Traits of selected Clostridium strains for syngas fermentation to ethanol.

Syngas fermentation is an anaerobic bioprocess that could become industrially relevant as a biorefinery platform for sustainable production of fuels and chemicals. An important prerequisite for commercialization is adequate performance of the biocatalyst (i.e., sufficiently high production rate, titer, selectivity, yield, and stability of the fermentation). Here, we compared the performance of three potential candidate Clostridium strains in syngas-to-ethanol conversion: Clostridium ljungdahlii PETC, C. ljungdahlii ERI-2, and Clostridium autoethanogenum JA1-1. Experiments were conducted in a two-stage, continuously fed syngas-fermentation system that had been optimized for stable ethanol production. The two C. ljungdahlii strains performed similar to each other but different from C. autoethanogenum. When the pH value was lowered from 5.5 to 4.5 to induce solventogenesis, the cell-specific carbon monoxide and hydrogen consumption (similar rate for all strains at pH 5.5), severely decreased in JA1-1, but hardly in PETC and ERI-2. Ethanol production in strains PETC and ERI-2 remained relatively stable while the rate of acetate production decreased, resulting in a high ethanol/acetate ratio, but lower overall productivities. With JA1-1, lowering the pH severely lowered rates of both ethanol and acetate production; and as a consequence, no pronounced shift to solventogenesis was observed. The highest overall ethanol production rate of 0.301 g · L(-1) · h(-1) was achieved with PETC at pH 4.5 with a corresponding 19 g/L (1.9% w/v) ethanol concentration and a 5.5:1 ethanol/acetate molar ratio. A comparison of the genes relevant for ethanol metabolism revealed differences between C. ljungdahlii and C. autoethanogenum that, however, did not conclusively explain the different phenotypes.

Chain Elongation with Reactor Microbiomes: Open-Culture Biotechnology To Produce Biochemicals.

Chain elongation into medium-chain carboxylates, such as n-caproate and n-caprylate, with ethanol as an electron donor and with open cultures of microbial consortia (i.e., reactor microbiomes) under anaerobic conditions is being developed as a biotechnological production platform. The goal is to use the high thermodynamic efficiency of anaerobic fermentation to convert organic biomass or organic wastes into valuable biochemicals that can be extracted. Several liter-scale studies have been completed and a first pilot-plant study is underway. However, the underlying microbial pathways are not always well understood. In addition, an interdisciplinary approach with knowledge from fields ranging from microbiology and chemical separations to biochemistry and environmental engineering is required. To bring together research from different fields, we reviewed the literature starting with the microbiology and ending with the bioprocess engineering studies that already have been performed. Because understanding the microbial pathways is so important to predict and steer performance, we delved into a stoichiometric and thermodynamic model that sheds light on the effect of substrate ratios and environmental conditions on product formation. Finally, we ended with an outlook.

Biocatalytic reduction of short-chain carboxylic acids into their corresponding alcohols with syngas fermentation.

Short-chain carboxylic acids generated by various mixed- or pure-culture fermentation processes have been considered valuable precursors for production of bioalcohols. While conversion of carboxylic acids into alcohols is routinely performed with catalytic hydrogenation or with strong chemical reducing agents, here, a biological conversion route was explored. The potential of carboxydotrophic bacteria, such as Clostridium ljungdahlii and Clostridium ragsdalei, as biocatalysts for conversion of short-chain carboxylic acids into alcohols, using syngas as a source of electrons and energy is demonstrated. Acetic acid, propionic acid, n-butyric acid, isobutyric acid, n-valeric acid, and n-caproic acid were converted into their corresponding alcohols. Furthermore, biomass yields and fermentation stoichiometry from the experimental data were modeled to determine how much metabolic energy C. ljungdahlii generated during syngas fermentation. An ATP yield of 0.4-0.5 mol of ATP per mol CO consumed was calculated in the presence of hydrogen. The ratio of protons pumped across the cell membrane versus electrons transferred from ferredoxin to NAD(+) via the Rnf complex is suggested to be 1.0. Based on these results, we provide suggestions how n-butyric acid to n-butanol conversion via syngas fermentation can be further improved.

