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Microbial Electrosynthesis (MES) has been widely applied for acetic acid (HA) production from CO 2 and electricity. Ethanol (EtOH) has a higher market value than HA, and wide application in industry and as a biofuel. However, it has only been obtained sporadically and at low concentrations, probably due to sub-optimal operating conditions. This study aimed at enhancing EtOH productivity in MES cells by jointly optimising key operation parameters, including pH, H 2 and CO 2 partial pressure (pH 2 and pCO 2 ), and HA concentration, to promote solventogenesis. Two H-type cells were operated in fed-batch mode at -0.8 V vs. SHE with CO 2 as the sole carbon source. A mixed culture, enriched with Clostridium ljungdahlii was used as the biocatalyst. The combination of low pH (<4.5) and pCO 2 (<0.3 atm), along with high HA concentration (about 6 g L -1 ) and pH 2 (>3 atm), were mandatory conditions for maintaining an efficient solventogenic culture, dominated by Clostridium sp., capable of high-rate EtOH production. The maximum EtOH production rate was 10.95 g m -2 d -1 , and a concentration of 5.28 g L -1 was achieved. Up to 30 % of the electrons and 15.2 % of the carbon provided were directed towards EtOH production, and 28.1 kWh were required for the synthesis of 1 kg of EtOH from CO 2 . These results highlight that strict conditions are required for a continuous, reliable, EtOH production in MES cells. Future investigation should focus on improving cell configuration to achieve EtOH production at higher current densities while minimizing the electric energy input., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Research efforts aimed at increasing ethanol and butanol productivity from syngas are currently gaining attention. For most model carboxydotrophic bacteria, production rates, yields and maximum product titres have been studied in detail, but little is known on alcohol toxicity in these bacteria. The aim of this work was to investigate the inhibitory effects of ethanol and butanol on the growth of Clostridium ljungdahlii PETC, C. carboxidivorans P7, and 'Butyribacterium methylotrophicum DSM3468'. Experiments to determine inhibitory effects due to product accumulation were carried out using a synthetic mixture of CO:CO2:H2 as a substrate. These conditions were chosen to mimic gaseous effluents of biomass and waste gasification plants. Inhibition effects were recorded as changes in growth parameters. No significant inhibition was observed for ethanol at concentrations below 15 g/L. The three species exhibited higher sensitivity to butanol. Half inhibition constants for butanol could be estimated for P7 (IC50 = 4.12 g/L), DSM3468 (IC50 = 1.79 g/L), and PETC (IC50 = 9.75 g/L). In conclusion, at least for the tested strains, alcohol toxicity is not an immediate handicap for increasing alcohol production of the tested homoacetogenic strains.

Incubation of carboxydotrophs at 37 °C provides optimal conditions for their growth. However, a fast accumulation of organic acids, specifically acetate, during the exponential growth phase may result in low alcohol production and substrate consumption due to a phenomenon known as "acid crash". The present work investigates growth and productivity of Clostridium carboxidivorans P7 at two incubation temperatures. At 37 °C the culture was not able to override the "acid crash", resulting in low ethanol titers (1.56 mM). On the other hand, lower metabolic rates at 25 °C enhanced ethanol and butanol production (32.1 and 14.5 mM, respectively). Moreover, at low temperatures, hexanol and caproic acid were also produced at significant concentrations, 8.21 and 9.02 mM respectively, among the highest values reported for P7. Our results demonstrate that production of biofuels with longer carbon chains molecules may be enhanced incubating syngas-fermenting acetogenic bacteria at sub-optimal temperatures., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

The current energy model based on fossil fuels is coming to an end due to the increase in global energy demand. Biofuels such as ethanol and butanol can be produced through the syngas fermentation by acetogenic bacteria. The present work hypothesizes that formate addition would positively impact kinetic parameters for growth and alcohol production in Clostridium ljungdahlii PETC and Clostridium carboxidivorans P7 by diminishing the need for reducing equivalents. Fermentation experiments were conducted using completely anaerobic batch cultures at different pH values and formate concentrations. PETC cultures were more tolerant to formate concentrations than P7, specially at pH 5.0 and 6.0. Complete growth inhibition of PETC occurred at sodium formate concentrations of 30.0 mM; however, no differences in growth rates were observed at pH 7.0 for the two strains. Incubation at formate concentrations lower than 2.0 mM resulted in increased growth rates for both strains. The most recognizable effects of formate addition on the fermentation products were the increase in the total carbon fixed into acids and alcohols at pH 5.0 and pH 6.0, as well as, a higher ethanol to total products ratio at pH 7.0. Taken all together, these results show the ability of acetogens to use formate diminishing the energy demand for growth, and enhancing strain productivity., (Copyright© by the Spanish Society for Microbiology and Institute for Catalan Studies.)