Chapter 8: Photobiological Methods of Renewable Hydrogen; Production

Oxygenic photosynthetic microbes such as green algae and cyanobacteria normally absorb sunlight and store the energy in the form of polysaccharides such as starch (in green algae) or glycogen (in cyanobacteria). These storage biomolecules are mobilized, as required, to produce the energy needed to drive microbial metabolism. The conversion of light energy into chemical potential, "the light reactions of photosynthesis", is well described and has been reviewed recently (cf. Ref. 32,132,184, and references therein). Under certain conditions these microbes can use the energy from sunlight to produce H₂ gas instead of fixing carbon, and thus are at least partially able to sustain growth and cellular repair function although at lower than normal levels for up to several days.65 However, H₂ production is not the normal function of algal and cyanobacterial photosynthesis. Indeed, the H ₂-producing enzymes are not even synthesized under normal growth conditions (with the exception of some cyanobacteria), and their genes are only expressed following exposure to specific environmental conditions. Nevertheless, it is this type of biological function that offers the potential to efficiently generate renewable H₂ in direct light conversion processes. The photobiological production of H₂ by oxygenic, photosynthetic organisms occurs in four major sequential steps: 1. light absorption and transfer of excitation energy to reaction center (RC) molecules; 2. charge separation by the RCs, electron extraction from water, and charge equilibration between the photosystems; 3. transfer of low potential (high energy) electrons to intermediate acceptors such as ferredoxin or some combination of an organic carrier and ferredoxin; and 4. H₂ gas release associated with nitrogenase or hydrogenase enzymes. The overall light-conversion efficiency of the process is determined mainly by the absorption spectrum of the organism, the amount of that energy delivered to the H₂-producing enzyme, and the energetic requirements of the enzyme catalyzing the H₂-evolving step. The purpose of Sections 2 and 3 is to show how green algae and cyanobacteria approach each step and how the process might be mobilized to produce useful amounts of H₂. Section 4 will discuss other types of biohydrogen systems, Section 5 will examine the major current scientific issues preventing application of photobiological H₂-production technology, and Section 6 will outline some future directions.