Integrating syngas fermentation with the carboxylate platform and yeast fermentation to reduce medium cost and improve biofuel productivity.

To ensure economic implementation of syngas fermentation as a fuel-producing platform, engineers and scientists must both lower operating costs and increase product value. A considerable part of the operating costs is spent to procure chemicals for fermentation medium that can sustain sufficient growth of carboxydotrophic bacteria to convert synthesis gas (syngas: carbon monoxide, hydrogen, and carbon dioxide) into products such as ethanol. Recently, we have observed that wildtype carboxydotrophic bacteria (including Clostridium ljungdahlii) can produce alcohols with a longer carbon chain than ethanol via syngas fermentation when supplied with the corresponding carboxylic acid precursors, resulting in possibilities of increasing product value. Here, we evaluated a proof-of-concept system to couple syngas fermentation with the carboxylate platform to both lower medium costs and increase product value. Our carboxylate platform concept consists of an open culture, anaerobic fermentor that is fed with corn beer from conventional yeast fermentation in the corn kernel-to-ethanol industry. The mixed-culture anaerobic fermentor produces a mixture ofcarboxylic acids at dilute concentrations within the carboxylate platform effluent (CPE). Besides providing carboxylic acid precursors, this effluent may represent an inexpensive growth medium. An elemental analysis demonstrated that the CPE lacked certain essential trace metals, but contained ammonium, phosphate, sodium, chloride, potassium, magnesium, calcium, and sulphate at required concentrations. CPE medium with the addition of a trace metal solution supported growth and alcohol production of C. ljungdahlii at similar or better levels compared with an optimized synthetic medium (modified ATCC 1754 medium). Other expensive supplements, such as yeast extract or macro minerals (ammonium, phosphate), were not required. Finally, n-butyric acid and n-caproic acid within the CPE were converted into their corresponding medium-chain alcohols n-butanol and n-hexanol.

Prolonged conversion of n-butyrate to n-butanol with Clostridium saccharoperbutylacetonicum in a two-stage continuous culture with in-situ product removal.

n-Butanol was produced continuously in a two-stage fermentor system with integrated product removal from a co-feed of n-butyric acid and glucose. Glucose was always required as a source of ATP and electrons for the conversion of n-butyrate to n-butanol and for biomass growth; for the latter it also served as a carbon source. The first stage generated metabolically active planktonic cells of Clostridium saccharoperbutylacetonicum strain N1-4 that were continuously fed into the second (production) stage; the volumetric ratio of the two fermentors was 1:10. n-Butanol was removed continuously from the second stage via gas stripping. Implementing a two-stage process was observed to dramatically dampen metabolic oscillations (i.e., periodical changes of solventogenic activity). Culture degeneration (i.e., an irreversible loss of solventogenic activity) was avoided by periodical heat shocking and re-inoculating stage 1 and by maintaining the concentration of undissociated n-butyric acid in stage 2 at 3.4 mM with a pH-auxostat. The system was successfully operated for 42 days during which 93% of the fed n-butyrate was converted to n-butanol at a production rate of 0.39 g/(L × h). The molar yields Y(n-butanol/n-butyrate) and Y(n-butanol/glucose) were 2.0, and 0.718, respectively. For the same run, the molar ratio of n-butyrate to glucose consumed was 0.358. The molar yield of carbon in n-butanol produced from carbon in n-butyrate and glucose consumed (Y(n-butanol/carbon) ) was 0.386. These data illustrate that conversion of n-butyrate into n-butanol by solventogenic Clostridium species is feasible and that this can be performed in a continuous system operating for longer than a month. However, our data also demonstrate that a relatively large amount of glucose is required to supply electrons and ATP for this conversion and for cell growth in a continuous culture.

