Acetogenic Microorganisms Research Articles

Investigation on the microbial community of an accelerating stabilization landfill by aeration engineering.

The microbial community of the landfill undergoing aerobic stabilization process by aeration engineering was investigated. The municipal solid wastes (MSWs) were sampled from two aeration well sites with different landfill temperatures (65.5°C and 41.7°C) under higher and lower stabilization level. The physical component, chemical property, and microbial population of MSWs were analyzed and compared. The result showed that the phylum Firmicutes was dominant in the aerobic landfill; and the genus Weissella and Syntrophaceticus were more abundant in high, and low temperature site, respectively. The bacterial distribution showed difference on two temperature sites and four landfill depths, mainly affected by the ammonia-nitrogen and moisture content of MSWs. The ecological profiles of the microorganisms responded the aeration engineering were predicted. The anaerobic hydrolytic and acetogenic microorganisms were decreased in abundance, while the facultative Lactobacillus increased when the landfill under a higher stabilization level. The function abundances of methane oxidation, sulfide oxidation, and aerobic chemoheterotrophy were enriched by aeration engineering, which was the microbial mechanism for accelerating the stabilization process of landfill.

Power to biogas upgrading: Effects of different H2/CO2 ratios on products and microbial communities in anaerobic fermentation system

Two anaerobic reactors with and without Ca2+ were operated at 35 °C to investigate the effects of different H2/CO2 ratios on products and microbial communities. Through the investigation of various parameters, it was shown that the change of pH triggered by the variations of H2/CO2 is the decisive factor affecting the product selection in anaerobic fermentation system. During the biosynthesis of ATP for cell growth and reproduction, protons (H+) were pumped from extracellular to intracellular by proton pump, which caused an increase of intrinsic pH of fermentative system. When the pH below 9.5, the methanogenic pathway was more prevalent. While the pH above 10.0 was conducive to the homoacetogenesis. Microbial community analysis showed that with the changes of H2/CO2 ratio, a turnover had occurred. When the ratio of H2/CO2 was 4, the main methanogen was Methanobacterium with the dominant interspecies electron transfer bacteria (IETB) of Thermovirga and DMER64. The turnover of microbial community occurred when the H2/CO2 ratio was 4.5 and 4.25. The dominant acetogenic microorganisms were norank_o_Clostridia_UCG-014 (homoacetogen) and Natronincola (obligately alkaliphilic acetogen). When the H2/CO2 ratio returned to 4, the dominant methanogens were hydrotropic Methanobacterium and Methanobrevibacter with four interspecies electron transfer bacteria including DMER64, Thermovirga, Dechlorobacter and Achromobacter.

Leveraging substrate flexibility and product selectivity of acetogens in two-stage systems for chemical production.

Carbon dioxide (CO2 ) stands out as sustainable feedstock for developing a circular carbon economy whose energy supply could be obtained by boosting the production of clean hydrogen from renewable electricity. H2 -dependent CO2 gas fermentation using acetogenic microorganisms offers a viable solution of increasingly demonstrated value. While gas fermentation advances to achieve commercial process scalability, which is currently limited to a few products such as acetate and ethanol, it is worth taking the best of the current state-of-the-art technology by its integration within innovative bioconversion schemes. This review presents multiple scenarios where gas fermentation by acetogens integrate into double-stage biotechnological production processes that use CO2 as sole carbon feedstock and H2 as energy carrier for products' synthesis. In the integration schemes here reviewed, the first stage can be biotic or abiotic while the second stage is biotic. When the first stage is biotic, acetogens act as a biological platform to generate chemical intermediates such as acetate, formate and ethanol that become substrates for a second fermentation stage. This approach holds the potential to enhance process titre/rate/yield metrics and products' spectrum. Alternatively, when the first stage is abiotic, the integrated two-stage scheme foresees, in the first stage, the catalytic transformation of CO2 into C1 products that, in the second stage, can be metabolized by acetogens. This latter scheme leverages the metabolic flexibility of acetogens in efficient utilization of the products of CO2 abiotic hydrogenation, namely formate and methanol, to synthesize multicarbon compounds but also to act as flexible catalysts for hydrogen storage or production.

