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[Abstract] Clostridium kluyveri was used for chain elongation of C2C4 fatty acids in stirred tank bioreactors. The influence of different electron acceptors (acetic acid, butyric acid and the mixture of both) on C6 fatty acid production was evaluated in presence of ethanol using similar molar alcohol/acid ratios around 3.5. Bottle batch assays without pH regulation and with only acetic acid as electron acceptor yielded a final C6 fatty acid concentration of 6.8 ± 0.6 g L−1. Then, pH-regulated bioreactors were operated at constant pH of 6.8. Under such conditions, the maximum growth rate was 0.039 h−1 obtained using acetic acid and butyric acid as electron acceptors, whereas the lowest growth rate was 0.010 h−1 with only butyric acid as electron acceptor. The maximum growth rate with acetic acid only, was similar, though slightly lower, as with the mixture of C2C4 fatty acids. Besides, the maximum productions of hexanoic acid were 11.8 g L−1, 13.1 g L−1 and 21.2 g L−1 using, respectively, acetic acid, butyric acid and the mixture of both acids as electron acceptors. Thus, the use of a mixture of acetic acid and butyric acid in presence of ethanol for chain elongation, at constant pH, proved to be efficient for hexanoic acid production.

[Abstract] Clostridium carboxidivorans can produce acids and/or alcohols through syngas fermentation. In that C1-gas fermentation process, the production of acids takes place at higher pH (acetogenesis) (e.g., around 6.00), while the conversion of accumulated acids into alcohols (solventogenesis) is more favourable at a lower pH (e.g., 4.75–5.00). The pH drop, when switching from acetogenesis to solventogenesis, can either be natural—and result from the production of acids—or artificial. In the latter case, for the acidification process, a strong acid (HCl) was added to a syngas fermenting bioreactor in this study, while NaOH was added to increase the pH whenever needed. Cycles of high and low pH were applied in order to switch from acetogenic to solventogenic stages. This pH adjustment procedure leads to the accumulation of salts. The possible inhibitory effect exerted by changes in salinity in the bioreactor was estimated in batch bottles assays, carried out with different salinities (media with different concentrations of sodium chloride) using C. carboxidivorans and CO as a carbon source. At NaCl concentrations below 9 g/L, maximum growth rates around 0.055 h−1 were obtained, whereas increasing the concentration of sodium chloride had a negative effect on bacterial growth and CO consumption. In the case of the most concentrated bottles, above 15 g/L NaCl no relevant growth was observed. Also, the IC50, i.e. concentration yielding 50% growth inhibition, was estimated, and reached a value of 11 g/L sodium chloride.

[Abstract] The search for new energy sources or fuels has increased in recent years. Besides, the increasing demand and greater shortage of conventional fuels, creates the need to replace them by renewable energy sources. Some possible renewable energy sources are (higher) alcohols such as ethanol, butanol, and hexanol. The biological production of ethanol and higher alcohols can be a good and inexpensive way for the obtention of new renewable energy sources. The anaerobic fermentation of waste materials or waste gases is an energy-inexpensive and feedstock-flexible technology for the production of those bioalcohols. The use of those raw materials in biorefineries has two major advantages : it is an efficient method for the elimination of pollutants and compounds with a greenhouse effect (CO, CO2) and it allows to produce solvents with a high commercial value. The main objective of this dissertation is the obtention of higher alcohols using different carbon sources (glucose, carbon monoxide and syngas) with the anaerobic bacterium Clostridium carboxidivorans as biocatalyst. The experimental results are reported in the different chapters of this dissertation, aiming at optimizing the fermentation conditions and operating parameters of the bioconversion of these different carbon sources into ethanol and higher alcohols., [Resumen] La busqueda de nuevas fuentes de energía o combustibles ha incrementado recientemente. Además el incremento de la demanda y de la gran escasez de combustibles convencionales, crea la necesidad de reemplazarlos por fuentes de energía renovables. Algunas de estas posibles fuentes de energía renovables son los alcoholes como etanol, butanol y hexanol La producción biológica de etanol y alcoholes de cadena larga puede ser una buena y barata forma de obtener nuevas fuentes de energía renovable. La fermentación de materiales y gases residuales es una energía barata y una tecnología de materias primas flexibles para la producción de esos bioalcoholes. El uso de esos materiales residuales en biorefinerías t iene dos ventajas principales: es un método eficiente para la eliminación de contaminantes y gases de efecto invernadero (CO, CO2) y permite producir solventes de elevado valor comercial. El principal objetivo de esta tesis es la obtención de alcoholes de cadena larga empleando diferentes fuentes de carbono (glucosa, monóxido de carbono y gas de síntesis) empleando a Clostridium carboxidivorans como biocatalizador. Los resultados están recogidos en los distintos capítulos de esta tesis doctoral, con el objetivo de optimizar las condiciones de fermentación y los parámetros operativos de bioconversión de estas fuentes de carbono en alcoholes., [Resumo] A busqueda de novas fontes de enerxía ou combustibles alternativos incrementou recentemente. Ademais o incremento da demanda e da gran escaseza de combustibles convencionais, crea a necesidade de reemplazarlos por fontes de enerxía renovables. Algunhas destas posibles fontes de enerxía renovables son os alcohois como etanol, butanol e hexanol. A produción biolóxica de etanol e alcohois de cadea longa pode ser unha boa e barata forma de obter novas fontes de enerxía renovables. A fermentación de materiais e gases residuais é unha enerxía barata e unha tecnoloxía de materias primas flexibles para a produción deses bioalcohois. O uso de todos eses materiais residuais en biorefinerías ten dúas vantaxes principais: é un método eficiente para a eliminación de contaminantes e gases de efecto invernadoiro (CO, CO2) e isto permite producir liquides con alto valor comercial. O principal obxectivo desta tese é a obtención de alcohois de cadea longa empregando diferentes fontes de carbono (glucosa, monóxido de carbono e gas de síntese) empregando a Clostridium carboxidivorans como biocatalizador. Os resultados están recollidos nos distintos capítulos desta tese doctoral, co obxectivo de optimizar as condicións de fermentación e os parámetros operativos da bioconversión destas fontes de carbono en al cohois.