Effect of salinity on C1-gas fermentation by Clostridium carboxidivorans producing acids and alcohols

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 IC 50 , i.e. concentration yielding 50% growth inhibition, was estimated, and reached a value of 11 g/L sodium chloride.

Quantitative Bioreactor Monitoring of Intracellular Bacterial Metabolites in Clostridium autoethanogenum Using Liquid Chromatography–Isotope Dilution Mass Spectrometry

“…The accumulation of pyruvate and acetyl-CoA may play an indirect role in the production of ethanol from acetate and acetaldehyde. The increased level of ethanol associated with the decreased level of acetate during pH reduction was as a result of ferredoxin-dependent acetaldehyde oxidoreductase (AFOR) and alcohol dehydrogenase (ADH), the enzymes responsible for the conversion of acetate to ethanol, which was consistent with a similar microorganism …”

Residual Gas for Ethanol Production by Clostridium carboxidivorans in a Dual Impeller Stirred Tank Bioreactor (STBR)

“…The maximum biomass concentration for all media was detected after about 10 h due to fast glucose consumption, followed by an accented drop until approximately 50 h. After that, it stabilized at approximately 0.5−0.6 g/L. This growth profile has been shown for syngas batch fermentations with C. carboxidivorans in other works [22,36,37]. Since syngas is not continuously fed in serum bottles, cell growth reaches the stationary phase, when there is a balance between growth and death of cells.…”

“…Efforts to increase the gas-liquid mass transfer usually include the study of different reactor designs such as stirred tank reactor (STR) [14,22], bubble column reactor (BCR) [23], hollow fiber membrane reactor (HFMR) [24], monolithic biofilm reactor (MBR) [23], trickle bed reactor (TBR) [25], and horizontal rotating packed bed biofilm reactor (h-RPB) [26]. STR is the most usual bioreactor used for biotechnology due to its good mixing and simple operation.…”

Effect of Humin and Chemical Factors on CO2-Fixing Acetogenesis and Methanogenesis

“…NaCl concentrations above 6 g/L inhibited acetogenesis. This inhibitory threshold was lower than that reported previously [55], in which Clostridium carboxidivorans, a CO 2 -fixing acetogen, was inhibited by NaCl at concentrations higher than 9 g/L. For methanogenesis, the optimal concentration of NaCl for the acceleration of methane production was 6 g/L.…”