Photobiological Hydrogen Production – Prospects and Challenges Efforts to scale up the capacity of green algae and cyanobacteria to use sunlight to convert water into hydrogen gas for energy use

G lobal energy demand is projected to increase 50% or more by the year 2030, with coal remaining the primary fuel for developing countries. Because of the deleterious effects of fossil fuels and the uncertainties of future sources, the development of alternative renewable energy supplies such as hydrogen is critical. Hydrogen, the simplest and most abundant element, can be produced as a gas (H2) from various resources and, when combusted, generates water as the only byproduct. Its enabling technology, the H2-based fuel cell, is twice as efficient as the traditional internal combustion engine currently used in automobiles. However, nearly all hydrogen gas being produced industrially comes from steam-reforming of natural gas, a fossil fuel. Thus the development of alternative and renewable pathways for producing hydrogen fuels is of utmost importance. Green algae and cyanobacteria can use solar energy to convert water into hydrogen gas, an energy carrier whose use does not emit greenhouse gases. Barriers to microbially based, large-scale production of hydrogen include (a) inherent properties of the microbes that preclude continuity and efficiency of H2 production; (b) underlying limitations of photosynthetic efficiency; (c) limitations of the hydrogenase catalyic function; and (d) engineering challenges.