The partial pressure of syngas components have a major influence on microbial growth and product profiles because the enzymes involved are sensitive to substrate exposure [194]. Due to the low solubility of CO and H2 in water, the growth of dense bacterial cell cultures can face mass transfer limitations, so increasing the partial pressure of gaseous substrates can help alle­viate this problem. For instance, studies in which the CO partial pressure (PCo) increased from 0.35 to 2.0 atm showed that this resulted in a 440% increase in maximum cell density, a signifi­cant increase in ethanol productivity and a decrease in acetate production in C. carboxidivorans strain P7 [195]. In another study involving C. ljungdahlii, the increase of PCO from 0.8 to 1.8 atm had a positive effect on ethanol production, and the microbe did not exhibit any substrate in­hibition at high PCO [196].In less CO-tolerant microorganisms, the effect of increasing PCO parti­al pressure range from non-appreciable in the case of Rhodospirillum rubrum [197], to negative impact on doubling time of Peptostreptococcus productus (now: Blautia product) [194] and Eubac — terium limosum [198]. Similar to CO, the increase in partial pressure of H2 (pH2) to 1700 mbar en­hanced acetate productivity of A. woodii to 7.4g acetate/ l/day [199].