Xylose anaerobic conversion by open-mixed cultures

Caproate production from xylose via the fatty acid biosynthesis pathway by genus Caproiciproducens dominated mixed culture fermentation

Caproate production from organic wastes is deemed as a novel strategy in mixed culture fermtation (MCF). However, producing caproate from natural sugar of xylose by MCF is seldom reported and the metabolic pathway is still unclear. Thus, the caproate production from xylose was investigated in this study by mesophilic MCF. The results showed that the caproate concentration from xylose (10 g/L) was 1.2 ± 0.17 g/L (equal to 2.7 gCOD/L) under pH 5.0. Dosing extra ethanol of 5 g/L could slightly increase the caproate production by ∼ 30% (i.e., 1.6 g/L). While dosing extra acetate of 5 g/L negatively affected the caproate production, which was just 0.2 g/L. The microbial analysis illustrated that genus Caproiciproducens was a main identified caproate producer, occupying over 80% of enriched mixed culture. The fatty acid biosynthesis pathway was identified via metagenomic analysis. These unexpected differences extended the understanding of caproate production from organic wastes.

Machine learning-assisted identification of bioindicators predicts medium-chain carboxylate production performance of an anaerobic mixed culture

BACKGROUND

The ability to quantitatively predict ecophysiological functions of microbial communities provides an important step to engineer microbiota for desired functions related to specific biochemical conversions. Here, we present the quantitative prediction of medium-chain carboxylate production in two continuous anaerobic bioreactors from 16S rRNA gene dynamics in enriched communities.

RESULTS

By progressively shortening the hydraulic retention time (HRT) from 8 to 2 days with different temporal schemes in two bioreactors operated for 211 days, we achieved higher productivities and yields of the target products n-caproate and n-caprylate. The datasets generated from each bioreactor were applied independently for training and testing machine learning algorithms using 16S rRNA genes to predict n-caproate and n-caprylate productivities. Our dataset consisted of 14 and 40 samples from HRT of 8 and 2 days, respectively. Because of the size and balance of our dataset, we compared linear regression, support vector machine and random forest regression algorithms using the original and balanced datasets generated using synthetic minority oversampling. Further, we performed cross-validation to estimate model stability. The random forest regression was the best algorithm producing more consistent results with median of error rates below 8%. More than 90% accuracy in the prediction of n-caproate and n-caprylate productivities was achieved. Four inferred bioindicators belonging to the genera Olsenella, Lactobacillus, Syntrophococcus and Clostridium IV suggest their relevance to the higher carboxylate productivity at shorter HRT. The recovery of metagenome-assembled genomes of these bioindicators confirmed their genetic potential to perform key steps of medium-chain carboxylate production.

CONCLUSIONS

Shortening the hydraulic retention time of the continuous bioreactor systems allows to shape the communities with desired chain elongation functions. Using machine learning, we demonstrated that 16S rRNA amplicon sequencing data can be used to predict bioreactor process performance quantitatively and accurately. Characterizing and harnessing bioindicators holds promise to manage reactor microbiota towards selection of the target processes. Our mathematical framework is transferrable to other ecosystem processes and microbial systems where community dynamics is linked to key functions. The general methodology used here can be adapted to data types of other functional categories such as genes, transcripts, proteins or metabolites. Video Abstract.

Decoupling mechanism of Acid Orange 7 decolorization and sulfate reduction by a Caldanaerobacter dominated extreme-thermophilic consortium

The biological treatment of textile wastewater discharged from the dye baths and rinsing processes are challenged by both high temperatures of 50-80 °C and sulfate reduction. At present, most studies report azo dyes can be removed under mesophilic conditions, but the sulfate reduction is inevitable, consuming extra electron donors and producing undesirable sulfide. In this work, a Caldanaerobacter (> 97%) dominated extreme-thermophilic consortium (EX-AO7) was enriched using xylose as the substrate. The typical sulfate-reducing enzymes such as sulfite oxidase and sulfite reductase were not identified in enriched EX-AO7 by the metagenomic analysis. Then, the decolorization and sulfate reduction were expectedly decoupled by enriched EX-AO7 in extreme-thermophilic conditions, in which no sulfide was detected during the AO7 decolorization process. AO7 of 100 and 200 mg/L could be totally decolorized by EX-AO7. However, when 400 mg/L AO7 was added, the residual AO7 concentration was 22 ± 19 mg/L after 24 h, which was mainly due to the toxicity of AO7. Dosing zero-valent iron (ZVI) could also promote AO7 decolorization by 1.7 times since the addition of ZVI could provide a proliferative environment for EX-AO7 growth. Thereby, our work provides a new paradigm to promote the AZO dyes decolorization and minimize sulfate reduction.

