Photobiological Hydrogen Production by Cyanobacteria Utilizing Nitrogenase Systems -Present Status and Future Development-

Abstract

For photobiological hydrogen (H2) production, cyanobacteria are among the ideal candidates because they grow with minimal nutritional requirements (only mineral salts and water), with sun light as the sole source of energy, and with H2 and O2 as the major products. In cyanobacteria, H2 production is catalyzed by either nitrogenase or hydrogenase. The merits of the nitrogenase reaction are that H2 production is simultaneous with photosynthetic O2 evolution and no anaerobic treatment of the cells is required. One of the major obstacles to efficient photobiological production of H2 in heterocystous cyanobacteria might be the presence of hydrogenases that reabsorb the H2 produced by nitrogenase in the heterocysts. Many heterocystous cyanobacteria contain both uptake hydrogenase (Hup) and bidirectional (or reversible) hydrogenase (Hox) though a few have only Hup. In the experiment described in the chapter, three hydrogenase mutants from Anabaena sp. PCC 7120 (∆hupL, ∆hoxH, and ∆hupL/∆hoxH) were constructed and the effects of deletion of hydrogenase genes on hydrogen production were evaluated. Disruption of uptake hydrogenase gene (∆hupL mutant) was effective in elevating hydrogen productivity in Anabaena sp. PCC 7120 cells. The efficiency of visible light energy conversion to H2 by the ∆hupL mutant at its highest H2 production stage was 1.0–1.6 % at an incident light intensity of 50 W/m2.