title: Upgrading syngas fermentation effluent usingClostridium kluyveriin a continuous fermentation.
abstract: BACKGROUND
  The product of current syngas fermentation systems is an ethanol/acetic acid mixture and the goal is to maximize ethanol recovery. However, ethanol currently has a relatively low market value and its separation from the fermentation broth is energy intensive. We can circumvent these disadvantages of ethanol production by converting the dilute ethanol/acetic acid mixture into products with longer carbon backbones, which are of higher value and are more easily extracted than ethanol. Chain elongation, which is the bioprocess in which ethanol is used to elongate short-chain carboxylic acids to medium-chain carboxylic acids (MCCAs), has been studied with pure cultures and open cultures of microbial consortia (microbiomes) with several different substrates. While upgrading syngas fermentation effluent has been studied with open cultures, to our knowledge, no study exists that has performed this with pure cultures.
  
  
  RESULTS
  Here, pure cultures ofClostridium kluyveriwere used in continuous bioreactors to convert ethanol/acetic acid mixtures into MCCAs. Besides changing the operating conditions in regards to substrate loading rates and composition, the effect of in-line product extraction, pH, and the use of real syngas fermentation effluent on production rates were tested. Increasing the organic loading rates resulted in proportionally higher production rates ofn-caproic acid, which were up to 40 mM day-1(4.64 g L-1day-1) at carbon conversion efficiencies of 90% or higher. The production rates were similar for bioreactors with and without in-line product extraction. Furthermore, a lower ethanol/acetic acid ratio (3:1 instead of 10:1) enabled faster and more efficientn-caproic acid production. In addition,n-caprylic acid production was observed for the first time withC. kluyveri(up to 2.19 ± 0.34 mM in batch). Finally, the use of real effluent from syngas fermentation, without added yeast extract, but with added defined growth factors, did maintain similar production rates. Throughout the operating period, we observed that the metabolism ofC. kluyveriwas inhibited at a mildly acidic pH value of 5.5 compared to a pH value of 7.0, while reactor microbiomes perform successfully at mildly acidic conditions.
  
  
  CONCLUSIONS
  Clostridium kluyverican be used as a biocatalyst to upgrade syngas fermentation effluent into MCCAs at pH values above 5.5.
authors.affiliation: Cornell University, Biological and Environmental Engineering, Riley-Robb Hall, Ithaca, NY 14853 USA.|Ghent University, Center for Microbial Ecology and Technology (CMET), Coupure Links 653, 9000 Ghent, Belgium.|Organic Waste SystemsDok Noord 5, 9000 Ghent, Belgium.
authors.forename: Sylvia
authors.initials: S
authors.lastname: Gildemyn
authors.affiliation: Cornell University, Biological and Environmental Engineering, Riley-Robb Hall, Ithaca, NY 14853 USA.
authors.forename: Bastian
authors.initials: B
authors.lastname: Molitor
authors.affiliation: Cornell University, Biological and Environmental Engineering, Riley-Robb Hall, Ithaca, NY 14853 USA.
authors.forename: Joseph G
authors.initials: JG
authors.lastname: Usack
authors.affiliation: Cornell University, Biological and Environmental Engineering, Riley-Robb Hall, Ithaca, NY 14853 USA.
authors.forename: Mytien
authors.initials: M
authors.lastname: Nguyen
authors.affiliation: Ghent University, Center for Microbial Ecology and Technology (CMET), Coupure Links 653, 9000 Ghent, Belgium.
authors.forename: Korneel
authors.initials: K
authors.lastname: Rabaey
authors.affiliation: Cornell University, Biological and Environmental Engineering, Riley-Robb Hall, Ithaca, NY 14853 USA.|University of Tübingen, Center for Applied GeoSciences, Hölderlinstr. 12, 72074 Tübingen, Germany.
authors.forename: Largus T
authors.identifier: 0000-0003-0180-1865
authors.initials: LT
authors.lastname: Angenent
keywords: Carboxylate platform; Carboxylic acids; Chain elongation; Syngas fermentation; n-Caproic acid; n-Caprylic acid