An advantageous feature of fermentative H2 production is its temporal separation from photosynthesis (Figure 1). That is, 02 evolution and C02 photoassimilation by photosynthesis occur during the day, whereas fermentative H2 production by degradation of photosynthetic product (starch) can occur during the night. By taking advantage of this temporal separation between 02 evolution (day) and H2 production (night), one can potentially develop an algal H2 production technology that avoids the problem of gas product separation. This approach is a major project by scientists in Japan (22, 29, 33, 34), and important progress has been made. Miura et al. (34) have demonstrated algal fermentative H2 production that is temporally separated from photosynthesis (C02 fixation and 02 evolution), using a combination of green algae and photosynthetic bacteria comprising over 100 L of green algae.

The challenge, as always, in this fermentative H2 production approach is the efficiency. Figure 5 presents a typical measurement of photosynthesis and fermentative H2 production in wild-type Chlamydomonas in a helium atmosphere in the presence of C02 under cycles of 12 h of moderate actinic illumination (PAR, 200 pE — m’2* s1) and 12 h of darkness, using the flow-detection system (Figure 2). From the data, it can be clearly seen that the rate of fermentative H2 production is very slow—less that 5% of the rate of oxygen evolution during the day. Therefore, enhancing the rate of dark fermentative H2 production is the key challenge.