Photosynthetic hydrogen production: Mechanisms and approaches

The development of renewable fuels of the future is important for the replacement of depleting oil and natural gas reserves. Hydrogen is one of the most promising clean fuels, since its combustion yields only water. One of the visionary methods to obtain hydrogen at the expanse of solar energy is the use of photosynthetic microorganisms. Hydrogen production in phototrophs is coupled to the oxygenic and anoxygenic photosynthesis involving hydrogen-evolving enzymes, hydrogenases and nitrogenases. At the present time the efficiency of hydrogen photoproduction is not sufficiently high. Most hydrogen-evolving enzymes are inhibited by molecular oxygen, which creates a major barrier for the sustained hydrogen photoproduction in oxygenic phototrophs, such as green algae and cyanobacteria. However, several strategies have been applied to solve this problem, including spatial and temporal separation of water splitting and hydrogen evolution, and regulation of water splitting activity and respiration to maintain anoxic conditions. Anoxygenic photosynthesis can be used to drive hydrogen photoproduction in integrated systems including fermentative anaerobic organisms. In this review different mechanisms for hydrogen production in photosynthetic organisms and the latest advances in this area are discussed.