO2/CH4) values of RW and RF were 1.057 and 1.058, respectively, which were significantly lower than 1.062 in DR (P<0.05). The fac values of RF ranged from 84% to 98%, being 34%-38% and 20%-23% higher than those of RW and DR, respectively (P<0.05).

Date Fruits as Raw Material for Vinegar and Non-Alcoholic Fermented Beverages.

Currently, foods and beverages with healthy and functional properties, especially those that claim to prevent chronic diseases, are receiving more and more interest. As a result, numerous foods and beverages have been launched onto the market. Among the products with enhanced properties, vinegar and fermented beverages have a high potential for growth. Date palm fruits are a versatile raw material rich in sugars, dietary fibers, minerals, vitamins, and phenolic compounds; thus, they are widely used for food production, including date juice, jelly, butter, and fermented beverages, such as wine and vinegar. Furthermore, their composition makes them suitable for the formulation of functional foods and beverages. Microbial transformations of date juice include alcoholic fermentation for producing wine as an end-product, or as a substrate for acetic fermentation. Lactic fermentation is also documented for transforming date juice and syrup. However, in terms of acetic acid bacteria, little evidence is available on the exploitation of date juice by acetic and gluconic fermentation for producing beverages. This review provides an overview of date fruit’s composition, the related health benefits for human health, vinegar and date-based fermented non-alcoholic beverages obtained by acetic acid bacteria fermentation.

Bacteria–photocatalyst sheet for sustainable carbon dioxide utilization

The clean conversion of carbon dioxide and water to a single multicarbon product and O2 using sunlight via photocatalysis without the assistance of organic additives or electricity remains an unresolved challenge. Here we report a bio-abiotic hybrid system with the non-photosynthetic, CO2-fixing acetogenic bacterium Sporomusa ovata grown on a scalable and cost-effective photocatalyst sheet consisting of a pair of particulate semiconductors (La and Rh co-doped SrTiO3 (SrTiO3:La,Rh) and Mo-doped BiVO4 (BiVO4:Mo)). The biohybrid effectively produces acetate (CH3COO–) and oxygen (O2) using only sunlight, CO2 and H2O, achieving a solar-to-acetate conversion efficiency of 0.7% at ambient conditions (298 K, 1 atm). The photocatalyst sheet oxidizes water to O2 and provides electrons and hydrogen (H2) to S. ovata for the selective synthesis of CH3COO– from CO2. To demonstrate utility in a closed carbon cycle, the solar-generated acetate was used directly as feedstock in a bioelectrochemical system for electricity generation. These semi-biological approaches thus offer a promising strategy for sustainably and cleanly fixing CO2 and closing the carbon cycle. Conversion of CO2 to fuels or chemicals via artificial photosynthesis usually requires the assistance of organic additives or electricity. Now, a biohybrid system is reported consisting of a photocatalyst sheet and bacteria producing acetate and O2 from CO2 and H2O using sunlight as the sole energy input.

The Total Content of Polyphenols and the Antioxidant Properties of several Berry Vinegars

Berry fruits represent a valuable source of natural aroma and antioxidants for vinegars. The aim of this research was the evaluation of the&#x0D; total polyphenol content (TPC), total anthocyanin content (TAC), and antioxidant properties (by ABTS and DPPH assay) of several vinegars&#x0D; produced by an acetous fermentation process, using substrates that contain berry (raspberries, blueberries, blackberries) juice (40%; 60%) inoculated with acetous bacteria (0.5%; 1%). Experimental results indicated that blueberry vinegar samples obtained using 0.5% acetous bacteria and 60% berry juices led to the highest mean values for TPC, TAC, and levels of antioxidant activity, but all these variants had significantly the lowest values of acetic acid content.

Recent Progress in Electronic Noses for Fermented Foods and Beverages Applications

Fermented foods and beverages have become a part of daily diets in several societies around the world. Emitted volatile organic compounds play an important role in the determination of the chemical composition and other information of fermented foods and beverages. Electronic nose (E-nose) technologies enable non-destructive measurement and fast analysis, have low operating costs and simplicity, and have been employed for this purpose over the past decades. In this work, a comprehensive review of the recent progress in E-noses is presented according to the end products of the main fermentation types, including alcohol fermentation, lactic acid fermentation, acetic acid fermentation and alkaline fermentation. The benefits, research directions, limitations and challenges of current E-nose systems are investigated and highlighted for fermented foods and beverage applications.

