Fig. 2

The metabolic pathways for biological hydrogen production by dark fermentation. The general substrate glucose is broken down to pyruvate through the glycolytic pathway. The fate of pyruvate differs depending on the corresponding microorganisms. The PFL pathway is employed by facultative anaerobes (shown on the right). The PFOR pathway is typical for strict anaerobes (shown on the left). Abbreviations for enzymes shown are: NFOR, NADH:ferredoxin oxidoreductase; HYD, ferredoxin-dependent hydrogenase; PFOR, pyruvate:ferredoxin oxidoreductase; ACK, acetate kinase; PTA, phosphotransacetylase; LDH, lactate dehydrogenase; PFL, formate lyase. FHL, formate:hydrogen lyase complex; AAD, aldehyde alcohol dehydrogenase; THL, thiolase; ADC, acetoacetate decarboxylase; HBD, β-hydroxybutyryl-CoA dehydrogenase; BCD, butyryl-CoA dehydrogenase; BUK, butyrate kinase