Role of secondary transporters and phosphotransferase systems in glucose transport by Oenococcus oeni.

Glucose uptake by the heterofermentative lactic acid bacterium Oenococcus oeni B1 was studied at the physiological and gene expression levels. Glucose- or fructose-grown bacteria catalyzed uptake of [(14)C]glucose over a pH range from pH 4 to 9, with maxima at pHs 5.5 and 7. Uptake occurred in two-step kinetics in a high- and low-affinity reaction. The high-affinity uptake followed Michaelis-Menten kinetics and required energization. It accumulated the radioactivity of glucose by a factor of 55 within the bacteria. A large portion (about 80%) of the uptake of glucose was inhibited by protonophores and ionophores. Uptake of the glucose at neutral pH was not sensitive to degradation of the proton potential, Δp. Expression of the genes OEOE_0819 and OEOE_1574 (here referred to as 0819 and 1574), coding for secondary transporters, was induced by glucose as identified by quantitative real-time (RT)-PCR. The genes 1574 and 0819 were able to complement growth of a Bacillus subtilis hexose transport-deficient mutant on glucose but not on fructose. The genes 1574 and 0819 therefore encode secondary transporters for glucose, and the transports are presumably Δp dependent. O. oeni codes, in addition, for a phosphotransferase transport system (PTS) (gene OEOE_0464 [0464] for the permease) with similarity to the fructose- and mannose-specific PTS of lactic acid bacteria. Quantitative RT-PCR showed induction of the gene 0464 by glucose and by fructose. The data suggest that the PTS is responsible for Δp-independent hexose transport at neutral pH and for the residual Δp-independent transport of hexoses at acidic pH.

Metabolic engineering of Rhodopseudomonas palustris for the obligate reduction of n-butyrate to n-butanol.

BACKGROUND: Rhodopseudomonas palustris is a versatile microbe that encounters an innate redox imbalance while growing photoheterotrophically with reduced substrates. The resulting excess in reducing equivalents, together with ATP from photosynthesis, could be utilized to drive a wide range of bioconversions. The objective of this study was to genetically modify R. palustris to provide a pathway to reduce n-butyrate into n-butanol for maintaining redox balance. RESULTS: Here, we constructed and expressed a plasmid-based pathway for n-butanol production from Clostridium acetobutylicum ATCC 824 in R. palustris. We maintained the environmental conditions in such a way that this pathway functioned as the obligate route to re-oxidize excess reducing equivalents, resulting in an innate selection pressure. The engineered strain of R. palustris grew under otherwise restrictive redox conditions and achieved concentrations of 1.5 mM n-butanol at a production rate of 0.03 g L-1 day-1 and a selectivity (i.e., products compared to the consumed substrate) of close to 40%. Since the theoretical maximum selectivity is 45%, the engineered strain converted close to its maximum selectivity. CONCLUSIONS: The innate redox imbalance of R. palustris can be used to drive the reduction of n-butyrate into n-butanol after expression of a plasmid-based enzyme from a butanol-producing Clostridium strain.

A Narrow pH Range Supports Butanol, Hexanol, and Octanol Production from Syngas in a Continuous Co-culture of Clostridium ljungdahlii and Clostridium kluyveri with In-Line Product Extraction.