Development of CO gas conversion system using high CO tolerance biocatalyst

Acetogenic bacteria (acetogens) are microorganisms with high potential as biocatalysts that can convert C1 gas, such as carbon dioxide (CO2) or carbon monoxide (CO), into biomass or biochemicals; however, high CO concentrations cause growth retardation in these microorganisms. In this study, we performed adaptive evolution of Eubacterium limosum, an acetogen, for approximately 400 generations under high CO synthetic gas (syngas) conditions to obtain E. limosum ECO2, which can rapidly proliferate even under high CO conditions. A mutation was revealed in H636R in the Acetyl-CoA synthase (ACS) protein of the ECO2 strain through whole-genome re-sequencing, and the mutation increased the CO gas conversion rate. By analyzing the transcript profiling of the ECO2 strain under high CO syngas conditions, the transcript levels of the methyl branch in the Wood–Ljungdahl pathway and the energy conservation system increased at high CO concentration conditions. Finally, to convert CO into value-added biochemicals, 2,3-BDO (2,3-butanediol) biosynthesis pathways were constructed and transformed into the E. limosum ECO2 strain. This strain showed capability to produce 2.60 mg L-1h−1 of 2,3-BDO through CO 66% syngas fed-batch fermentation. This was 6.5-fold higher than that of the wild type. We developed a microorganism with high CO tolerance, and this strain showed high application potential as a biocatalyst that can efficiently convert CO gas into value-added chemicals.

Production of prickly pear (Opuntia ficus-indica) vinegar in submerged culture using Acetobacter malorum and Gluconobacter oxydans: Study of volatile and polyphenolic composition

Prickly pear (Opuntia ficus-indica (L). Mill.) vinegar has been produced by submerged culture using scale laboratory fermenters at three acetification temperatures (30 °C, 37 °C, and 40 °C). Pure thermotolerant Acetic Acid Bacteria (AAB) which had been previously submitted to molecular identification by PCR–RFLP of the 16S–23S rDNA regions and 16S–23S ITS rDNA sequencing, were used. The AAB used were identified as Acetobacter malorum and Gluconobacter oxydans. The acetic fermentation achieved by A. malorum at 30 °C and 37 °C was more efficient, in terms of acidity, than that accomplished by G. oxydans. The chemical analysis revealed the presence of 85 individual volatile compounds and 17 polyphenolic compounds. The concentration of approximately half the volatile compounds was significantly affected by fermentation temperature, with clearly lower concentrations as temperature increased, whereas few significant differences were observed when comparing the vinegars produced by the two AAB species. Regarding phenolic compounds, significant increases were registered when temperature changed from 30 °C to 40 °C. Furthermore, the vinegars produced by A. malorum presented a greater concentration of phenolic compounds than those produced by G. oxydans.

A Third Way of Energy Conservation in Acetogenic Bacteria.

ABSTRACTAcetogenic bacteria are a group of strictly anaerobic bacteria that make a living from acetate formation from two molecules of CO2 via the Wood-Ljungdahl pathway (WLP). The free energy change of this reaction is very small and allows the synthesis of only a fraction of an ATP. How this pathway is coupled to energy conservation has been an enigma since its discovery ~90 years ago. Here, we describe an electron transport chain in the cytochrome- and quinone-containing acetogen Sporomusa ovata that leads from molecular hydrogen as an electron donor to an intermediate of the WLP, methylenetetrahydrofolate (methylene-tetrahydrofolate [THF]), as an electron acceptor. The catalytic site of the hydrogenase is periplasmic and likely linked cytochrome b to the membrane. We provide evidence that the MetVF-type methylenetetrahydrofolate reductase is linked proteins MvhD and HdrCBA to the cytoplasmic membrane. Membrane preparations catalyzed the H2-dependent reduction of methylene-THF to methyl-THF. In our model, a transmembrane electrochemical H+ gradient is established by both scalar and vectorial protons that leads to the synthesis of 0.5 mol ATP/mol methylene-THF by a H+-F1Fo ATP synthase. This H2- and methylene-THF-dependent electron transport chain may be present in other cytochrome-containing acetogens as well and represents a third way of chemiosmotic energy conservation in acetogens, but only in addition to the well-established respiratory enzymes Rnf and Ech.IMPORTANCE Acetogenic bacteria grow by making acetate from CO2 and are considered the first life forms on Earth since they couple CO2 reduction to the conservation of energy. How this is achieved has been an enigma ever since. Recently, two respiratory enzymes, a ferredoxin:NAD+ oxidoreductase (Rnf) and a ferredoxin:H+ oxidoreductase (Ech), have been found in cytochrome-free acetogenic model bacteria. However, some acetogens contain cytochromes in addition, and there has been a long-standing assumption of a cytochrome-containing electron transport chain in those acetogens. Here, we provide evidence for a respiratory chain in Sporomusa ovata that has a cytochrome-containing hydrogenase as the electron donor and a methylenetetrahydrofolate reductase as the terminal electron acceptor. This is the third way of chemiosmotic energy conservation found in acetogens.

