Prospects of ethanol and biodiesel as substitutes for conventional fuels will not be discussed here; these two aspects are presented in sufficient detail in Chaps. 3, 4, 5, and 6. One of the promising approaches for future fuel is, perhaps, hydrogen and methane, both of which could be obtained from living, particularly microbial resources.

Photosynthesis is the main route through which oxidized carbon is reduced and again oxidized back to carbon dioxide for the generation of energy. Based on this principle, we can utilize a few steps from this life chain. This topic could be called biophotolysis—alternatively, photobiolysis.

In the system, direct electron transport from water to hydrogen has not been demonstrated as a technically feasible reaction. For this, con­tinued research is required to elucidate the basic nature of FeS (PEA, ferredoxin, and hydrogenase). This may lead ultimately to the practical feasibility of production of hydrogen (ideally 20 ^L/h). Section 1.16 dis­cusses hydrogen in detail. One inherent problem is the stability of the hydrogenase system because of its sensitivity to molecular oxygen pro­duced during photosynthesis.

However, one may design a two-step or two-compartment system. Reduced Co II is the oxygen-stable electron carrier between photo­synthesis and hydrogenase. A higher ratio of reduced Co II or Co II helps the evolution of hydrogen, in spite of the unfavorable redox potential of the coenzyme. Only Co II (reduced) can be pumped or transported from one stage (compartment) to the other. Photosynthesis and hydrogenase systems have to be encapsulated or immobilized sep­arately in order to retain their respective activity; the two stages or compartments may be connected through fiber filters. An example could be to use appropriate algae to produce reduced organic com­pounds which can be pumped into bath of photosynthetic bacteria of hydrogen fermentation.

One partial modification will be to collect oxygen during the day and hydrogen at night, at the expense of accumulated reduced coenzymes, made operative by anaerobically adapted microalgae or nonheterocystous nitrogen — fixing blue-green algae. For product separation, the enzyme technology or immobilization is inapplicable for biophotolysis. However, there are potential practical applications of immobilized hydrogenase in biochemical hydrogen—oxygen fuel cells. If such enzymes can be immobi­lized on an electrode surface, an inexpensive fuel cell might be developed, which would increase the energy recoverable for hydrogen to save fuels.

Awareness of the limitations due to efficiency, engineering, and the economy of these principles will save disappointment and encourage con­tinued research. Geographical location and frequency of weather change limits the insolation. The best photosynthetic efficiency is only 6% of the total incident solar radiation, i. e., 5 kg/(m2 • yr) of H2 by biophotolysis. Half of this could be a very satisfactory achievement.