Carboxydotrophic bacteria (CTB) have received attention due to their ability to synthesize commodity chemicals from producer gas and synthesis gas (syngas). CTB have an important advantage of a high product selectivity compared to chemical catalysts. However, the product spectrum of wild-type CTB is narrow. Our objective was to investigate whether a strategy of combining two wild-type bacterial strains into a single, continuously fed bioprocessing step would be promising to broaden the product spectrum. Here, we have operated a syngas-fermentation process with Clostridium ljungdahlii and Clostridium kluyveri with in-line product extraction through gas stripping and product condensing within the syngas recirculation line. The main products from C. ljungdahlii fermentation at a pH of 6.0 were ethanol and acetate at net volumetric production rates of 65.5 and 431 mmol C·L-1·d-1, respectively. An estimated 2/3 of total ethanol produced was utilized by C. kluyveri to chain elongate with the reverse ß-oxidation pathway, resulting in n-butyrate and n-caproate at net rates of 129 and 70 mmol C·L-1·d-1, respectively. C. ljungdahlii likely reduced the produced carboxylates to their corresponding alcohols with the reductive power from syngas. This resulted in the longer-chain alcohols n-butanol, n-hexanol, and n-octanol at net volumetric production rates of 39.2, 31.7, and 0.045 mmol C·L-1·d-1, respectively. The continuous production of the longer-chain alcohols occurred only within a narrow pH spectrum of 5.7-6.4 due to the pH discrepancy between the two strains. Regardless whether other wild-type strains could overcome this pH discrepancy, the specificity (mol carbon in product per mol carbon in all other liquid products) for each longer-chain alcohol may never be high in a single bioprocessing step. This, because two bioprocesses compete for intermediates (i.e., carboxylates): (1) chain elongation; and (2) biological reduction. This innate competition resulted in a mixture of n-butanol and n-hexanol with traces of n-octanol.

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

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

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

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

Chain elongation in anaerobic reactor microbiomes to recover resources from waste.

Different microbial pathways can elongate the carbon chains of molecules in open cultures of microbial populations (i.e. reactor microbiomes) under anaerobic conditions. Here, we discuss three such pathways: 1. homoacetogenesis to combine two carbon dioxide molecules into acetate; 2. succinate formation to elongate glycerol with one carbon from carbon dioxide; and 3. reverse ß oxidation to elongate short-chain carboxylates with two carbons into medium-chain carboxylates, leading to more energy-dense and insoluble products (e.g. easier to separate from solution). The ability to use reactor microbiomes to treat complex substrates can simultaneously address two pressing issues: 1. providing proper waste management; and 2. producing renewable chemicals and fuels.

Genome-scale dynamic modeling of the competition between Rhodoferax and Geobacter in anoxic subsurface environments.

The advent of rapid complete genome sequencing, and the potential to capture this information in genome-scale metabolic models, provide the possibility of comprehensively modeling microbial community interactions. For example, Rhodoferax and Geobacter species are acetate-oxidizing Fe(III)-reducers that compete in anoxic subsurface environments and this competition may have an influence on the in situ bioremediation of uranium-contaminated groundwater. Therefore, genome-scale models of Geobacter sulfurreducens and Rhodoferax ferrireducens were used to evaluate how Geobacter and Rhodoferax species might compete under diverse conditions found in a uranium-contaminated aquifer in Rifle, CO. The model predicted that at the low rates of acetate flux expected under natural conditions at the site, Rhodoferax will outcompete Geobacter as long as sufficient ammonium is available. The model also predicted that when high concentrations of acetate are added during in situ bioremediation, Geobacter species would predominate, consistent with field-scale observations. This can be attributed to the higher expected growth yields of Rhodoferax and the ability of Geobacter to fix nitrogen. The modeling predicted relative proportions of Geobacter and Rhodoferax in geochemically distinct zones of the Rifle site that were comparable to those that were previously documented with molecular techniques. The model also predicted that under nitrogen fixation, higher carbon and electron fluxes would be diverted toward respiration rather than biomass formation in Geobacter, providing a potential explanation for enhanced in situ U(VI) reduction in low-ammonium zones. These results show that genome-scale modeling can be a useful tool for predicting microbial interactions in subsurface environments and shows promise for designing bioremediation strategies.

Electricity generation by Geobacter sulfurreducens attached to gold electrodes.