Role of xylose from acidic hydrolysates of agave bagasse during biohydrogen production

This study compares the H₂ production from glucose, xylose, and acidic hydrolysates of Agave tequilana bagasse as substrates. The fermentation was performed in a granular sludge reactor operated in two phases: (1) model substrates (glucose and xylose) and (2) acidic hydrolysates at 35 °C, pH 4.5 and a hydraulic retention time of 5.5 h with glucose (10 g L^-1) and xylose (12 g L^-1). A sequencing batch reactor was used to acclimate the biomass between the glucose and xylose continuous fermentation (with a mixture of xylose-glucose) and acidic hydrolysates. During the discontinuous acclimating step, the xylose/glucose ratio increment negatively affected the H₂ productivity. Although the continuous H₂ production with xylose was negligible, the co-fermentation with glucose (88-12%) allowed H₂ productivity of 2,889 ± 502 mL H₂ L^-1d^-1. An acidic hydrolysate concentration of 3.3 g_carbohydrate L^-1 showed a three-fold higher H₂ productivity than with a concentration of 10 g L^-1. The results indicated that xylose, as the only substrate, was challenging to metabolize by the inoculum, and its mixture with glucose improved the H₂ productivity. Therefore, the low H₂ productivity with hydrolysates could be related to the presence of xylose.

Product Inhibition and pH Affect Stoichiometry and Kinetics of Chain Elongating Microbial Communities in Sequencing Batch Bioreactors

Anaerobic microbial communities can produce carboxylic acids of medium chain length (e.g., caproate, caprylate) by elongating short chain fatty acids through reversed β-oxidation. Ethanol is a common electron donor for this process. The influence of environmental conditions on the stoichiometry and kinetics of ethanol-based chain elongation remains elusive. Here, a sequencing batch bioreactor setup with high-resolution off-gas measurements was used to identify the physiological characteristics of chain elongating microbial communities enriched on acetate and ethanol at pH 7.0 ± 0.2 and 5.5 ± 0.2. Operation at both pH-values led to the development of communities that were highly enriched (>50%, based on 16S rRNA gene amplicon sequencing) in Clostridium kluyveri related species. At both pH-values, stably performing cultures were characterized by incomplete substrate conversion and decreasing biomass-specific hydrogen production rates during an operational cycle. The process stoichiometries obtained at both pH-values were different: at pH 7.0, 71 ± 6% of the consumed electrons were converted to caproate, compared to only 30 ± 5% at pH 5.5. Operating at pH 5.5 led to a decrease in the biomass yield, but a significant increase in the biomass-specific substrate uptake rate, suggesting that the organisms employ catabolic overcapacity to deal with energy losses associated to product inhibition. These results highlight that chain elongating conversions rely on a delicate balance between substrate uptake- and product inhibition kinetics.

Substrate availability drives mixed culture fermentation of glucose to lactate at steady state

Mixed-culture fermentation (MCF) enables carbon recycling from complex organic waste streams into valuable feedstock chemicals. Using complex microbial consortia, MCF systems can be tuned to produce a range of biochemicals to meet market demand. However, the metabolic mechanisms and community interactions which drive biochemical production changes under differing conditions are currently poorly understood. These mechanisms are critical to useful MCF production models. Furthermore, predictable product transitions are currently limited to pH-driven changes between butyrate and ethanol, and chain-elongation (fed by lactate, acetate, and ethanol) to butyrate, valerate, and hexanoate. Lactate, a high-value biopolymer feedstock chemical, has been observed in transition states, but sustained production has not been described. In this study, steady state lactate production was achieved by increasing the organic loading rate of a butyrate-producing system from limiting to nonlimiting conditions at pH 5.5. Crucially, butyrate production resumed upon return to substrate-limited conditions. 16S ribosomal DNA community profiling combined with metaproteomics demonstrated that the butyrate-producing lineage Megasphaera redirected carbon flow through the methylglyoxal bypass when substrate was nonlimiting, which altered the community structure and metabolic expression toward lactate production. This metabolic mechanism can be included in future MCF models to describe the changes in product generation in substrate nonlimiting conditions.

