Hydrogen production by microalgae

Recent applied research on biophotolysis-hydrogen gas production by microalgae, from water and sunlight-is reviewed. Most approaches have proven to be limited, including those based on nitrogen-fixing microalgae and those producing hydrogen and oxygen gas simultaneously. The most plausible approaches are those that couple separate photosynthesis and fermentation stages. These involve fixation of carbon dioxide into storage carbohydrates, followed by conversion to hydrogen gas by reversible hydrogenase. Preliminary engineering and economic analyses suggest that a 10% conversion efficiency is required to be competitive with other sources of renewable hydrogen. This high efficiency could be achieved by genetically reducing the number of light harvesting chlorophylls and other pigments in microalgae. Hydrogen yields from dark fermentation could be greatly increased by incorporating metabolic engineering methods. Another avenue is scale-up of biohydrogen processes with economically viable bioreactors. Large-scale biohydrogen production will require long-term, high risk R&D. Until efficiencies are improved, near term use of microalgae for wastewater treatment and other processes may be feasible.