TABLE 3.

Comparison of theoretical H₂ thresholds of three model acetogens.

Strain	Acetobacterium woodii	Clostridium autoethanogenum	Moorella thermoacetica
Temperature optimum (°C)	25	37	55
pH optimum	7.0	5.5	7.0
ATP gain n (mole ATP/mole acetate)a	0.3b	1c	0.5d
Hydrogen threshold (Pa)e	14	1160	51

^aOnly ATP gains for the formation of acetate are considered here, ATP gains for the formation of other products are different (Bertsch and Müller, 2015; Mock et al., 2015). ^bThe energy conservation mechanism of A. woodii is completely unraveled and the exact ATP gain known (Schuchmann and Müller, 2014). ^cThis ATP gain is an assumed value (Mock et al., 2015), as the energy conservation mechanism of C. autoethanogenum is not yet completely unraveled. ^dThis ATP gain is an assumed value (Schuchmann and Müller, 2014; Basen and Müller, 2017), as the energy conservation mechanism of M. thermoacetica is not yet completely unraveled. ^eCalculated using Equation 6 and the ATP gain n and the pH and temperature given in the table and assuming an acetate concentration of 2 mM and CO₂ partial pressure of 0.2 atm. The effect of the temperature on ΔG⁰_acetogenesis and ΔG_ADP/ATP was neglected, as the temperature effect on ΔG_ADP/ATP is not known.