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Low Fermentation pH Is a Trigger to Alcohol Production, but a Killer to Chain Elongation

Gasification of organic wastes coupled to syngas fermentation allows the recovery of carbon in the form of commodity chemicals, such as carboxylates and biofuels. Acetogenic bacteria ferment syngas to mainly two-carbon compounds, although a few strains can also synthesize four-, and six-carbon molecules. In general, longer carbon chain products have a higher biotechnological (and commercial) value due to their higher energy content and their lower water solubility. However, de-novo synthesis of medium-chain products from syngas is quite uncommon in acetogenic bacteria. An alternative to de-novo synthesis is bioproduction of short-chain products (C2 and C4), and their subsequent elongation to C4, C6, or C8 through reversed β-oxidation metabolism. This two-step synergistic approach has been successfully applied for the production of up to C8 compounds, although the accumulation of alcohols in these mixed cultures remained below detection limits. The present work investigates the production of higher alcohols from syngas by open mixed cultures (OMC). A syngas-fermenting community was enriched from sludge of an anaerobic digester for a period of 109 days in a lab-scale reactor. At the end of this period, stable production of ethanol and butanol was obtained. C6 compounds were only transiently produced at the beginning of the enrichment phase, during which Clostridium kluyveri, a bacterium able to carry out carbon chain elongation, was detected in the community. Further experiments showed pH as a critical parameter to maintain chain elongation activity in the co-culture. Production of C6 compounds was recovered by preventing fermentation pH to decrease below pH 4.5–5. Finally, experiments showed maximal production of C6 compounds (0.8 g/L) and alcohols (1.7 g/L of ethanol, 1.1 g/L of butanol, and 0.6 g/L of hexanol) at pH 4.8. In conclusion, low fermentation pH is critical for the production of alcohols, although detrimental to C. kluyveri. Fine control of fermentation pH to final values around 4.8 could allow sustained production of higher alcohols

The authors would like to thank the Spanish Ministry (BestEnergy, CTQ2011-23632 and Bescarbox, CTM2013-43454-R for its financial support in this study. LEQUIA and EcoAqua have been recognized as consolidated research groups by the Catalan Government (2014-SGR-1168 and 2014-SGR-0484). RG gratefully acknowledges support from Ghent University BOF postdoctoral fellowship (BOF15/PDO/068)

Frontiers in Microbiology, 2016, vol.7, art.702

Frontiers Media

Author: Ganigué Pagès, Ramon
Sánchez Paredes, Patricia
Bañeras Vives, Lluís
Colprim Galceran, Jesús
Date: 2016 May 24
Abstract: Gasification of organic wastes coupled to syngas fermentation allows the recovery of carbon in the form of commodity chemicals, such as carboxylates and biofuels. Acetogenic bacteria ferment syngas to mainly two-carbon compounds, although a few strains can also synthesize four-, and six-carbon molecules. In general, longer carbon chain products have a higher biotechnological (and commercial) value due to their higher energy content and their lower water solubility. However, de-novo synthesis of medium-chain products from syngas is quite uncommon in acetogenic bacteria. An alternative to de-novo synthesis is bioproduction of short-chain products (C2 and C4), and their subsequent elongation to C4, C6, or C8 through reversed β-oxidation metabolism. This two-step synergistic approach has been successfully applied for the production of up to C8 compounds, although the accumulation of alcohols in these mixed cultures remained below detection limits. The present work investigates the production of higher alcohols from syngas by open mixed cultures (OMC). A syngas-fermenting community was enriched from sludge of an anaerobic digester for a period of 109 days in a lab-scale reactor. At the end of this period, stable production of ethanol and butanol was obtained. C6 compounds were only transiently produced at the beginning of the enrichment phase, during which Clostridium kluyveri, a bacterium able to carry out carbon chain elongation, was detected in the community. Further experiments showed pH as a critical parameter to maintain chain elongation activity in the co-culture. Production of C6 compounds was recovered by preventing fermentation pH to decrease below pH 4.5–5. Finally, experiments showed maximal production of C6 compounds (0.8 g/L) and alcohols (1.7 g/L of ethanol, 1.1 g/L of butanol, and 0.6 g/L of hexanol) at pH 4.8. In conclusion, low fermentation pH is critical for the production of alcohols, although detrimental to C. kluyveri. Fine control of fermentation pH to final values around 4.8 could allow sustained production of higher alcohols
The authors would like to thank the Spanish Ministry (BestEnergy, CTQ2011-23632 and Bescarbox, CTM2013-43454-R for its financial support in this study. LEQUIA and EcoAqua have been recognized as consolidated research groups by the Catalan Government (2014-SGR-1168 and 2014-SGR-0484). RG gratefully acknowledges support from Ghent University BOF postdoctoral fellowship (BOF15/PDO/068)
Format: application/pdf
Citation: https://doi.org/10.3389/fmicb.2016.00702
ISSN: 1664-302X
Document access: http://hdl.handle.net/10256/14009
Language: eng
Publisher: Frontiers Media
Collection: MICINN/PN 2012-2014/CTQ2011-23632
MINECO/PE 2014-2015/CTM2013-43454-R
Reproducció digital del document publicat a: https://doi.org/10.3389/fmicb.2016.00702
Articles publicats (D-B)
Is part of: Frontiers in Microbiology, 2016, vol.7, art.702
Rights: Attribution 4.0 Spain
Rights URI: http://creativecommons.org/licenses/by/4.0/es/
Subject: Biotecnologia
Biotechnology
Residus orgànics -- Eliminació
Organic waste disposal
Title: Low Fermentation pH Is a Trigger to Alcohol Production, but a Killer to Chain Elongation
Type: info:eu-repo/semantics/article
Repository: DUGiDocs

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