ABSTRACT
Most of the organisms can produce hydrogen naturally. In fact, for some of them, hydrogen production is a survival mechanism under stress conditions. Under anaerobic conditions, they can maintain the redox balance by releasing the electrons as molecular hydrogen. However, some organism such as methanogenic bacteria can consume hydrogen by oxidative metabolism to obtain energy from the H−H bond. They are involved in recycling the hydrogen in nature (Zeikus, 1977). The chemical equation for methanogenesis can be summarized as follows (Equation 3.1):
4H2 + CO2 → CH4 + 2H2O (3.1)
For every methane that is generated, one ATP is also generated (Conrad et al., 1985). They oxidize H2 to a proton (H+) and a hydride ion (H−) via a hydrogenase enzyme. Besides, some hydrogen-producing microorganisms also possess bidirectional hydrogenase and uptake hydrogenase by which they can re-consume hydrogen and maintain the redox potential of the cell. To increase the yield of hydrogen, consumption of hydrogen is not desirable during hydrogen generation. Thus the reaction broth in a reactor should be devoid of methanogens and uptake hydrogenase activity of the hydrogenproducing organisms to increase the yield of hydrogen.
