Photosynthesis is the fundamental driving force that supports all biofuel synthetic processes by conversion of solar energy into carbon storage products and/or H2. Hydrogen photoproduction by micro-organisms combines the photosynthetic properties of oxygenic and non-oxygenic microbes with the activity of H2-producing enzymes in nature: hydrogenases and nitrogenases. To introduce the subject of biohydrogen production, the main types of photobiological H2-producing processes found in nature are presented: 1) direct biophotolysis which couples oxygenic photosynthesis to H2 production by hydrogenases (found in green algae and cyanobacteria; 2) indirect biophotolysis with separated oxygen formation and H2 production catalysed by hydrogenase or nitrogenase; 3) nonoxygenic photosynthesis coupled to nitrogenase-catalyzed H2 production present in many purple photosynthetic bacteria. Additionally, alternative methods to circumvent the current limitations, the integration of two or more biological processes into a single system are discussed.
