The Effect of Carbon Source
Anaerobic microorganisms are able to grow either chemoorgano — trophically with carbon sources such as fructose, acetate, malate, glutamate, furmarate, succinate and pyruvate, or chemolithotrophi — cally on substrates such as CO and H2/CO2, which are considered as the main syngas constituents. During the syngas fermentation these anaerobic microorganisms utilize CO or CO2 during the fermentation process to provide energy for bacterial growth and maintenance and also metabolize the production of alcohols and acetate. Cotter et al. first reported a comparison of a carbohydrate carbon source with syngas [30]. In this study the growth of C. Ijung — dahlii was measured in syngas (50% N2, 20% CO, 20% CO2 and 10% H2), as well as utilizing fructose as the carbon source. They found that bacterial growth on sugar-based carbon resulted in a dense culture of about 1 g/L, whereas the syngas produced only 0.562 g/L. Moreover, the ethanol concentration of fructose-fed culture was 13 mM and in the syngas-fed culture was 3.8 mM. This type of large difference in growth performance was most likely due to the diffusion limitations at the gas-liquid interface and/or efficiency of the uptake and transport mechanism of the gaseous substrate. In addition to this, they observed [30] that syngas fermentation inoculated with cells pre-cultured on fructose resulted in a higher cell density (0.850 g/l) in comparison to the cells pre-cultured on syngas (0.562 g/l). It was inferred that such difference in culture performance was likely attributed to a greater availability of intracellular cofactors, enzymes, and maintenance energy in cells adapted to sugar substrate.