Metabolic engineering of Clostridium ljungdahlii for the production of hexanol and butanol from CO2 and H2

BackgroundThe replacement of fossil fuels and petrochemicals with sustainable alternatives is necessary to mitigate the effects of climate change and also to counteract diminishing fossil resources. Acetogenic microorganisms such as Clostridium spp. are promising sources of fuels and basic chemical precursors because they efficiently utilize CO and CO2 as carbon source. However the conversion into high titers of butanol and hexanol is challenging.ResultsUsing a metabolic engineering approach we transferred a 17.9-kb gene cluster via conjugation, containing 13 genes from C. kluyveri and C. acetobutylicum for butanol and hexanol biosynthesis, into C. ljungdahlii. Plasmid-based expression resulted in 1075 mg L−1 butanol and 133 mg L−1 hexanol from fructose in complex medium, and 174 mg L−1 butanol and 15 mg L−1 hexanol from gaseous substrate (20% CO2 and 80% H2) in minimal medium. Product formation was increased by the genomic integration of the heterologous gene cluster. We confirmed the expression of all 13 enzymes by targeted proteomics and identified potential rate-limiting steps. Then, we removed the first-round selection marker using CRISPR/Cas9 and integrated an additional 7.8 kb gene cluster comprising 6 genes from C. carboxidivorans. This led to a significant increase in the hexanol titer (251 mg L−1) at the expense of butanol (158 mg L−1), when grown on CO2 and H2 in serum bottles. Fermentation of this strain at 2-L scale produced 109 mg L−1 butanol and 393 mg L−1 hexanol.ConclusionsWe thus confirmed the function of the butanol/hexanol biosynthesis genes and achieved hexanol biosynthesis in the syngas-fermenting species C. ljungdahlii for the first time, reaching the levels produced naturally by C. carboxidivorans. The genomic integration strain produced hexanol without selection and is therefore suitable for continuous fermentation processes.

Electron Recycle Concept in a Microbial Electrolysis Cell for Biogas Upgrading

AbstractAn innovative strategy to control the metabolism of microorganisms is offered by bioelectrochemical systems in which a graphite‐based cathode can be used as electron donor or acceptor. An advanced microbial electrolysis cell is developed to combine CO2 removal from a synthetic biogas in a biocathode and the organic matter oxidation in a bioanode. A novel biogas upgrading approach is presented in which an electron recycle concept is obtained by the combination of CO2 reduction and oxidation. While the bioelectrochemical anodic chemical oxygen demand oxidation provides the electrons necessary for the cathodic CO2 reduction into methane and acetate, the acetate produced by acetogenic microorganisms migrates from the cathode to the anode being oxidized again by the bioanode.

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.

Microbiomes in Soils Exposed to Naturally High Concentrations of CO2 (Bossoleto Mofette Tuscany, Italy).

Direct and indirect effects of extremely high geogenic CO2 levels, commonly occurring in volcanic and hydrothermal environments, on biogeochemical processes in soil are poorly understood. This study investigated a sinkhole in Italy where long-term emissions of thermometamorphic-derived CO2 are associated with accumulation of carbon in the topsoil and removal of inorganic carbon in low pH environments at the bottom of the sinkhole. The comparison between interstitial soil gasses and those collected in an adjacent bubbling pool and the analysis of the carbon isotopic composition of CO2 and CH4 clearly indicated the occurrence of CH4 oxidation and negligible methanogenesis in soils at the bottom of the sinkhole. Extremely high CO2 concentrations resulted in higher microbial abundance (up to 4 × 109 cell g–1 DW) and a lower microbial diversity by favoring bacteria already reported to be involved in acetogenesis in mofette soils (i.e., Firmicutes, Chloroflexi, and Acidobacteria). Laboratory incubations to test the acetogenic and methanogenic potential clearly showed that all the mofette soil supplied with hydrogen gas displayed a remarkable CO2 fixation potential, primarily due to the activity of acetogenic microorganisms. By contrast, negligible production of acetate occurred in control tests incubated with the same soils, under identical conditions, without the addition of hydrogen. In this study, we report how changes in diversity and functions of the soil microbial community – induced by high CO2 concentration – create peculiar biogeochemical profile. CO2 emission affects carbon cycling through: (i) inhibition of the decomposition of the organic carbon and (ii) promotion of CO2-fixation via the acetyl-CoA pathway. Sites naturally exposed to extremely high CO2 levels could potentially represent an untapped source of microorganisms with unique capabilities to catalytically convert CO2 into valuable organic chemicals and fuels.

