2.The work of Gaffron and Rubin demonstrated that Scenedesmus produced hydrogen gas not only under light conditions, but also produced it fermentatively under dark anaerobic conditions, with intracellular starch as a reducing source. Although the rate of fermentative hydrogen production per unit of dry cell weight, was less than that obtained through light-dependent hydrogen production, hydrogen production was sustainable due to the absence of oxygen. On the basis of experiments conducted on fermentative hydrogen production under dark conditions, Miura and Miyamoto's group (6) proposed hydrogen production in a light/dark cycle. According to their proposal, CO2 is reduced to starch by Photosynthesis in the daytime (under light conditions) and the starch thus formed, is decomposed to hydrogen gas and organic acids and/or alcohols under anaerobic conditions during nighttime (under dark conditions). The technological merits of this proposal include the fact that oxygen-inactivation of Hydrogenase can be prevented through maintenance of Green algae under anaerobic conditions, nighttime hours are used effectively, temporal separation of hydrogen and oxygen production does not require gas separation for simultaneous water-splitting, and organic acids and alcohols can be converted to hydrogen gas by photosynthetic bacteria under light conditions (Section 5.3). A pilot plant using a combined system of green algae and photosynthetic bacteria was operated within a power plant of Kansai Electric Power Co. Ltd. (Nankoh, Osaka, Japan). Miyamoto and co-workers (7) recently proposed chemical digestion of Algal Biomass as a means of producing substrates for photosynthetic bacteria, thus improving the yield of starch degradation.
3. Asada and Kawamura determined that Cyanobacteria also produce hydrogen gas auto-fermentatively under dark and anaerobic conditions. Spirulina species were demonstrated to have the highest activity among cyanobacteria tested. The nature of the electron carrier for hydrogenase in cyanobacteria is still unclear. Hydrogenases have been purified and partially characterized in a few cyanobacteria and microalgae