An innovative strategy to enhance the ensiling quality and methane production of excessively wilted wheat straw: Using acetic acid or hetero-fermentative lactic acid bacterial community as additives

Ensiling is an effective storage strategy for agricultural biomass, especially for energy crops (mainly energy grasses and maize). However, the ensiling of excessively wilted crop straw is limited due to material characteristics, such as a high lignocellulosic content and low water-soluble carbohydrate and moisture contents. In this study, acetic acid or hetero-fermentative lactic acid bacterial community (hetero-fermentative LAB) were employed as silage additives to improve the ensiling process of excessively wilted wheat straw (EWS). The results showed that the additives inhibited the growth of Enterobacteriaceae and Clostridium_sensu_stricto_12, whose abundances decreased from 55.8% to 0.03–0.2%, respectively. The growth of Lactobacillus was accelerated, and the abundances increased from 1.3% to 80.1–98.4% during the ensiling process. Lactic acid fermentation was the dominant metabolic pathway in the no additive treatment. The additives increased acetic acid fermentation and preserved the hemicellulose and cellulose contents, increasing the methane yield by 17.7–23.9%. This study shows that ensiling with acetic acid or hetero-fermentative LAB is an effective preservation and storage strategy for efficient methane production from EWS.

The Potential of Sequential Fermentations in Converting C1 Substrates to Higher-Value Products.

Today production of (bulk) chemicals and fuels almost exclusively relies on petroleum-based sources, which are connected to greenhouse gas release, fueling climate change. This increases the urgence to develop alternative bio-based technologies and processes. Gaseous and liquid C1 compounds are available at low cost and often occur as waste streams. Acetogenic bacteria can directly use C1 compounds like CO, CO2, formate or methanol anaerobically, converting them into acetate and ethanol for higher-value biotechnological products. However, these microorganisms possess strict energetic limitations, which in turn pose limitations to their potential for biotechnological applications. Moreover, efficient genetic tools for strain improvement are often missing. However, focusing on the metabolic abilities acetogens provide, they can prodigiously ease these technological disadvantages. Producing acetate and ethanol from C1 compounds can fuel via bio-based intermediates conversion into more energy-demanding, higher-value products, by deploying aerobic organisms that are able to grow with acetate/ethanol as carbon and energy source. Promising new approaches have become available combining these two fermentation steps in sequential approaches, either as separate fermentations or as integrated two-stage fermentation processes. This review aims at introducing, comparing, and evaluating the published approaches of sequential C1 fermentations, delivering a list of promising organisms for the individual fermentation steps and giving an overview of the existing broad spectrum of products based on acetate and ethanol. Understanding of these pioneering approaches allows collecting ideas for new products and may open avenues toward making full use of the technological potential of these concepts for establishment of a sustainable biotechnology.

Lactobacillus fermentum as a new inhibitor to control advanced glycation end-product formation during vinegar fermentation

The inhibitory activity of lactic acid bacteria (LAB) toward advanced glycation end-products (AGEs) during vinegar fermentation was studied, and its relationships with the substrate consumption, antioxidant capacity, total phenolic content, total flavonoid compounds, α-glucosidase, and α-amylase activity inhibition were evaluated. The vinegar was made from rice powder flour by liquid-state fermentation (LSF). The selected LAB strains were separately co-cultivated with Saccharomyces cerevisiae and Acetobacter pasteurianus 1.41 in alcoholic and acetic acid fermentation, respectively. Among 3 strains, Lactobacillus fermentum showed the strongest inhibitory effect on the formation of total fluorescent AGEs and carboxymethyl lysine (CML)/carboxyethyl lysine (CEL) in the fermentation process. The corresponding mechanisms included the acceleration of substrate consumption, improvement of antioxidant activities, and inhibition of α-glucosidase and α-amylase. In addition, the fluorescent AGEs and the CML/CEL were negatively correlated with the antioxidant activities, while the α-glucosidase and α-amylase activities were positively correlated with the total phenols and total flavonoids. Moreover, the variety of main flavor compounds increased, including esters, alcohols, phenols and acids. The results of the study support the potential use of screened LAB strains to inhibit the formation of fluorescent AGEs, CML and CEL on fermented products and in the food processing industry, without associated risks to consumers.

