Like other organisms, acetogens have a limited range of pH for optimal growth so the pH of the fermentation medium needs to be closely controlled. The extracellular pH directly influ­ences the intracellular pH, membrane potential, proton motive force, and consequently sub­strate utilization and product profile [208, 209]. In most studies, lowering pH medium divert carbon and electron flow from cell and acid formation towards alcohol production [113, 209—211]. By applying this knowledge, Gaddy and Clausen performed a two-stage CSTR syngas fermentation systems using C. ljungdahlii where they set the first reactor at pH 5 to promote cell growth, and pH 4 — 4.5 in the second reactor to induce ethanol production [212]. One recent study with C. Ijungdahlii showed conflicting results in which cell density and ethanol production were both higher at pH 6.8 when compared to pH 5.5 [213].

1.5. Temperature

The optimum temperature for mesophilic acetogens are between 30-40°C, while thermophil­ic acetogens grow best between 55 and 58°C. The fermentation temperature not only affects substrate utilization, growth rate and membrane lipid composition of the acetogens, but also gas substrate availability because gas solubility increases with decreasing temperature [24, 211]. "C. ragsdalef’was reported to produce more ethanol at 32°C than at the optimum growth temperature of 37°C [211].