The problem of light saturation has been a subject of research in photosynthesis for almost 5 decades, with the report by Kok (1953) that microalgae cultures exposed to short (milliseconds) flashes of bright light, followed by longer periods of darkness, exhibited the same light conversion efficiencies as cultures exposed to the same total photon flux averaged for the entire period. The interpretation was straightforward: only a limited number of photons can be used per unit time, and the millisecond light/dark periods allow averaging high photon fluxes. A large body of literature has developed on this subject, including laboratory work by the ASP (Terry 1984, 1986; see also Section II). The mass culture work in Hawaii (Section III. B.2.), among many others, attempted to use this phenomenon to increase algal productivities. However, practical applications are not plausible because of the very short time periods involved. Another approach, central to the Japanese microalgae program (Section IV. B.1.c.), has been to diffuse light throughout the depth of the culture, using optical fibers, thus avoiding high a surface irradiance. But this approach is also not practical for biodiesel production because of the very high cost of the system.

A potential practical solution to the light saturation problem, and also probably to photoinhibition, has been recognized for many years (e. g., Kok 1973): reduce the number of chlorophyll molecules cooperating in photosynthesis (the so-called “antenna” chlorophylls) from a few hundred to a few dozen. This would allow the photosynthetic apparatus to absorb only as much light as it can use. The benefits of reduced absorption are that it would:

• reduce waste,

• limit photooxidative damage to the photosynthetic reaction center, and

• increase the overall productivity of an algal culture, by a factor of at least 3 (see Benemann and Oswald 1996 for a recent discussion).

However, it has only recently become possible to consider achieving this objective, through the detailed understanding of photosynthesis at the molecular level, and the development of genetic engineering tools that could now allow us to redesign the photosynthetic apparatus. Recent work by Melis et al. (1998) and Neidhardt et al. (1998) demonstrated, at the physiological level, the feasibility of obtaining high efficiencies and high light saturation levels with algal cultures. Much more research is required, but the molecular and genetic tools are available to achieve the

desired high photosynthetic efficiencies by algal mass cultures. Such tools can also be used to direct the flow of photosynthate to desired metabolic products, such as lipids (see Section II).

Future R&D should demonstrate the feasibility of genetically engineering an improved photosynthesis system using algae for which such genetic systems are already well established. Once proven, these techniques can then be transferred to strains suitable for mass culture.