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Abstract: Wheat straw is an abundant agricultural residue which can be used as raw material to produce hydrogen (H2), a promising alternative energy carrier, at a low cost. Bioconversion of lignocellulosic biomass to produce H2 usually involves three main operations: pretreatment, hydrolysis and fermentation. In this study, the efficiency of exogenous enzyme addition on fermentative H2 production from wheat straw was evaluated using mixed-cultures in two experimental systems: a one-stage system (direct enzyme addition) and a two-stage system (enzymatic hydrolysis prior to dark fermentation). H2 production from untreated wheat straw ranged from 5.18 to 10.52 mL-H2 g-VS−1. Whatever the experimental enzyme addition procedure, a two-fold increase in H2 production yields ranging from 11.06 to 19.63 mL-H2 g-VS−1 was observed after enzymatic treatment of the wheat straw. The high variability in H2 yields in the two step process was explained by the consumption of free sugars by indigenous wheat straw microorganisms during enzymatic hydrolysis. The direct addition of exogenous enzymes in the one-stage dark fermentation stage proved to be the best way of significantly improving H2 production from lignocellulosic biomass. Finally, the optimal dose of enzyme mixture added to the wheat straw was evaluated between 1 and 5 mg-protein g-raw wheat straw−1. [Copyright &y& Elsevier]

Abstract: Dark fermentation using mixed cultures is an attractive biological process for producing hydrogen (H2) from lignocellulosic biomass at a low cost. Physicochemical pretreatment is generally used to convert lignocellulosic materials into monosaccharides. However, the processes also involved release degradation byproducts which can, in turn, inhibit microbial growth and metabolism and, hence, impact substrate conversion. In this study, the impact on H2 production of lignocellulose-derived compounds (i.e. furan derivatives, phenolic compounds and lignins) was assessed along with their effect on bacterial communities and metabolisms. Batch tests were carried out using xylose as model substrate (1.67molH2 molxylose −1 in the control test). All the putative inhibitory compounds showed a significant negative impact on H2 production performance (ranging from 0.34 to 1.39molH2 molxylose −1). The H2 yields were impacted more strongly by furan derivatives (0.40–0.51molH2 molxylose −1) than by phenolic compounds (1.28–1.39molH2 molxylose −1). Except for the batch tests supplemented with lignins, the lag phase was shorter for inhibitors having the highest molecular weight (8 days versus 22 days for the lowest MW). Variability of the lag phase was clearly related to a shift in bacterial community structure, as shown by multivariate ordination statistics. The decrease in H2 yield was associated with a decrease in the relative abundance of several H2-producing clostridial species. Interestingly, Clostridium beijerinkii was found to be more resistant to the inhibitors, making this bacterium an ideal candidate for H2 production from hydrolyzates of lignocellulosic biomass. [Copyright &y& Elsevier]

Abstract: Dark fermentation using mixed cultures is a promising biotechnology for producing hydrogen (H2) from renewable organic waste at a low cost. The impact of the characteristics of carbohydrates was evaluated on H2 production and the associated changes in clostridial populations. A series of H2-producing batch experiments was performed from mono-, di- to tri-saccharides (i.e. fructose, glucose, sucrose, maltose, cellobiose, maltotriose). Both chain length and alpha- or beta-linkage of carbohydrates impacted H2 production performance as well as the patterns of hydrogenases. The H2 yield, ranging from 1.38 to 1.84 mol-H2/mol-hexose, decreased with the increasing chain length of the carbohydrates, showing a negative effect of the hydrolysis step on H2 production efficiency. Changes in H2 yield were associated with a specialization of clostridial species, which used different metabolic routes. The rise in H2 production was associated with butyrate and acetate increases while H2 consumption was related to caproate formation. Both clostridial [FeFe]- and [NiFe]-hydrogenases were identified in cellobiose cultures by a proteomic approach. This is the first study that combines genetic and proteomic analyses focused on H2-producing bacteria under various conditions and it opens very interesting perspectives to better understand and optimize H2 production using mixed cultures. [Copyright &y& Elsevier]

