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Upgrading syngas fermentation effluent using Clostridium kluyveri in a continuous fermentation

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 disa...

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Autores principales: Gildemyn, Sylvia, Molitor, Bastian, Usack, Joseph G., Nguyen, Mytien, Rabaey, Korneel, Angenent, Largus T.
Formato: Online Artículo Texto
Lenguaje:English
Publicado: BioMed Central 2017
Materias:
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5372331/
https://www.ncbi.nlm.nih.gov/pubmed/28367228
http://dx.doi.org/10.1186/s13068-017-0764-6
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author Gildemyn, Sylvia
Molitor, Bastian
Usack, Joseph G.
Nguyen, Mytien
Rabaey, Korneel
Angenent, Largus T.
author_facet Gildemyn, Sylvia
Molitor, Bastian
Usack, Joseph G.
Nguyen, Mytien
Rabaey, Korneel
Angenent, Largus T.
author_sort Gildemyn, Sylvia
collection PubMed
description 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 of Clostridium kluyveri were 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 of n-caproic acid, which were up to 40 mM day(−1) (4.64 g L(−1) day(−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 efficient n-caproic acid production. In addition, n-caprylic acid production was observed for the first time with C. 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 of C. kluyveri was 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 kluyveri can be used as a biocatalyst to upgrade syngas fermentation effluent into MCCAs at pH values above 5.5. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s13068-017-0764-6) contains supplementary material, which is available to authorized users.
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spelling pubmed-53723312017-03-31 Upgrading syngas fermentation effluent using Clostridium kluyveri in a continuous fermentation Gildemyn, Sylvia Molitor, Bastian Usack, Joseph G. Nguyen, Mytien Rabaey, Korneel Angenent, Largus T. Biotechnol Biofuels Research 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 of Clostridium kluyveri were 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 of n-caproic acid, which were up to 40 mM day(−1) (4.64 g L(−1) day(−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 efficient n-caproic acid production. In addition, n-caprylic acid production was observed for the first time with C. 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 of C. kluyveri was 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 kluyveri can be used as a biocatalyst to upgrade syngas fermentation effluent into MCCAs at pH values above 5.5. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s13068-017-0764-6) contains supplementary material, which is available to authorized users. BioMed Central 2017-03-29 /pmc/articles/PMC5372331/ /pubmed/28367228 http://dx.doi.org/10.1186/s13068-017-0764-6 Text en © The Author(s) 2017 Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
spellingShingle Research
Gildemyn, Sylvia
Molitor, Bastian
Usack, Joseph G.
Nguyen, Mytien
Rabaey, Korneel
Angenent, Largus T.
Upgrading syngas fermentation effluent using Clostridium kluyveri in a continuous fermentation
title Upgrading syngas fermentation effluent using Clostridium kluyveri in a continuous fermentation
title_full Upgrading syngas fermentation effluent using Clostridium kluyveri in a continuous fermentation
title_fullStr Upgrading syngas fermentation effluent using Clostridium kluyveri in a continuous fermentation
title_full_unstemmed Upgrading syngas fermentation effluent using Clostridium kluyveri in a continuous fermentation
title_short Upgrading syngas fermentation effluent using Clostridium kluyveri in a continuous fermentation
title_sort upgrading syngas fermentation effluent using clostridium kluyveri in a continuous fermentation
topic Research
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5372331/
https://www.ncbi.nlm.nih.gov/pubmed/28367228
http://dx.doi.org/10.1186/s13068-017-0764-6
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