title: A Narrow pH Range Supports Butanol, Hexanol, and Octanol Production from Syngas in a Continuous Co-culture ofClostridium ljungdahliiandClostridium kluyveriwith In-Line Product Extraction.
abstract: 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 withClostridium ljungdahliiandClostridium kluyveriwith in-line product extraction through gas stripping and product condensing within the syngas recirculation line. The main products fromC. ljungdahliifermentation 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 byC. kluyverito chain elongate with the reverse β-oxidation pathway, resulting inn-butyrate andn-caproate at net rates of 129 and 70 mmol C·L-1·d-1, respectively.C. ljungdahliilikely reduced the produced carboxylates to their corresponding alcohols with the reductive power from syngas. This resulted in the longer-chain alcoholsn-butanol,n-hexanol, andn-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 ofn-butanol andn-hexanol with traces ofn-octanol.
authors.affiliation: Department for Biological and Environmental Engineering, Cornell University Ithaca, NY, USA.
authors.forename: Hanno
authors.initials: H
authors.lastname: Richter
authors.affiliation: Department for Biological and Environmental Engineering, Cornell University Ithaca, NY, USA.
authors.forename: Bastian
authors.initials: B
authors.lastname: Molitor
authors.affiliation: Laboratory of Microbiology, Wageningen University Wageningen, Netherlands.
authors.forename: Martijn
authors.initials: M
authors.lastname: Diender
authors.affiliation: Laboratory of Microbiology, Wageningen University Wageningen, Netherlands.
authors.forename: Diana Z
authors.initials: DZ
authors.lastname: Sousa
authors.affiliation: Department for Biological and Environmental Engineering, Cornell UniversityIthaca, NY, USA; Atkinson Center for a Sustainable Future, Cornell UniversityIthaca, NY, USA.
authors.forename: Largus T
authors.initials: LT
authors.lastname: Angenent
keywords: Clostridium kluyveri; Clostridium ljungdahlii; alcohol; co-culture; hexanol; octanol; syngas