The versatility of gold for electrode manufacture suggests that it could be an ideal material for some microbial fuel cell applications. However, previous studies have suggested that microorganisms that readily transfer electrons to graphite do not transfer electrons to gold. Investigations with Geobacter sulfurreducens demonstrated that it could grow on gold anodes producing current nearly as effectively as with graphite anodes. Current production was associated with the development of G. sulfurreducens biofilms up to 40 microm thick. No current was produced if pilA, the gene for the structural protein of the conductive pili of G. sulfurreducens, was deleted. The finding that gold is a suitable anode material for microbial fuel cells offers expanded possibilities for the construction of microbial fuel cells and the electrochemical analysis of microbe-electrode interactions.

Lack of electricity production by Pelobacter carbinolicus indicates that the capacity for Fe(III) oxide reduction does not necessarily confer electron transfer ability to fuel cell anodes.

The ability of Pelobacter carbinolicus to oxidize electron donors with electron transfer to the anodes of microbial fuel cells was evaluated because microorganisms closely related to Pelobacter species are generally abundant on the anodes of microbial fuel cells harvesting electricity from aquatic sediments. P. carbinolicus could not produce current in a microbial fuel cell with electron donors which support Fe(III) oxide reduction by this organism. Current was produced using a coculture of P. carbinolicus and Geobacter sulfurreducens with ethanol as the fuel. Ethanol consumption was associated with the transitory accumulation of acetate and hydrogen. G. sulfurreducens alone could not metabolize ethanol, suggesting that P. carbinolicus grew in the fuel cell by converting ethanol to hydrogen and acetate, which G. sulfurreducens oxidized with electron transfer to the anode. Up to 83% of the electrons available in ethanol were recovered as electricity and in the metabolic intermediate acetate. Hydrogen consumption by G. sulfurreducens was important for ethanol metabolism by P. carbinolicus. Confocal microscopy and analysis of 16S rRNA genes revealed that half of the cells growing on the anode surface were P. carbinolicus, but there was a nearly equal number of planktonic cells of P. carbinolicus. In contrast, G. sulfurreducens was primarily attached to the anode. P. carbinolicus represents the first Fe(III) oxide-reducing microorganism found to be unable to produce current in a microbial fuel cell, providing the first suggestion that the mechanisms for extracellular electron transfer to Fe(III) oxides and fuel cell anodes may be different.

Pyruvate fermentation by Oenococcus oeni and Leuconostoc mesenteroides and role of pyruvate dehydrogenase in anaerobic fermentation.

The heterofermentative lactic acid bacteria Oenococcus oeni and Leuconostoc mesenteroides are able to grow by fermentation of pyruvate as the carbon source (2 pyruvate --> 1 lactate + 1 acetate + 1 CO(2)). The growth yields amount to 4.0 and 5.3 g (dry weight)/mol of pyruvate, respectively, suggesting formation of 0.5 mol ATP/mol pyruvate. Pyruvate is oxidatively decarboxylated by pyruvate dehydrogenase to acetyl coenzyme A, which is then converted to acetate, yielding 1 mol of ATP. For NADH reoxidation, one further pyruvate molecule is reduced to lactate. The enzymes of the pathway were present after growth on pyruvate, and genome analysis showed the presence of the corresponding structural genes. The bacteria contain, in addition, pyruvate oxidase activity which is induced under microoxic conditions. Other homo- or heterofermentative lactic acid bacteria showed only low pyruvate fermentation activity.

Use of the mannitol pathway in fructose fermentation of Oenococcus oeni due to limiting redox regeneration capacity of the ethanol pathway.

The heterolactic bacterium Oenococcus oeni ferments fructose by a mixed heterolactic/mannitol fermentation. For heterolactic fermentation of fructose, the phosphoketolase pathway is used. The excess NAD(P)H from the phosphoketolase pathway is reoxidized by fructose (yielding mannitol). It is shown here that, under conditions of C-limitation or decreased growth rates, fructose can be fermented by heterolactic fermentation yielding nearly stoichiometric amounts of lactate, ethanol and CO(2). Quantitative evaluation of NAD(P)H-producing (phosphoketolase pathway) and -reoxidizing (ethanol, mannitol and erythritol pathways) reactions demonstrated that at high growth rates or in batch cultures the ethanol pathway does not have sufficient capacity for NAD(P)H reoxidation, requiring additional use of the mannitol pathway to maintain the growth rate. In addition, insufficient capacities to reoxidize NAD(P)H causes inhibition of growth, whereas increased NAD(P)H reoxidation by electron acceptors such as pyruvate increases the growth rate.

