Photosynthesis is the fundamental biological process that converts the electromagnetic energy of sunlight into stored chemical energy that supports essentially all life on Earth. In green algae as in higher plants, photosynthesis occurs in a specialized organelle, the chloroplast. Light energy captured by the photosynthetic reaction centers is stored predominately by reduction of C02, using water as the source of electrons. As illustrated in Figure 1, the key components of the photosynthetic apparatus involved in light absorption and energy conversion are embedded in thylakoid membranes inside the chloroplast. They are two chlorophyll (chl)-protein complexes, Photosystem I (PSI) with a reaction center, P700, and Photosystem II (PSII) with another distinct reaction center, P680. According to the current and prevailing concept of oxygenic photosynthe­sis, the Z-scheme, first proposed by Hill and Bendall (2) and now described in many textbooks (5-7), PSII can split water and reduce the plastoquinone (PQ) pool, the cytochrome (Cyt) b/f complex, and plastocyanin (PC), while PSI can reduce ferredoxin (Fd)/nicotinamide adenine dinucleotide phosphate (NADP+) and oxidize PC, the Cyt b/f complex, and the PQ pool. As a result, the electrons derived from water splitting are transferred to Fd/NADP+, which provides the reducing power for reduction of C02 to carbohydrate in the stromal region of the chloroplast by a series of enzymatic reactions collectively called the Calvin cycle. Electron transport in the membrane is coupled with proton transport from the stroma into the lumen, generating a proton gradient across the thylakoid membrane. The proton gradient drives phosphorylation through the coupling factor CF0-CF, to make essential ATP for the reduction of C02. This is the common description of oxygenic photosynthesis.

In many green algae, such as Chlamydomonas, there is a hydrogenase that can be induced under anaerobic conditions (8, 9). The hydrogenase can catalyze the reduction of protons to produce H2 using electrons from the reduced Fd as shown in Figure 1. Since protons are also produced by water splitting at PSII, the net result of this Fd/hydrogenase pathway is simultaneous photoevolution of H2 and 02, using water as the substrate and light energy as the driving force.

Under anaerobic conditions and darkness, H2 may be produced by fermentative metabolic degradation of organic reserves such as starch (Figure 1). This fermentative metabolic process has been well studied (70, 77). Since the substrate of the fermenta­tive pathway is generated photosynthetically by reduction of C02, the net result of the

Figure 1. Conventional photosynthetic pathway based on the Z-scheme.

sequential process, photosynthesis and fermentation, is still splitting water to H2 and 02, with C02 as an intermediate.