Carbon dioxide as key player in chain elongation and growth of Clostridium kluyveri: Insights from batch and bioreactor studies.

• C. kluyveri managed to grow in a medium with different levels of available CO 2. • 11.0 g/L C 6 was reached from acetate and ethanol in a bioreactor with CO 2 supply. • There is a direct correlation between CO 2 concentration and C. kluyveri 's growth. • CO 2 uptake by C. kluyveri at bioreactor level was demonstrated. Chain elongation technology allows medium-chain fatty acids (MCFAs) production as an alternative to fossil resources. Clostridium kluyveri generates n -caproate primarily from ethanol and acetate, presumably requiring CO 2 for growth. Here, the impact of CO 2 on C. kluyveri was explored. Bottle studies revealed the bacterium's adaptability to low CO 2 levels, even in conditions with minimal dissolved NaHCO 3 (0.0003 M) and unfavorable pH (below 6) under 1 bar CO 2. Bioreactor investigations demonstrated a direct correlation between CO 2 availability and bacterial growth. The highest n -caproate production (11.0 g/L) with 90.1 % selectivity was achieved in a bioreactor with continuous CO 2 supply at 3 mL/min. Additional bottle experiments pressurized with 1 bar CO 2 and varying ethanol:acetate ratios (1:1, 2:1, 4:1) also confirmed CO 2 consumption by C. kluyveri. However, increasing the ethanol:acetate ratio did not enhance n -caproate selectivity, likely due to overly acidic pH conditions. These findings provide insights into chain-elongators responses under diverse conditions. [ABSTRACT FROM AUTHOR]