A clean in-frame knockout system for gene deletion in Acetobacterium woodii

Acetogenic bacteria produce acetate following the fixation of CO2 via the Wood-Ljungdahl pathway. As such, they represent excellent process organisms for the production of novel chemicals and fuels from this waste greenhouse gas. Acetobacterium woodii is the model acetogen and numerous studies have been conducted investigating its biochemistry, gas consumption and use as a production chassis. However, there are a dearth of available tools for A. woodii gene modification which limits the research options available for genetic studies. Here, the previously proposed Clostridia Roadmap is implemented in A. woodii leading to the derivation of a knockout system for the generation of clean, in-frame deletions. The replicon of the Gram-positive plasmid pCD6 that originated in Clostridioides difficile was identified as being replication-defective in A. woodii, a property that was exploited to construct a pseudo-suicide knockout plasmid which was used to generate an auxotrophic, pyrE mutant. This allowed the subsequent use of a heterologous pyrE gene (from Clostridium acetobutylicum) as a counter selection marker and the deletion of a number of genes by allelic exchange. Specific mutants generated were affected in growth on glucose, fructose and ethanol as a consequence of deletion of fruA, pstG and adhE, respectively.

Unraveling the metabolic shift in anaerobic digestion pathways associated with the alteration of onion skin waste concentration

Onion skin waste (OSW) is common waste in developing countries, which can cause severe environmental pollution when not properly treated. Value-added products can be chemically extracted from OSW; however, that process is not economically feasible. Alternatively, dry anaerobic digestion (DAD) of OSW is a promising approach for both energy recovery and environment protection. The main hurdles during DAD of OSW can be the hydrolysis and acidification. In batch tests, sludge digestate (SD) rich with methanogens was co-digested with different fractions of OSW for enhancing hydrolysis and raising biogas productivity. The cumulative biogas production (CBP) was 36.6 ± 0.3 mL for sole DAD of SD (100% SD) and increased up to 281.9 ± 14.1 mL for (50% SD: 50% OSW) batch. Self-delignification of OSW took place by SD addition, where the lignin removal reached 75.3 ± 10.5% for (85% SD: 15% OSW) batch. Increasing the fraction of OSW (45% SD: 55% OSW) reduced the delignification by a value of 68.8%, where initial lignin concentration was 9.48 ± 1.6% in dry weight. Lignin breaking down resulted a high fraction of phenolic compounds (345.6 ± 58.8 mg gallic acid equivalent/g dry weight) in the fermentation medium, causing CBP drop (219.0 ± 28.5 mL). The presence of elements (K, Ca, Mg, Fe, Zn, Mn, S and P) in OSW improved the enzymatic activity, facilitated phenolic compounds degradation, shifted the metabolism towards acetate fermentation pathway, and raised biogas productivity. Acidogenesis was less affected by phenolic compounds than methanogenesis, causing higher H2 contents and lower CH4 contents, at batches with high share of OSW.

The characteristics and influencing factors of dissolved methane concentrations in Chongqing's central urban area in the Three Gorges Reservoir, China.

Methane (CH4) emissions from reservoirs have received widespread attention. The central urban area of Chongqing in the Three Gorges Reservoir area was selected as the study area in 2020. The temporal and spatial distribution of dissolved CH4 concentration and flux, key generation pathways, and influencing factors have been studied. The dissolved CH4 concentration in low-water-level period and impoundment period varied from 0.037~0.12 μmol·L-1 and 0.11~0.23 μmol·L-1, with the average values of (0.066 ± 0.0067) μmol·L-1 and (0.13 ± 0.034) μmol·L-1. The CH4 flux was (0.941 ± 0.217) μmol·m-2·h-1 in low-water-level period and (1.915 ± 0.204) μmol·m-2·h-1 in impoundment period. CH4 was produced by CO2 reduction and acetic acid fermentation, accounting for 17.95% and 82.05% of the total CH4 production, respectively. The dissolved CH4 concentration was significantly positively correlated with DO and NO3--N, and it is opposite with dissolved inorganic carbon. The dissolved CH4 concentration in this study area is affected by water environment (33.42%), inorganic nitrogen (29.60%), organic carbon (23.88%), and inorganic carbon (13.10%), and anthropogenic influences promoted dissolved CH4 concentration.