The impact of mixtures of xylose and glucose on the microbial diversity and fermentative metabolism of sequencing-batch or continuous enrichment cultures

Efficient industrial fermentation of lignocellulosic waste containing a large part of glucose and xylose is desirable to implement a circular economy. Mixed culture biotechnologies can aid in realizing this goal. The effect of feeding equivalent substrates to a microbial community, such a xylose and glucose, is not well understood in terms of the number of dominant species and how these species compete for the substrates. We compared the metabolism and microbial community structure in a continuous-flow stirred tank reactor (CSTR) and a sequencing batch reactor (SBR) fed with a mixture of xylose and glucose, inoculated with bovine rumen at pH 8, 30°C and a hydraulic retention time of 8 h. We hypothesised that a CSTR will select for generalist species, taking up both substrates. We used 16S rRNA gene sequencing and fluorescent in situ hybridisation to accurately determine the microbial community structures. Both enrichments were stoichiometrically and kinetically characterised. The CSTR enrichment culture was dominated by Clostridium intestinale (91% ± 2%). The SBR showed an abundance of Enterobacteriaceae (75% ± 8%), dominated by Citrobacter freundii and a minor fraction of Raoultella ornithinolytica. C. freundii ferments xylose and glucose in a non-diauxic fashion. Clearly, a non-diauxic generalist outcompetes specialists and diauxic generalists in SBR environments.

Trace elements effect on hydrolytic stage towards biogas production of model lignocellulosic substrates

The effect and the response of several trace elements (TE) addition to the anaerobic degradation of key compounds of lignocellulosic biomass were evaluated. Lignin, cellulose and xylose were selected as principal compounds of lignocellulosic biomass. Lignin degradation was only improved by the addition of 1000 mg Fe/L, which allowed an improvement on the methane yield coefficient of 28% compared to control. SEM images from an abiotic assay showed that this effect is more likely related with a chemical effect induced by the Fe solution, instead of an enzymatic response. Pre-treatments focused on breaking the recalcitrant structure of the lignin could be more promising than TE addition for rich lignin-content substrates. Unlike to the response observed with lignin, cellulose showed a clear effect of the TE addition on methane production rate, indicating a higher preponderance of the enzymatic activity compared to the lignin biomethanization. Experiments with xylose resulted in a strong accumulation of volatile fatty acids. TE addition should be adapted to the substrate composition given the different response of each lignocellulosic compound to the different TE addition.

Insights into Xylan Degradation and Haloalkaline Adaptation through Whole-Genome Analysis of Alkalitalea saponilacus, an Anaerobic Haloalkaliphilic Bacterium Capable of Secreting Novel Halostable Xylanase

The obligately anaerobic haloalkaliphilic bacterium Alkalitalea saponilacus can use xylan as the sole carbon source and produce propionate as the main fermentation product. Using mixed carbon sources of 0.4% (w/v) sucrose and 0.1% (w/v) birch xylan, xylanase production from A. saponilacus was 3.2-fold greater than that of individual carbon sources of 0.5% (w/v) sucrose or 0.5% (w/v) birch xylan. The xylanse is halostable and exhibits optimal activity over a broad salt concentration (2–6% NaCl). Its activity increased approximately 1.16-fold by adding 0.2% (v/v) Tween 20. To understand the potential genetic mechanisms of xylan degradation and molecular adaptation to saline-alkali extremes, the complete genome sequence of A. saponilacus was performed with the pacBio single-molecule real-time (SMRT) and Illumina Misseq platforms. The genome contained one chromosome with a total size of 4,775,573 bps, and a G+C genomic content of 39.27%. Ten genes relating to the pathway for complete xylan degradation were systematically identified. Furthermore, various genes were predicted to be involved in isosmotic cytoplasm via the “compatible-solutes strategy” and cytoplasmic pH homeostasis though the “influx of hydrogen ions”. The halostable xylanase from A. saponilacus and its genomic sequence information provide some insight for potential applications in industry under double extreme conditions.

