FIGURE 6.

Model of fermentative pathways involved in dark anaerobic H₂ production in C. reinhardtii. In the wild type, PFL1 is the major fermentative enzyme in short term anaerobiosis and cleaves pyruvate into formate and acetyl-CoA. The latter can be reduced to ethanol via bifunctional acetaldehyde-alcohol dehydrogenase (ADH1) (92) or converted to acetate via phosphotransacetylase and acetate kinase (PAT/ACK). In addition to PFL1, PFR1 is capable of pyruvate oxidation. In a pfl1 mutant or in long term anaerobiosis, pyruvate oxidation might be preferably catalyzed by PFR1. PFR1 transfers electrons to ferredoxins and thereby allows H₂ generation via the [Fe-Fe]-hydrogenase. In addition to pyruvate, PFR1 is able to use oxaloacetate as a substrate. This might link the oxidation of other substrates such as fatty acids or amino acids to fermentative H₂ production, possibly via an anaerobically operating glyoxylate cycle or parts thereof.