Acetogenic microorganisms in operating biogas plants depending on substrate combinations

In recent years, various substrates for increasing the production of biomethane are increasingly exploring. The effect of such substrates on methanogenesis is investigated mainly in small volume digesters. However, operating acetogenic biogas plants depending on substrates combinations and monitoring of microbial communities in the anaerobic digesters located in the Czech Republic have never been studied yet. The variation in the distribution of main acetogenic genera in eight biogas plants based on different input substrates ratio was studied. The genera composition was identified by sequencing of the 16S rRNA gene. In total, 33 genera of this group of microorganisms were found out. The highest percentage (30.2%) of acetogens compared to other biogas plants was observed in Modřice biogas plant with the input substrate biological sludge and primary sludge (50/50, w/w %). The smallest percentage (1.14%) and variation in the distribution of acetogenic microorganisms was found in Cejc biogas plant, with the input substrate maize silage and liquid pig manure (80/20, w/w %). For the biogas plant operator it would be important to know the composition of a consortium of anaerobic microorganisms that are present in anaerobic digesters (in two regions in the Czech Republic) processing different input combinations of a raw material. This paper characterizes the effect of the input raw materials on the variation in the distribution of the microbial community.

Metabolite Profile Analysis of Aurantiochytrium limacinum SR21 Grown on Acetate-based Medium for Lipid Fermentation.

Thraustochytrids, a group of marine protists, are continuously gaining attention due to their capability in producing lipids for various biotechnological applications towards foods, medicines, chemicals, and biofuels. Although various substrates, predominantly glucose, have been used as carbon source for this microalga, it is desirable to adopt cheaper and more diversified substrate to expand their application range. In this study, we aimed to examine the ability of acetate, which can be easily generated from various resources by acetogenic microorganisms, as a substrate of Aurantiochytrium limacinum SR21. As a result of flask-scale analysis, specific growth rates (µ) of the strain SR21 grown in 3% acetate- or glucose-based medium were 0.55 and 0.98 h-1, respectively. The maximum yield of total fatty acid in acetate medium was 4.8 g/L at 48 h while that in glucose medium was 6.8 g/L at 30 h, indicating that acetate has potential as substrate. Metabolome analysis was performed to comprehensively elucidate characteristic metabolic fluctuations caused by acetate assimilation and identify targets to improve the fatty acid productivity from acetate. It was found that the use of glyoxylate cycle, which bypasses release of energy molecules such as NADH and GTP, and the inhibition of utilization of compounds from TCA cycle for anabolic reactions, may cause the slow growth in acetate which has an effect also in lipid productivity. The activity of the pentose phosphate pathway was found to be weak in acetate cultivation, thus NADPH was mainly produced in malate-pyruvate cycle. Lastly, mevalonate pathway was found to be activated in acetate cultivation which additionally competes with acetyl-CoA as starting material of fatty acid synthesis.

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

ABSTRACTBioaugmentation 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.

A Genetic System for the Thermophilic Acetogenic Bacterium Thermoanaerobacter kivui.

Thermoanaerobacter kivui is one of the very few thermophilic acetogenic microorganisms. It grows optimally at 66°C on sugars but also lithotrophically with H2 + CO2 or with CO, producing acetate as the major product. While a genome-derived model of acetogenesis has been developed, only a few physiological or biochemical experiments regarding the function of important enzymes in carbon and energy metabolism have been carried out. To address this issue, we developed a method for targeted markerless gene deletions and for integration of genes into the genome of T. kivui The strain naturally took up plasmid DNA in the exponential growth phase, with a transformation frequency of up to 3.9 × 10-6 A nonreplicating plasmid and selection with 5-fluoroorotate was used to delete the gene encoding the orotate phosphoribosyltransferase (pyrE), resulting in a ΔpyrE uracil-auxotrophic strain, TKV002. Reintroduction of pyrE on a plasmid or insertion of pyrE into different loci within the genome restored growth without uracil. We subsequently studied fructose metabolism in T. kivui The gene fruK (TKV_c23150) encoding 1-phosphofructosekinase (1-PFK) was deleted, using pyrE as a selective marker via two single homologous recombination events. The resulting ΔfruK strain, TKV003, did not grow on fructose; however, growth on glucose (or on mannose) was unaffected. The combination of pyrE as a selective marker and the natural competence of the strain for DNA uptake will be the basis for future studies on CO2 reduction and energy conservation and their regulation in this thermophilic acetogenic bacterium.IMPORTANCE Acetogenic bacteria are currently the focus of research toward biotechnological applications due to their potential for de novo synthesis of carbon compounds such as acetate, butyrate, or ethanol from H2 + CO2 or from synthesis gas. Based on available genome sequences and on biochemical experiments, acetogens differ in their energy metabolism. Thus, there is an urgent need to understand the carbon and electron flows through the Wood-Ljungdahl pathway and their links to energy conservation, which requires genetic manipulations such as deletion or overexpression of genes encoding putative key enzymes. Unfortunately, genetic systems have been reported for only a few acetogenic bacteria. Here, we demonstrate proof of concept for the genetic modification of the thermophilic acetogenic species Thermoanaerobacter kivui The genetic system will be used to study genes involved in biosynthesis and energy metabolism, and may further be applied to metabolically engineer T. kivui to produce fuels and chemicals.