Electron bifurcation mechanism and homoacetogenesis explain products yields in mixed culture anaerobic fermentations

Anaerobic fermentation of organic wastes using microbial mixed cultures is a promising avenue to treat residues and obtain added-value products. However, the process has some important limitations that prevented so far any industrial application. One of the main issues is that we are not able to predict reliably the product spectrum (i.e. the stoichiometry of the process) because the complex microbial community behaviour is not completely understood. To address this issue, in this work we propose a new metabolic network of glucose fermentation by microbial mixed cultures that incorporates electron bifurcation and homoacetogenesis. Our methodology uses NADH balances to analyse published experimental data and evaluate the new stoichiometry proposed. Our results prove for the first time the inclusion of electron bifurcation in the metabolic network as a better description of the experimental results. Homoacetogenesis has been used to explain the discrepancies between observed and theoretically predicted yields of gaseous H₂ and CO₂ and it appears as the best solution among other options studied. Overall, this work supports the consideration of electron bifurcation as an important biochemical mechanism in microbial mixed cultures fermentations and underlines the importance of considering homoacetogenesis when analysing anaerobic fermentations.

Galacturonate Metabolism in Anaerobic Chemostat Enrichment Cultures: Combined Fermentation and Acetogenesis by the Dominant sp. nov. "Candidatus Galacturonibacter soehngenii"

Agricultural residues such as sugar beet pulp and citrus peel are rich in pectin, which contains galacturonic acid as a main monomer. Pectin-rich residues are underexploited as feedstocks for production of bulk chemicals or biofuels. The anaerobic, fermentative conversion of d-galacturonate in anaerobic chemostat enrichment cultures provides valuable information toward valorization of these pectin-rich feedstocks. Replicate anaerobic chemostat enrichments, with d-galacturonate as the sole limiting carbon source and inoculum from cow rumen content and rotting orange peels, yielded stable microbial communities, which were dominated by a novel Lachnospiraceae species, for which the name "Candidatus Galacturonibacter soehngenii" was proposed. Acetate was the dominant catabolic product, with formate and H₂ as coproducts. The observed molar ratio of acetate and the combined amounts of H₂ and formate deviated significantly from 1, which suggested that some of the hydrogen and CO₂ formed during d-galacturonate fermentation was converted into acetate via the Wood-Ljungdahl acetogenesis pathway. Indeed, metagenomic analysis of the enrichment cultures indicated that the genome of "Candidatus G. soehngenii" encoded enzymes of the adapted Entner-Doudoroff pathway for d-galacturonate metabolism as well as enzymes of the Wood-Ljungdahl pathway. The simultaneous operation of these pathways may provide a selective advantage under d-galacturonate-limited conditions by enabling a higher specific ATP production rate and lower residual d-galacturonate concentration than would be possible with a strictly fermentative metabolism of this carbon and energy source.IMPORTANCE This study on d-galacturonate metabolism by open, mixed-culture enrichments under anaerobic, d-galacturonate-limited chemostat conditions shows a stable and efficient fermentation of d-galacturonate into acetate as the dominant organic fermentation product. This fermentation stoichiometry and population analyses provide a valuable baseline for interpretation of the conversion of pectin-rich agricultural feedstocks by mixed microbial cultures. Moreover, the results of this study provide a reference for studies on the microbial metabolism of d-galacturonate under different cultivation regimes.

