Hydrogen-producing bacteria are classified into 2 types by the difference in reaction enzymes. One is bacteria with hydrogenase, and the other with nitrogenase.

Hydrogenase: 2H+ + X2- —— H2 + X (5.4.1)

Nitrogenase: 2H+ + 2e — + 4ATP —► H2 + 4ADP + Pi (5.4.2)

X: electron carrier, Pi: inorganic phosphate

As shown in above reactions, hydrogenase catalyzes a reversible reaction of evolution and uptake of hydrogen. On the other hand, the reaction by nitrogenase needs energy (ATP). In anaerobic fermentation, the reaction by hydrogenase is mainly investigated. Representative

CeHiaOe + 2H2O -► 2CH3COOH + 2CO2 + 4H2 AGO’ = -184 kJ (5.4.3)

C6H12O6 -► CH3CH2CH2COOH + 2CO2 + 2H2 AGO’ = -257 kJ (5.4.4)

Figure 5.4.1 shows the pathway of hydrogen fermentation. Hydrogen is formed from hydrogenase via both NADH and ferredoxin, via only ferredoxin, or via formate-lyase. In hydrogen fermentation, hydrogen is produced from oxidative decomposition of organic substrates. Therefore, hydrogen fermentation is utilized as treatment of wastes and waste water. In such cases, the following treatment such as methane fermentation or activated sludge method is needed, since hydrogen fermentation accompanies the production of organic acids. The reaction rate of hydrogen fermentation is fast compared with methane fermentation. It may be promising pre-treatment method of methane fermentation.

Fig.5.4.1. Pathway of hydrogen fermentation