Biofuels in Perspective

Biological Hydrogen Production by Anaerobic Microorganisms 199
is the case but hydrogen is only formed when it is efficiently taken away by methanogens.
Thermodynamically, acetate can only be oxidized to carbon dioxide at a very low hydrogen
partial pressure (P(H2)). Thus, hydrogen inhibits its own formation. This is not only the
case for acetate, but also for a variety of other organic substrates. 1
Thus far, formation of hydrogen-rich gas in dark fermentation is only feasible from
sugars, although it is restricted by thermodynamics. This is also true for thermophilic
microorganisms, even though they harbor the biochemical potential to convert complex
organic carbon completely to carbon dioxide and hydrogen. Thus, energy input is needed
to overcome the thermodynamic barrier.
11.3 Thermodynamics of Hydrogen Formation
Carbohydrates are the main substrates for fermentative bacteria that produce hydrogen. 3–5
Cellulose and hemicellulose are the most abundant polysaccharides available in nature, and
therefore, glucose and xylose are the predominant monomeric sugars used for hydrogen
formation. Starch and sucrose may also be available in large amounts as these are used as
storage material in various plants. Fermentation of glucose and xylose results in hydrogen
formation, but in addition, depending on the microorganism, also acetate, butyrate,
lactate, ethanol and some other organic compounds (e.g. formate, butanediol, succinate)
are produced. Under standard conditions complete oxidation of glucose to CO2 and H2 is
thermodynamically not possible:
glucose + 12 H2O → 6HCO3 − + 6H + + 12 H2 �G 0′
=+3.2kJ/mol
The oxidation of glucose proceeds only to acetate, and even in this case other end
products are usually produced as well. Intermediates of the metabolism are used as electron
acceptors resulting in branched pathways, leading to butyrate, lactate, ethanol, alanine etc.
(Figure 11.1). Production of acetate is coupled to the synthesis of ATP by substrate-levelphosphorylation,
whereas production of the other products yields less or no ATP. These
branched pathways enable the microorganism to adjust the metabolism so that an optimal
ATP gain and thermodynamic efficiency of ATP synthesis are accomplished. 6
The oxidation of glucose to 2 acetate and 4 H2 is thermodynamically feasible, but in
natural environments this only occurs when the hydrogen concentration is kept low by
hydrogen-consuming methanogens. 1
glucose + 2H2O → 2 acetate − + 2HCO3 − + 4H2
�G 0′
=−206.1 kJ/mol
The need for a low hydrogen partial pressure can be explained best when we look at the
hydrogen producing reactions. During catabolism (e.g. the Embden-Meyerhof pathway)
reducing equivalents are produced in the form of NADH (glyceraldehyde-3-P dehydrogenase
reaction) and in the form of reduced ferredoxin (pyruvate:ferredoxin oxidoreductase
reaction) (Figure 11.1). The midpoint redox potential of the couples NAD + /NADH and
oxidized ferredoxin/reduced ferredoxin are −320 mV and −398 mV, respectively. 6 These
electron carriers must be recycled continuously for catabolism to proceed. Various fermentation
reactions exist that can accomplish this recycling of electron mediators, but the