Optimization and Molecular Characterization of Syngas Fermenting Anaerobic Mixed Microbial Consortium TERI SA1

The present study focused on the optimization and molecular characterization of anaerobic mixed consortium TERI SA1 that can utilize synthesis gas as sole carbon source for volatile fatty acids production. Optimization study using Box- Behnken design and RSM methodology was carried out in order to investigate the effect of three medium factors on metabolite formation from synthesis gas bioconversion: (yeast extract (0.0–2.0 g/L), ammonium chloride (0.0–1.5 g/L) and corn steep liquor (0.0-10 g/L). Optimized parameters enhanced the production of volatile fatty acids upto 3.9 g/L, which indicated an increase of around 289 % from the non-optimized conditions. Furthermore, two approaches were used for isolation and phylogenetic identification of anaerobic consortium TERI SA1 involving 16S rRNA sequencing of culturable bacterial isolates as well as meta-genomic approach (by making a 16S rRNA gene library of total community DNA). Based on similarity search with NCBI database selected positive clones were most closely related with acetogenic microorganisms Clostridium scatalogenes, Clostridium carboxydivorans, Clostridium drakei and Uncultured Clostridium sp. and strains isolated by culturable method (ASH051 and ASH 052) with Clostridium scatalogenes, and Clostridium drakei. These strains have previously been reported for acetic acid production from syngas bioconversion.Article History: Received July 16th 2017; Received in revised form September 13rd 2017; Accepted Sept 28th 2017; Available onlineHow to Cite This Article: Singla, A., Kumar, S., Lavania, M., Chhipa, H., Kapardar, R., Rastogi, S., Lal, B., and Sarma, P.M. (2017) Optimization and Molecular Characterization of Syngas Fermenting Anaerobic Mixed Microbial Consortium TERI SA1.International Journal of Renewable Energy Development, 6(3), 241-251.https://doi.org/10.14710/ijred.6.3.241-251

Investigation of availability of a high throughput screening method for predicting butanol solvent -producing ability of Clostridium beijerinckii.

BackgroundCurrently, efficient screening methods for selection of desired bacterial phenotypes from large populations are not easy feasible or readily available due to the complicated physiological and metabolic networks of solventogenic clostridia. In this study, to contribute to the improvement of methods for predicting the butanol-producing ability of Clostridium beijerinckii based on starch substrate, we further investigate a simple, visualization screening method for selecting target strains from mutant library of Clostridium beijerinckii NCIMB 8052 by using trypan blue dye as an indicator in solid starch via statistical survey and validation of fermentation experiment with controlling pH.ResultsTo verify an effective, efficient phenotypic screening method for isolating high butanol-producing mutants, the revalidation process was conducted based on Trypan Blue was used for visualization, and starch was used as the bacterial metabolic substrate. The availability of the screening system was further evaluated based on the relationship between characteristics of mutant strains and their α-amylase activities. Mutant clones were analyzed in detail based on their distinctive growth patterns and rate of fermentation of soluble starch to form butanol and were compared by statistical method. Significant correlations were identified between colony morphology and changes in butanol concentrations. The screening method was validated via statistical analysis for characterizing phenotypic parameters. The fermentation experiment of mutant strains with controlling pH value also demonstrated a positive correlation between increased α-amylase activity and increased solvent production by Clostridium beijerinckii was observed, and therefore indicated that the trypan blue dyeing method can be used as a fast method to screen target mutant strain for better solvent producers from, for instance, a mutant library.ConclusionsThe suitability of the novel screening procedure was validated, opening up a new indicator of approach to select mutant solventogenic clostridia with improved fermentation of starch to increase butanol concentrations. The applicability can easily be broadened to a wide range of interesting microbes such as cellulolytic or acetogenic microorganisms, which produce biofuels from feedstock rich in starch.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-016-0776-6) contains supplementary material, which is available to authorized users.

