Dark fermentation: isolation and characterization of hydrogen-producing strains from sludges.

To improve bacterial hydrogen production, ten hydrogen-producing strains belonging to Clostridium spp. were isolated from various sludges under low vacuum. Hydrogenogenesis by dark fermentation in batch cultures of these strains was optimal at about 35 degrees C and an initial pH of 6.5, which for all strains gradually dropped to ca. pH 4 during the fermentation. Clostridium roseum H5 and C. diolis RT2 had the highest hydrogen yields per total substrate (120 ml H2/g initial COD). Substrate consumption alone by C. beijerinckii UAM and C. diolis RT2 reached 573 and 475 ml H2/g consumed COD, respectively. Butyric acid fermentation was predominant, with butyrate and acetate as the major by-products and propionate, ethanol, and lactate as secondary metabolites. The acetate:butyrate ratios and fermentation pathways varied depending on the strains and environmental conditions. Hydrogenogenesis was studied in greater detail in C. saccharobutylicum H1. In butyric acid fermentation by this representative strain, acetoacetate was detected as an intermediate metabolite. Hydrogenogenesis was also analyzed in an enrichment culture, which behaved similarly to the axenic cultures.

Dark fermentation: isolation and characterization of hydrogen-producing strains from sludges

To improve bacterial hydrogen production, ten hydrogen-producing strains belonging to Clostridium spp. were isolated from various sludges under low vacuum. Hydrogenogenesis by dark fermentation in batch cultures of these strains was optimal at about 35 ºC and an initial pH of 6.5, which for all strains gradually dropped to ca. pH 4 during the fermentation. Clostridium roseum H5 and C. diolis RT2 had the highest hydrogen yields per total substrate (120 ml H2/g initial COD). Substrate consumption alone by C. beijerinckii UAM and C. diolis RT2 reached 573 and 475 ml H2/g consumed COD, respectively. Butyric acid fermentation was predominant, with butyrate and acetate as the major by-products and propionate, ethanol, and lactate as secondary metabolites. The acetate: butyrate ratios and fermentation pathways varied depending on the strains and environmental conditions. Hydrogenogenesis was studied in greater detail in C. saccharobutylicum H1. In butyric acid fermentation by this representative strain, acetoacetate was detected as an intermediate metabolite. Hydrogenogenesis was also analyzed in an enrichment culture, which behaved similarly to the axenic cultures (AU)

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Microbial consortia for hydrogen production enhancement.

Ten efficient hydrogen-producing strains affiliated to the Clostridium genus were used to develop consortia for hydrogen production. In order to determine their saccharolytic and proteolytic activities, glucose and meat extract were tested as fermentation substrates, and the best hydrogen-producing strains were selected. The C. roseum H5 (glucose-consuming) and C. butyricum R4 (protein-degrading) co-culture was the best hydrogen-producing co-culture. The end-fermentation products for the axenic cultures and co-cultures were analyzed. In all cases, organic acids, mainly butyrate and acetate, were produced lowering the pH and thus inhibiting further hydrogen production. In order to replace the need for reducing agents for the anaerobic growth of clostridia, a microbial consortium including Clostridium spp. and an oxygen-consuming microorganism able to form dense granules (Streptomyces sp.) was created. Increased yields of hydrogen were achieved. The effect of adding a butyrate-degrading bacteria and an acetate-consuming archaea to the consortia was also studied.