Vulnerability curves from conifer sapwood sections exposed over solutions with known water potentials.

The cohesion-tension (CT) theory requires stability of liquid water in conducting elements under high tensions. This stability has been measured using different methods, some of which yielded contradictory results. In this study a method is presented to establish known tensions in the water inside conifer tracheids, to detect cavitation events under these conditions and to construct vulnerability curves. Tangential sapwood sections of Juniperus virginiana L. were placed closely over the surface of NaCl solutions with water potentials ranging from -0.91 to -7.57 MPa. Water potentials were measured with a thermocouple hygrometer in contact with the section, and ultrasound acoustic emissions (UAE) from the sections were registered with an ultrasound transducer. The emission rate of signals increased with the concentration of the solution. Exposure of 100 microm sections in the airspace over a solution provided optimal conditions for the rupture of the water column: many tracheid walls bordered on air, and water in the lumen came under high tension. Nevertheless, the water remained in the metastable liquid state for periods of many hours. The vulnerability obtained from simultaneous measurements of water potentials and ultrasound acoustic emissions on sapwood sections was substantially higher than from conventionally measured curves of detached branches. It is argued that the isolation of tracheids in a massive organ as well as the rate of potential decline will influence the probability of cavitations at a given water potential and thus the parameters of the vulnerability curve.

Significance of phosphoglucose isomerase for the shift between heterolactic and mannitol fermentation of fructose by Oenococcus oeni.

The bacterium Oenococcus oeni employs the heterolactic fermentation pathway (products lactate, ethanol, CO(2)) during growth on fructose as a substrate, and the mannitol pathway when using fructose as an electron acceptor. In this study, [U-(13)C]glucose, [U-(13)C]fructose, HPLC, NMR spectroscopy, and enzyme analysis were applied to elucidate the use of both pathways by the hexoses. In the presence of glucose or pyruvate, fructose was metabolized either by the mannitol or the phosphoketolase pathways, respectively. Phosphoglucose isomerase, which is required for channeling fructose into the phosphoketolase pathways, was inhibited by a mixed-type inhibition composed of competitive ( K(i)=180 microM) and uncompetitive ( K'(i)=350 microM) inhibition by 6-phosphogluconate. Erythrose 4-phosphate inhibited phosphoglucose isomerase competitively ( K(i)=1.3 microM) with a low contribution of uncompetitive inhibition ( K'(i)=13 microM). The cellular 6-phosphogluconate content during growth on fructose plus pyruvate (<75 microM) was significantly lower than during growth on fructose alone or fructose plus glucose (550 and 480 microM). We conclude that competitive inhibition of phosphoglucose isomerase by 6-phosphogluconate (and possibly erythrose 4-phosphate) is responsible for exclusion of fructose from the phosphoketolase pathway during growth on fructose plus glucose, but not during growth on fructose plus pyruvate.

A new method to determine the oxygen concentration inside the sapwood of trees.

Research into the short-term fluctuations of oxygen concentrations in tree stems has been hampered by the difficulty of measuring oxygen inside tissues. A new method, which is based on fluorescence quenching of a ruthenium complex in the presence of oxygen, has been applied to measure changes of oxygen concentration in the sapwood of trees. During a field day-course oxygen increased with the radiation load and fell during the night (in Fagus orientalis from 20.3% in the afternoon to 17.5% in the morning next day). In a greenhouse experiment the sapwood oxygen concentration of Laurus nobilis could be influenced by flooding the root system. The very fast response, high resolution (better than 0.1%), easy calibration, and dependence only on oxygen and temperature make the technique well suited for measurements of oxygen concentrations in the sapwood.