Determination of the archaeal and bacterial communities in two-phase and single-stage anaerobic systems by 454 pyrosequencing

2-Phase anaerobic digestion (AD), where the acidogenic phase was operated at 2day hydraulic retention time (HRT) and the methanogenic phase at 10days HRT, had been evaluated to determine if it could provide higher organic reduction and methane production than the conventional single-stage AD (also operated at 12days HRT). 454 pyrosequencing was performed to determine and compare the microbial communities. The acidogenic reactor of the 2-phase system yielded a unique bacterial community of the lowest richness and diversity, while bacterial profiles of the methanogenic reactor closely followed the single-stage reactor. All reactors were predominated by hydrogenotrophic methanogens, mainly Methanolinea. Unusually, the acidogenic reactor contributed up to 24% of total methane production in the 2-phase system. This could be explained by the presence of Methanosarcina and Methanobrevibacter, and their activities could also help regulate reactor alkalinity during high loading conditions through carbon dioxide production. The enrichment of hydrolytic and acidogenic Porphyromonadaceae, Prevotellaceae, Ruminococcaceae and unclassified Bacteroidetes in the acidogenic reactor would have contributed to the improved sludge volatile solids degradation, and ultimately the overall 2-phase system's performance. Syntrophic acetogenic microorganisms were absent in the acidogenic reactor but present in the downstream methanogenic reactor, indicating the retention of various metabolic pathways also found in a single-stage system. The determination of key microorganisms further expands our understanding of the complex biological functions in AD process.

Control of amphibious weed ipomoea (Ipomoea carnea) by utilizing it for the extraction of volatile fatty acids as energy precursors

Volatile fatty acids (VFAs), comprising mainly of acetic acid and lesser quantities of propionic and butyric acids, are generated when zoomass or phytomass is acted upon by acidogenic and acetogenic microorganisms. VFAs can be utilized by methanogens under anaerobic conditions to generate flammable methane–carbon dioxide mixtures known as ‘biogas’. Acting on the premise that this manner of VFA utilization for generating relatively clean energy can be easily accomplished in a controlled fashion in conventional biogas plants as well as higher-rate anaerobic digesters, we have carried out studies aimed to generate VFAs from the pernicious weed ipomoea (Ipomoea carnea). The VFA extraction was accomplished by a simple yet effective technology, appropriate for use even by laypersons. For this acid-phase reactors were set, to which measured quantities of ipomoea leaves were charged along with water inoculated with cow dung. The reactors were stirred intermittently. It was found that VFA production started within hours of the mixing of the reactants and peaked by the 10th or 11th day in all the reactors, effecting a conversion of over 10% of the biomass into VFAs. The reactor performance had good reproducibility and the process appeared easily controllable, frugal and robust.

Sulfide-Driven Microbial Electrosynthesis

Microbial electrosynthesis, the conversion of carbon dioxide to organic molecules using electricity, has recently been demonstrated for acetogenic microorganisms, such as Sporomusa ovata. The energy for reduction of carbon dioxide originates from the hydrolysis of water on the anode, requiring a sufficiently low potential. Here we evaluate the use of sulfide as an electron source for microbial electrosynthesis. Abiotically oxidation of sulfide on the anode yields two electrons. The oxidation product, elemental sulfur, can be further oxidized to sulfate by Desulfobulbus propionicus, generating six additional electrons in the process. The eight electrons generated from the combined abiotic and biotic steps were used to reduce carbon dioxide to acetate on a graphite cathode by Sporomusa ovata at a rate of 24.8 mmol/day · m(2). Using a strain of Desulfuromonas as biocatalyst on the anode resulted in an acetate production rate of 49.9 mmol/day · m(2), with a Coulombic efficiency of over 90%. These results demonstrate that sulfide can serve effectively as an alternative electron donor for microbial electrosynthesis.

Evaluation of two-component Fe(0) fixed bed filters with porous materials for reductive dechlorination

Although successfully applied, Fe(0) fixed bed reactors, so called permeable reactive barriers, bear the risk of reactivity and permeability losses. Mixing of corrosive cast iron granules with composite filter materials was assumed as promising countermeasure to both problems. In the present study, inert sand, gravel and porous pumice and anthracite were tested in long term column experiments as composite materials in two component reactive barriers treating TCE contaminated groundwater. Tracer breakthrough analyses and gravimetric measurements confirmed that porosity differed for the tested materials but gas was entrapped in all columns. Hydrogen was collected at the effluent of all columns within 50days but was then completely consumed by hydrogenotrophic sulfate reducing and acetogenic microorganisms. Trichloroethylene was completely removed but overall contaminant removal decreased in the long term due to increased effluent concentrations of the metabolite cis-dichloroethylene.

