Schematic representation of the metabolism underlying ‘photofermentation’, based on the introduction of a fermentation pathway or a hydrogen evolution pathway (i.e. a hydrogenase) from a chemotrophic organism into a cyanobacterium. Coupling between the endogenous metabolism of the phototrophic organism and the (heterologously encoded) pathways may occur through central metabolites like glyceraldehyde-3-phosphate or NADPH (and ATP).