The chemostat metabolite spectra of alkaline mixed culture fermentation under mesophilic, thermophilic, and extreme-thermophilic conditions

Alkaline mixed culture fermentation (MCF) is a promising technology for reducing organic waste and producing biochemicals. However, chemostat metabolite spectra that are necessary for constructing a model and analyzing factors are seldom reported. In the present study, the effects of pH on the metabolites distribution in mesophilic (35 °C), thermophilic (55 °C), and extreme-thermophilic (70 °C) alkaline MCF were demonstrated. A chemical oxygen demand balance above 80% was achieved, and the main metabolites included acetate, ethanol, propionate, lactate, and formate. The yields of ethanol and formate increased as pH was increased from 7.5 to higher pH under mesophilic and thermophilic conditions, while the yields of acetate, lactate, and/or propionate decreased. The yields of ethanol, acetate, and formate increased under extreme-thermophilic conditions as pH was increased from 7.5 to 9.0, whereas lactate and hydrogen yields decreased. Low biomass yield under thermophilic and extreme-thermophilic conditions benefited higher metabolite production and minimized the accumulation of sludge.

Low carbon renewable natural gas production from coalbeds and implications for carbon capture and storage

Isotopic studies have shown that many of the world’s coalbed natural gas plays are secondary biogenic in origin, suggesting a potential for gas regeneration through enhanced microbial activities. The generation of biogas through biostimulation and bioaugmentation is limited to the bioavailability of coal-derived compounds and is considered carbon positive. Here we show that plant-derived carbohydrates can be used as alternative substrates for gas generation by the indigenous coal seam microorganisms. The results suggest that coalbeds can act as natural geobioreactors to produce low carbon renewable natural gas, which can be considered carbon neutral, or perhaps even carbon negative depending on the amount of carbon sequestered within the coal. In addition, coal bioavailability is no longer a limiting factor. This approach has the potential of bridging the gap between fossil fuels and renewable energy by utilizing existing coalbed natural gas infrastructure to produce low carbon renewable natural gas and reducing global warming.

Coalbeds produce natural gas, which has been observed to be enhanced by in situ microbes. Here, the authors add plant-derived carbohydrates (monosaccharides) to coal seams to be converted by indigenous microbes into natural gas, thus demonstrating a potential low carbon renewable natural gas resource.

Anaerobic treatment of hydrothermally solubilised sugarcane bagasse and its kinetic modelling

The aim of this study was the evaluation of anaerobic treatment for the soluble organics generated from a steam-explosion pre-treatment of sugarcane bagasse. The batch analysis revealed that about 50% of the organics was possible to be degraded into methane whilst the rest was biologically inert and composed of mostly lignin. Based on the experiment a kinetic model composed of 14 kinds of soluble substances and 5 kinds of anaerobic microorganisms was developed. The model was used to simulate the process performance of a continuous anaerobic bioreactor with MLSS concentration at 2500-15,000mg/L. The simulation indicated that the bioreactor could receive the influent until 0.4kg-COD/kg-MLSS/d of loading without significant deterioration of methane conversion. By addition of powdered activated carbon, the rest of unbiodegradable soluble organics and dark brown colour in the effluent were removed to 840mg-C/L and 760 unit respectively at adsorption of 190mg-C/g-PAC and 1200unit/g-PAC.

Valorization of a Pulp Industry By-Product through the Production of Short-Chain Organic Acids

In this work, hardwood sulfite spent liquor (HSSL)—a by-product from a pulp and paper industry—was used as substrate to produce short-chain organic acids (SCOAs) through acidogenic fermentation. SCOAs have a broad range of applications, including the production of biopolymers, bioenergy, and biological removal of nutrients from wastewaters. A continuous stirred tank reactor (CSTR) configuration was chosen to impose selective pressure conditions. The CSTR was operated for 88 days at 30 °C, without pH control, and 1.76 days of hydraulic and sludge retention times were imposed. The culture required 46 days to adapt to the conditions imposed, reaching a pseudo-steady state after this period. The maximum concentration of SCOAs produced occurred on day 71—7.0 g carbon oxygen demand (COD)/L that corresponded to a degree of acidification of 36%. Acetate, propionate, butyrate, valerate, and lactate were the SCOAs produced throughout the 88 days, with an average proportion of 59:17:19:1.0:4.0%, respectively.