Electrosynthesis of organic compounds from carbon dioxide is catalyzed by a diversity of acetogenic microorganisms.

Microbial electrosynthesis, a process in which microorganisms use electrons derived from electrodes to reduce carbon dioxide to multicarbon, extracellular organic compounds, is a potential strategy for capturing electrical energy in carbon-carbon bonds of readily stored and easily distributed products, such as transportation fuels. To date, only one organism, the acetogen Sporomusa ovata, has been shown to be capable of electrosynthesis. The purpose of this study was to determine if a wider range of microorganisms is capable of this process. Several other acetogenic bacteria, including two other Sporomusa species, Clostridium ljungdahlii, Clostridium aceticum, and Moorella thermoacetica, consumed current with the production of organic acids. In general acetate was the primary product, but 2-oxobutyrate and formate also were formed, with 2-oxobutyrate being the predominant identified product of electrosynthesis by C. aceticum. S. sphaeroides, C. ljungdahlii, and M. thermoacetica had high (>80%) efficiencies of electrons consumed and recovered in identified products. The acetogen Acetobacterium woodii was unable to consume current. These results expand the known range of microorganisms capable of electrosynthesis, providing multiple options for the further optimization of this process.

Reaction engineering analysis of hydrogenotrophic production of acetic acid by Acetobacterium woodii

Great interest has emerged in biological CO₂-fixing processes in the context of current climate change discussions. One example for such a process is the hydrogenotrophic production of acetic acid by anaerobic microorganisms. Acetogenic microorganisms make use of carbon dioxide in the presence of hydrogen to produce acetic acid and biomass. In order to establish a process for the hydrogenotrophic production of acetic acid, the formation of acetate by Acetobacterium woodii was studied in a batch-operated stirred-tank bioreactor at different hydrogen partial pressures (pH₂) in the gas phase. The volumetric productivity of the batch processes increased with increasing hydrogen partial pressure. A maximum of the volumetric productivity of 7.4 g(acetate) L⁻¹ day⁻¹ was measured at a pH₂ of 1,700 mbar. At this pH(2) a final acetate concentration of 44 g L⁻¹ was measured after a process time of 11 days, if the pH was controlled at pH 7.0 (average cell density of 1.1 g L⁻¹ cell dry weight). The maximum cell specific actetate productivity was 6.9 g(acetate) g(cdw)⁻¹ day⁻¹ under hydrogenotrophic conditions.

New molecular tools for enhancing methane production, explaining thermodynamically limited lifestyles and other important biotechnological issues

a Planta, 14071, Cordoba, Spain. In this special issue a series of articles appear detailing methods of energy generation under different conditions; processes which are related to the future vision of microbes as a source of electric power (Lovley, 2009). Methanogenesis is identified as an important part of the biogeochemical carbon cycle in diverse anaerobic envi- ronments. Methane is an important fuel that can be gen- erated from several wastes in these anoxic environments and therefore, understanding how it is produced, which organisms produce it and the reactions required to achieve high yields is of major biotechnological interest. Alves and colleagues (2009) present a mini-review on the use of lipids as substrates for methane production. Conversion of lipids into methane is achieved by the syntrophic consortia of acetogenic bacteria and methano- genic archeae. The microbes involved are influenced by the nature of the incoming lipids and examples of different microbial communities when feeding reactors with oleate or palmitate are reviewed. Alves and colleagues (2009) point out that the number of acetogenic microorganisms that degrade butyrate or higher fatty acids is very low and includes microbes of the Syntrophaceae and Syntroph- omonadaceae families. These microbes live together with hydrogen-consuming methanogenic archeae and sulfate- reducing bacteria. The syntrophic cooperation seems to be optimal when microbes are organized as microcolo- nies, which is considered to favour interspecies hydrogen transfer. As in many biotechnological processes, lipid conversion into methane requires both the appropriate microbes and their performance optimization in the reac- tors, each of which still require some improvement. The authors note that previous failures to deal with high load lipids were related mainly to sludge flotation and biomass washout. To overcome these limitations the authors describe a new reactor concept which involves primary biomass retention through flotation and secondary biomass retention via settling. These reactors are being tested in scale and if, as expected, they solve the problem may soon come into industrial use. The seminal review of Bernhard Schinck (Schink, 1997)