Hydrogen production by microalgae

The production of H-2 gas from water and sunlight using microalgae, 'biophotolysis', has been a subject of applied research since the early 1970s. A number of approaches have been investigated, but most proved to have fundamental limitations or require unpredictable research breakthroughs. Examples are processes based on nitrogen-fixing microalgae and those producing H-2 and O-2 simultaneously from water ('direct biophotolysis'). The most plausible processes for future applied R & D are those which couple separate stages of microalgal photosynthesis and fermentations ('indirect biophotolysis'). These involve fixation of CO2 into storage carbohydrates followed by their conversion to H-2 by the reversible hydrogenase, both in dark and possibly light-driven anaerobic metabolic processes. Based on a preliminary engineering and economic analysis, biophotolysis processes must achieve close to an overall 10% solar energy conversion efficiency to be competitive with alternatives sources of renewable H-2, such as photovoltaic-electrolysis processes. Such high solar conversion efficiencies in photosynthetic CO2 fixation could be reached by genetically reducing the number of light harvesting (antenna) chlorophylls and other pigments in microalgae. Similarly, greatly increased yields of H-2 from dark fermentation by microalgae could be obtained through application of the techniques of metabolic engineering. Another challenge is to scale-up biohydrogen processes with economically viable bioreactors. Solar energy driven microalgae processes for biohydrogen production are potentially large-scale, but also involve long-term and economically high-risk R&D. In the nearer-term, it may be possible to combine microalgal H-2 production with wastewater treatment.