Consolidated Bioprocessing for Butyric Acid Production from Rice Straw with Undefined Mixed Culture

Lignocellulosic biomass is a renewable source with great potential for biofuels and bioproducts. However, the cost of cellulolytic enzymes limits the utilization of the low-cost bioresource. This study aimed to develop a consolidated bioprocessing without the need of supplementary cellulase for butyric acid production from lignocellulosic biomass. A stirred-tank reactor with a working volume of 21 L was constructed and operated in batch and semi-continuous fermentation modes with a cellulolytic butyrate-producing microbial community. The semi-continuous fermentation with intermittent discharging of the culture broth and replenishment with fresh medium achieved the highest butyric acid productivity of 2.69 g/(L· d). In semi-continuous operation mode, the butyric acid and total carboxylic acid concentrations of 16.2 and 28.9 g/L, respectively, were achieved. Over the 21-day fermentation period, their cumulative yields reached 1189 and 2048 g, respectively, corresponding to 41 and 74% of the maximum theoretical yields based on the amount of NaOH pretreated rice straw fed in. This study demonstrated that an undefined mixed culture-based consolidated bioprocessing for butyric acid production can completely eliminate the cost of supplementary cellulolytic enzymes.

Effects of sludge inoculum and organic feedstock on active microbial communities and methane yield during anaerobic digestion

Anaerobic digestion (AD) is a widespread microbial technology used to treat organic waste and recover energy in the form of methane ("biogas"). While most AD systems have been designed to treat a single input, mixtures of digester sludge and solid organic waste are emerging as a means to improve efficiency and methane yield. We examined laboratory anaerobic cultures of AD sludge from two sources amended with food waste, xylose, and xylan at mesophilic temperatures, and with cellulose at meso- and thermophilic temperatures, to determine whether and how the inoculum and substrate affect biogas yield and community composition. All substrate and inoculum combinations yielded methane, with food waste most productive by mass. Pyrosequencing of transcribed bacterial and archaeal 16S rRNA showed that community composition varied across substrates and inocula, with differing ratios of hydrogenotrophic/acetoclastic methanogenic archaea associated with syntrophic partners. While communities did not cluster by either inoculum or substrate, additional sequencing of the bacterial 16S rRNA gene in the source sludge revealed that the bacterial communities were influenced by their inoculum. These results suggest that complete and efficient AD systems could potentially be assembled from different microbial inocula and consist of taxonomically diverse communities that nevertheless perform similar functions.

High-rate volatile fatty acid (VFA) production by a granular sludge process at low pH

Volatile fatty acids (VFA) are proposed platform molecules for the production of basic chemicals and polymers from organic waste streams. In this study we developed a granular sludge process to produce VFA at high rate, yield and purity while minimizing potential operational costs. A lab-scale anaerobic sequencing batch reactor (ASBR) was fed with 10 g l(-1) glucose as model substrate. Inclusion of a short (2 min) settling phase before effluent discharge enabled effective granulation and very high volumetric conversion rates of 150-300 gCOD l(-1)  d(-1) were observed during glucose conversion. The product spectrum remained similar at the tested pH range with acetate and butyrate as the main products, and a total VFA yield of 60-70% on chemical oxygen demand (COD) basis. The requirement for base addition for pH regulation could be reduced from 1.1 to 0.6 mol OH(-) (mol glucose)(-1) by lowering the pH from 5.5 to 4.5. Solids concentrations in the effluent were 0.6 ± 0.3 g l(-1) but could be reduced to 0.02 ± 0.01 g l(-1) by introduction of an additional settling period of 5 min. The efficient production of VFA at low pH with a virtually solid-free effluent increases the economic feasibility of waste-based chemicals and polymer production. Biotechnol.