Abstract: The fermentative hydrogen production occurs during the anaerobic digestion of organic matter. It is a promising technology to produce renewable energy. However, the mixed culture fermentation performances vary considerably depending on the operating conditions such as pH. We investigated the potential of a molecular CE-SSCP (capillary electrophoresis-single strand conformation polymorphism) fingerprinting method based on the hydA functional genes to better describe the bacterial community dynamics, with regards to the standard 16S rDNA-based method. A series of batch experiments was performed from sucrose at different initial pH from 4 to 6. As expected, the highest H2 production potentials (H max) and rates (R max) were obtained at the highest pH. Changes in batch performances were clearly associated with shifts in the hydA diversity and structure. In contrast, 16S rDNA-based fingerprints were less sensitive to changes in H2 production performances. The H max was related to lower hydA diversity, with Clostridium sporogenes as the major H 2 producer. Communities harboring larger hydA diversities were found in experiments with the higher R max, suggesting that species coexistence may have positive effects on H2 production. [Copyright &y& Elsevier]

Abstract: A Capillary Electrophoresis Single Strand Conformation Polymorphism (CE-SSCP) method based on functional [Fe–Fe]-hydrogenase genes was developed for monitoring the hydrogen (H2)-producing clostridial population in mixed-culture bioprocesses. New non-degenerated primers were designed and then validated on their specific PCR detection of a broad range of clostridial hydA genes. The hydA-based CE-SSCP method gave a specific and discriminating profile for each of the Clostridium strains tested. This method was validated using H2-producing mixed cultures incubated at temperatures ranging from 25 °C to 45 °C. The hydA CE-SSCP profiles clearly differed between temperatures tested. Hence, they varied according to variations of the H2 production performances. The HydA sequences amplified with the new primer set indicated that diverse Clostridium strains impacted the H2 production yields. The highest performances were related to the dominance of Clostridium sporogenes-like hydA sequences. This CE-SSCP tool offers highly reliable and throughput analysis of the functional diversity and structure of the hydA genes for better understanding of the H2-producing clostridial population dynamics in H2 dark fermentation bioreactors. [ABSTRACT FROM AUTHOR]

The hydrogen-producing strain PROH2 pertaining to the genus Clostridium was successfully isolated from a shallow submarine hydrothermal chimney (Prony Bay, New Caledonia) driven by serpentinization processes. Cell biomass and hydrogen production performances during fermentation by strain PROH2 were studied in a series of batch experiments under various conditions of pH, temperature, NaCl and glucose concentrations. The highest hydrogen yield, 2.71 mol H 2 /mol glucose, was observed at initial pH 9.5, 37 °C, and glucose concentration 2 g/L, and was comparable to that reported for neutrophilic clostridial species. Hydrogen production by strain PROH2 reached the maximum production rate (0.55 mM-H 2 /h) at the late exponential phase. Yeast extract was required for growth of strain PROH2 and improved significantly its hydrogen production performances. The isolate could utilize various energy sources including cellobiose, galactose, glucose, maltose, sucrose and trehalose to produce hydrogen. The pattern of end-products of metabolism was also affected by the type of energy sources and culture conditions used. These results indicate that Clostridium sp. strain PROH2 is a good candidate for producing hydrogen under alkaline and mesothermic conditions. [ABSTRACT FROM AUTHOR]

Abstract: A series of batch tests was conducted to investigate the effect of a CO2 sparging pretreatment on hydrogen production from fermentation of the organic fraction of municipal solid waste. Tests were carried out in a stirred bioreactor without adding inoculum or supplementation solution, at 35 °C and with a pH regulated at 5.5. The influence of CO2 flowrate (30, 100 and 500 mL/min) was evaluated. Results showed a decrease in biological hydrogen production for a sparging flowrate of , similar performances for the control experiments and for a sparging flowrate of and an increase in biohydrogen production for a sparging flowrate of . For this latter operating condition, a substantial improvement in hydrogen production performances was obtained with an increase in the hydrogen yield by more than 20% and a decrease in the chemicals consumption used to maintain the pH by more than 20%. No shift in the metabolic pathways and in the bacterial community structure was observed but positive bacterial interactions occurred and enhanced the hydrogen production. Therefore, this study suggested that the growth of hydrogen producing bacteria could be either enhanced or slowed down by a CO2 sparging pretreatment depending on the level of CO2 flowrate, the shift being close to . [Copyright &y& Elsevier]