CHAPTER 4: The Physiology of the Bidirectional NiFe-hydrogenase in Cyanobacteria and the Role of Hydrogen Throughout the Evolution of Life

Hydrogen (H₂) probably played a central role as an electron donor for the fixation of inorganic carbon at the advent of life on Earth. LUCA, the last universal common ancestor, most likely possessed a NiFe-hydrogenase and transferred the enzyme to the first bacterial and archaean cell. At present, hydrogenases are widespread in the environment and are not restricted to anoxic habitats. The occurrence of the cyanobacterial bidirectional NiFe-hydrogenase shows strong correlation with the habitat. It is prevalent in freshwater, coastal surface waters, microbial mats, and hot springs, but is absent from the open ocean and terrestrial deserts. The enzyme is truly bidirectional, with a bias towards H₂ production. In the presence of oxygen, it forms two inactive states that require reactivation at distinct redox potentials. The physiology of cyanobacterial H₂ production by means of the bidirectional NiFe-hydrogenases is discussed at the onset of illumination and under fermentative conditions. Approaches to enhance cyanobacterial H₂ production based on the current knowledge are suggested.