Waste to resource: Converting paper mill wastewater to bioplastic

In this study we investigated the feasibility of producing polyhydroxyalkanoate (PHA) by microbial enrichments on paper mill wastewater. The complete process includes (1) paper mill wastewater acidogenic fermentation in a simple batch process, (2) enrichment of a PHA-producing microbial community in a selector operated in sequencing batch mode with feast-famine regime, (3) Cellular PHA content maximization of the enrichment in an accumulator in fed-batch mode. The selective pressure required to establish a PHA-producing microbial enrichment, as derived from our previous research on synthetic medium, was validated using an agro-industrial waste stream in this study. The microbial enrichment obtained could accumulate maximum up to 77% PHA of cell dry weight within 5 h, which is currently the best result obtained on real agro-industrial waste streams, especially in terms of biomass specific efficiency. Biomass in this enrichment included both Plasticicumulans acidivorans, which was the main PHA producer, and a flanking population, which exhibited limited PHA-producing capacity. The fraction of P. acidivorans in the biomass was largely dependent on the fraction of volatile fatty acids in the total soluble COD in the wastewater after acidification. Based on this observation, one simple equation was proposed for predicting the PHA storage capacity of the enrichment. Moreover, some crucial bottlenecks that may impede the successful scaling-up of the process are discussed.

Pleiotropic functions of catabolite control protein CcpA in Butanol-producing Clostridium acetobutylicum

Background

Clostridium acetobutylicum has been used to produce butanol in industry. Catabolite control protein A (CcpA), known to mediate carbon catabolite repression (CCR) in low GC gram-positive bacteria, has been identified and characterized in C. acetobutylicum by our previous work (Ren, C. et al. 2010, Metab Eng 12:446–54). To further dissect its regulatory function in C. acetobutylicum, CcpA was investigated using DNA microarray followed by phenotypic, genetic and biochemical validation.

Results

CcpA controls not only genes in carbon metabolism, but also those genes in solvent production and sporulation of the life cycle in C. acetobutylicum: i) CcpA directly repressed transcription of genes related to transport and metabolism of non-preferred carbon sources such as d-xylose and l-arabinose, and activated expression of genes responsible for d-glucose PTS system; ii) CcpA is involved in positive regulation of the key solventogenic operon sol (adhE1-ctfA-ctfB) and negative regulation of acidogenic gene bukII; and iii) transcriptional alterations were observed for several sporulation-related genes upon ccpA inactivation, which may account for the lower sporulation efficiency in the mutant, suggesting CcpA may be necessary for efficient sporulation of C. acetobutylicum, an important trait adversely affecting the solvent productivity.

Conclusions

This study provided insights to the pleiotropic functions that CcpA displayed in butanol-producing C. acetobutylicum. The information could be valuable for further dissecting its pleiotropic regulatory mechanism in C. acetobutylicum, and for genetic modification in order to obtain more effective butanol-producing Clostridium strains.

Enhancement of fermentative hydrogen production in an extreme-thermophilic (70°C) mixed-culture environment by repeated batch cultivation

Repeated batch cultivation was applied to enrich hydrogen fermentative microflora under extreme-thermophilic (70°C) environment. Initial inoculums received from a hydrogen producing reactor fed with organic fraction of household solid wastes. In total seven transfers was conducted and maximum hydrogen yield reached 296 ml H(2)/g (2.38 mol/mol) glucose and 252 ml H(2)/g (2.03 mol/mol) for 1 and 2 g/l glucose medium, respectively. It was found that hydrogen production was firstly decreased and got increased gradually from third generation. Acetate was found to be the main metabolic by-product in all batch cultivation. Furthermore, the diversity of bacterial community got decreased after repeated batch cultivation. It was proved that repeated batch cultivation was a good method to enhance the hydrogen production by enriching the mixed cultures of dominant species.

Efficient production of L-lactic acid from corncob molasses, a waste by-product in xylitol production, by a newly isolated xylose utilizing Bacillus sp. strain

Lignocellulosic biomass-derived sugars are considered nowadays to be an economically attractive carbohydrate feedstock for large-scale fermentations of bulk chemicals such as lactic acid. In the present study, corncob molasses containing a high content of xylose, which is one of the lignocellulosic biomasses and a waste by-product from xylitol production, was used for L-lactic acid production via a newly isolated xylose utilizing Bacillus sp. strain XZL9. Bacillus sp. strain XZL9 can utilize the mixture of sugars including xylose, arabinose, and glucose in corncob molasses for L-lactic acid production. High concentration of L-lactic acid (74.7 g l⁻¹) was obtained from corncob molasses (initial total sugars of 91.4 g l⁻¹) in fed-batch fermentation. This study provides an encouraging means of producing L-lactic acid from lignocellulosic resource such as the low-cost corncob molasses.