Hydrogen production by microalgae

The production of H2 gas from water and sunlightusing microalgae, `biophotolysis', has been a subjectof applied research since the early 1970s. A numberof approaches have been investigated, but most provedto have fundamental limitations or requireunpredictable research breakthroughs. Examples areprocesses based on nitrogen-fixing microalgae andthose producing H2 and O2 simultaneously fromwater (`direct biophotolysis'). The most plausibleprocesses for future applied R & D are those whichcouple separate stages of microalgal photosynthesisand fermentations (`indirect biophotolysis'). Theseinvolve fixation of CO2 into storagecarbohydrates followed by their conversion to H2by the reversible hydrogenase, both in dark andpossibly light-driven anaerobic metabolic processes. Based on a preliminary engineering and economicanalysis, biophotolysis processes must achieve closeto an overall 10% solar energy conversion efficiencyto be competitive with alternatives sources ofrenewable H2, such as photovoltaic-electrolysisprocesses. Such high solar conversion efficiencies inphotosynthetic CO2 fixation could be reached bygenetically reducing the number of light harvesting(antenna) chlorophylls and other pigments inmicroalgae. Similarly, greatly increased yields ofH2 from dark fermentation by microalgae could beobtained through application of the techniques ofmetabolic engineering. Another challenge is toscale-up biohydrogen processes with economicallyviable bioreactors.Solar energy driven microalgae processes forbiohydrogen production are potentially large-scale,but also involve long-term and economically high-riskR&D. In the nearer-term, it may be possible tocombine microalgal H2 production with wastewatertreatment.