Insights into coupling between in-situ coalbed water geochemical signatures and microbial communities

The degradation of coal into CH4 by microorganisms is a multi-stage process involving a variety of bacteria and archaea. However, little understood about the coupling between in-situ coalbed water geochemical signature and microbial communities involved in different degradation processes. Here, the relation is made between the geochemical signature of the in-situ coalbed water (pH, anions, light metal cations and trace metal elements) and the microbial community (Miquan area, southern Junggar Basin, China). The results show that when the hydrolytic bacteria are the dominant group, they cause an increase in the concentration of certain trace elements in the coalbed water, and different types of hydrolytic bacteria dissolve different types of trace metal elements. Azoarcus and Pseudomonas together dissolve 27 elements including Tl, Sm and Lu. Five trace elements including Nb and V dissolved by Luteolibacter. Even though different hydrolytic bacteria require different Sr concentrations, there is no doubt that hydrolytic bacteria are enriched in Sr-rich environments. As far as methanogenic archaea concerned, Methanobacterium and Methanocorpusculum, which utilize reduced CO2 as a methanogenic pathway, are unaffected by trace metal elements. Although the anions SO42−, NO3− and F− have opposite promoting and inhibiting effects on Methanocorpusculum and Methanobacterium, they do not affect the methanogenic pathway of the microorganisms. Methanothrix increases in relative abundance in low Sr and high Mo concentration and produces CH4 in an acetic acid fermentation pathway. Higher concentrations of the trace elements Cs and Mn increased the relative abundance of Methanolobus, which uses methyl fermentation as the methanogenic pathway. Trace metal elements interactions with bacteria in situ coalbed water and the influence on fluctuations in methanogenic pathways of archaea outweigh other geochemical signatures (pH, anions and light metal cations). This discovery is a significant contribution to the understanding of the interactions between microorganisms and trace metal elements involved in different methanogenic processes in situ coalbed water, and lays the foundation for research on microbial coalbed gas.

Mutations in degP and spoT Genes Mediate Response to Fermentation Stress in Thermally Adapted Strains of Acetic Acid Bacterium Komagataeibacter medellinensis NBRC 3288.

An acetic acid bacterium, Komagataeibacter medellinensis NBRC 3288, was adapted to higher growth temperatures through an experimental evolution approach in acetic acid fermentation conditions, in which the cells grew under high concentrations of ethanol and acetic acid. The thermally adapted strains were shown to exhibit significantly increased growth and fermentation ability, compared to the wild strain, at higher temperatures. Although the wild cells were largely elongated and exhibited a rough cell surface, the adapted strains repressed the elongation and exhibited a smaller cell size and a smoother cell surface than the wild strain. Among the adapted strains, the ITO-1 strain isolated during the initial rounds of adaptation was shown to have three indel mutations in the genes gyrB, degP, and spoT. Among these, two dispensable genes, degP and spoT, were further examined in this study. Rough cell surface morphology related to degP mutation suggested that membrane vesicle-like structures were increased on the cell surface of the wild-type strain but repressed in the ITO-1 strain under high-temperature acetic acid fermentation conditions. The ΔdegP strain could not grow at higher temperatures and accumulated a large amount of membrane vesicles in the culture supernatant when grown even at 30°C, suggesting that the degP mutation is involved in cell surface stability. As the spoT gene of ITO-1 lost a 3′-end of 424 bp, which includes one (Act-4) of the possible two regulatory domains (TGS and Act-4), two spoT mutant strains were created: one (ΔTGSAct) with a drug cassette in between the 5′-half catalytic domain and 3′-half regulatory domains of the gene, and the other (ΔAct-4) in between TGS and Act-4 domains of the regulatory domain. These spoT mutants exhibited different growth responses; ΔTGSAct grew better in both the fermentation and non-fermentation conditions, whereas ΔAct-4 did only under fermentation conditions, such as ITO-1 at higher temperatures. We suggest that cell elongation and/or cell size are largely related to these spoT mutations, which may be involved in fermentation stress and thermotolerance.

A functional Wood-Ljungdahl pathway devoid of a formate dehydrogenase in the gut acetogens Blautia wexlerae, Blautia luti and beyond.

