In this laboratory, our current research has focused on improving the energy efficiency of photosynthetic hydrogen production. Using mutants of Chlamydomonas that lack PSI but contain PSII, we have demonstrated a new type of photosynthesis: that is, photoevolution of 02 and H2 and photoassimilation of C02 by PSII light reaction alone (J5, 36). This work builds on the original demonstration by Biochenko et al. (53) of hydrogen and oxygen transients in PSI-deficient mutants of Chlamydomonas. Based on studies of the electron transport pathway (Lee and Greenbaum, 1996, unpublished), the newly discovered water-splitting reaction for H2 and 02 production (reaction 1) or for

C02 fixation and 02 evolution (reaction 2) may require only half the number of photons of conventional Z-scheme photosynthetic reactions 3 and 4.

Newly Discovered PSII Photosynthesis:

H20 + 2 hv——— > H2 + 1/2 02 AG* = — 115 kJ/mol (1)

Energy efficiency = 67.4% (X = 680 nm)

C02 + H20 + 4 hv———— > 1/6(C6HI206) + 02 AGe = — 224 kJ/mol (2)

Energy efficiency = 68.2% (X = 680 nm)

Conventional Z-scheme Photosynthesis:

H20 + 4 hv——— > H2 + 1/2 02 AGe = — 446 kJ/mol (3)

Energy efficiency = 33.7% (X = 680 nm)

C02 + H20 + 8 hv———— > 1/6(C6H1206) + 02 AG° = — 927 kJ/mol (4)

Energy efficiency = 34.1% (X = 680 nm)

Both reactions 1 and 2 have a significantly large negative value of AG*. They should be able to occur spontaneously. Therefore, although the discovery is surprising and novel, it still obeys the laws of thermodynamics. Since reactions 1 and 2 require half the number of photons of reactions 3 and 4, the discovery can potentially lead to H2 production and/or C02 fixation technology with twice the energy conversion efficiency of conventional Z-scheme photosynthesis.

The demonstration of photosynthesis by a single light reaction proved that a single light reaction can span the potential difference between water oxidation and proton reduction for sustained evolution of H2 and 02, which was previously thought to be difficult to achieve (54). Therefore, we can now propose a new reactor system containing biometallocatalysts that requires only a single type of photochemical reaction center (Figure 6) but is able to perform the same function as the reactor system in Figure 3. As illustrated in Figure 6, when water is split to 02 and protons by PSII, electrons from the reducing side of PSII should, neglecting resistive loss, be able to reduce protons on a platinum catalyst surface to evolve H2 in a separate compartment without PSI.

At low light intensity and under ideal laboratory conditions, the maximum sunlight to H2 energy conversion efficiency for Z-scheme photosynthesis has been measured to be about 10% (55). From a practical point of view, application of PSII photosynthesis can potentially double the sunlight conversion efficiency from 10 to 20% (35). This potentially higher efficiency can put photosynthetic H2 production in a much more competitive position vis-a-vis other solar technologies. Moreover, since PSII photosynthesis can also photoassimilate C02, it should also be able to improve the energy efficiency of photosynthesis (C02 fixation) in general. Therefore, the discovery also provides a new opportunity to improve energy efficiency for production of H2 by the photosynthesis/fermentation combined system of Miura et al. (34).

Figure 6. A proposed biometallocatalytic reactor system for production of H2 and 02 in separate compartments by a single light reaction (PSII).

Acknowledgments

This research was supported by the U. S. Department of Energy, the Pittsburgh Energy Technology Center, and the ORNL Laboratory Director’s R&D Fund. Oak Ridge National Laboratory is managed by Lockheed Martin Energy Research Corp., for the U. S. Department of Energy under contract DE-AC05-96OR22464.