Biofuels in Perspective

ΔG'
30
20
10
0
−10
−20
−30
−40
−50
1
0.1
Biological Hydrogen Production by Anaerobic Microorganisms 203
0.01
25
70
100
0.001
0.0001
Hydrogen partial pressure (Atm)
25
70
100
0.00001
Figure 11.2 Effect of the hydrogen partial pressure on the Gibbs free energy change of hydrogen production
from NADH (solid line) and reduced ferredoxin (dashed line) at three different temperatures (25, 70 and
100 ◦ C). Values calculated from data in (6, 34, 35).
reduction to form H2. There are three reasons that can explain these observations. Firstly,
�G ◦′ values are calculated for standard conditions (1 Molar concentration of the reactants,
25 ◦ C, pH 7), which may differ considerably from the average physiological conditions.
For instance, when the P(H2) is low, the situation becomes notably different. For example,
at a P(H2) of10 −4 atm proton reduction by NADH is even exergonic (−4.7 kJ/mol). Depending
on the NAD + /NADH ratio this value may become even more negative. Secondly,
also temperature affects the thermodynamics of a reaction ((�G 0 = �H − T�S 0 )). As
discussed by Stams, 33 hydrogen producing reactions become energetically more favorable
at higher temperatures. Figure 11.2 illustrates how the �G’ value of proton reduction by
NADH or ferredoxin changes as function of the P(H2) and temperature.
These data suggest that at elevated temperatures H2 production from NADH is even
more exergonic, which corresponds with the high H2 levels reported for Tt. maritima and
other hyperthermophiles (Table 11.2). A third explanation for hydrogen formation from