Species of the genus Blautia are typical inhabitants of the human gut and considered as beneficial gut microbes. However, their role in the gut microbiome and their metabolic features are poorly understood. Blautia schinkii was described as an acetogenic bacterium, characterized by a functional Wood-Ljungdahl pathway (WLP) of acetogenesis from H2 +CO2 . Here we report that two relatives, Blautia luti and Blautia wexlerae do not grow on H2 +CO2 . Inspection of the genome sequence revealed all genes of the WLP except genes encoding a formate dehydrogenase and an electron-bifurcating hydrogenase. Enzyme assays confirmed this prediction. Accordingly, resting cells neither converted H2 +CO2 nor H2 +HCOOH+CO2 to acetate. Carbon monoxide is an intermediate of the WLP and substrate for many acetogens. Blautia luti and B. wexlerae had an active CO dehydrogenase and resting cells performed acetogenesis from HCOOH+CO2 +CO, demonstrating a functional WLP. Bioinformatic analyses revealed that many Blautia strains as well as other gut acetogens lack formate dehydrogenases and hydrogenases. Thus, the use of formate instead of H2 +CO2 as an interspecies hydrogen and electron carrier seems to be more common in the gut microbiome.

Formate-driven H2 production by whole cells of Thermoanaerobacter kivui

BackgroundIn times of global warming there is an urgent need to replace fossil fuel-based energy vectors by less carbon dioxide (CO2)-emitting alternatives. One attractive option is the use of molecular hydrogen (H2) since its combustion emits water (H2O) and not CO2. Therefore, H2 is regarded as a non-polluting fuel. The ways to produce H2 can be diverse, but steam reformation of conventional fossil fuel sources is still the main producer of H2 gas up to date. Biohydrogen production via microbes could be an alternative, environmentally friendly and renewable way of future H2 production, especially when the flexible and inexpensive C1 compound formate is used as substrate.ResultsIn this study, the versatile compound formate was used as substrate to drive H2 production by whole cells of the thermophilic acetogenic bacterium Thermoanaerobacter kivui which harbors a highly active hydrogen-dependent CO2 reductase (HDCR) to oxidize formate to H2 and CO2 and vice versa. Under optimized reaction conditions, T. kivui cells demonstrated the highest H2 production rates (qH2 = 685 mmol g−1 h−1) which were so far reported in the literature for wild-type organisms. Additionally, high yields (Y(H2/formate)) of 0.86 mol mol−1 and a hydrogen evolution rate (HER) of 999 mmol L−1 h−1 were observed. Finally, stirred-tank bioreactor experiments demonstrated the upscaling feasibility of the applied whole cell system and indicated the importance of pH control for the reaction of formate-driven H2 production.ConclusionsThe thermophilic acetogenic bacterium T. kivui is an efficient biocatalyst for the oxidation of formate to H2 (and CO2). The existing genetic tool box of acetogenic bacteria bears further potential to optimize biohydrogen production in future and to contribute to a future sustainable formate/H2 bio-economy.

One substrate, many fates: different ways of methanol utilization in the acetogen Acetobacterium woodii.

Acetogenic bacteria such as Acetobacterium woodii use the Wood-Ljungdahl pathway (WLP) for fixation of CO2 and energy conservation. This pathway enables conversion of diverse substrates to the main product of acetogenesis, acetate. Methyl group containing substrates such as methanol or methylated compounds, derived from pectin, are abundant in the environment and a source for CO2 . Methyl groups enter the WLP at the level of methyltetrahydrofolic acid (methyl-THF). For methyl transfer from methanol to THF a substrate-specific methyltransferase system is required. In this study, we used genetic methods to identify mtaBC2A (Awo_c22760-Awo_c22740) as the methanol-specific methyltransferase system of A. woodii. After methyl transfer, methyl-THF serves as carbon and/or electron source and the respiratory Rnf complex is required for redox homeostasis if methanol+CO2 is the substrate. Resting cells fed with methanol+CO2 , indeed converted methanol to acetate in a 4:3 stoichiometry. When methanol was fed in combination with other electron sources such as H2 +CO2 or CO, methanol was converted Rnf-independently and the methyl group was condensed with CO to build acetate. When fed in combination with alternative electron sinks such as caffeate methanol was oxidized only and resulting electrons were used for non-acetogenic growth. These different pathways for the conversion of methyl-group containing substrates enable acetogens to adapt to various ecological niches and to syntrophic communities.