Lewis M. Brown
Biotechnology Research Branch
Solar Energy Research Institute
The approach of using multiple-sun light intensity to increase the yield of various products from autotrophically grown algae is relatively unexplored. There are some possibilities for advanced photobioreactor design that might result in increased culture density and hence lower culture volume. However, culture productivity and product yield (typically) do not increase linearly with light intensity, but rather saturate at fairly low light intensities for most algae in the range of 15–20% of full sun (300–500 µmol quanta m-2 s-1) depending on species and on growth irradiance. Instantaneous measurements of photo-synthesis indicate that algae grown under higher light intensities are saturated at higher light intensities and have higher photosynthetic rates than algae grown under lower light intensities. There is no obvious advantage to growing algae at full sun let alone multiple suns because no increase in productivity is typically realized. However, it may be possible to grow algae in more concentrated culture at multiple-sun irradiances if smaller culture volumes are desired. Such culture conditions may be economical for high-value products (e.g. pharmaceuticals, pigments), but are unlikely to be economical for low-to mid-value products (fuel and commodity chemicals). Another problem with utilization of concentrated solar photon irradiances is photoinhibition. The photosynthetic process is severely inhibited at full-sun or multiple-sun irradiance. While there are probably multiple sites affected by high light intensity, it is thought that a primary site for photoinhibition is at the QB protein of photosystem II. Thus, there is a limit to the amount of light that algae can be exposed to without cellular damage taking place. Some workers have proposed that higher rates of photosynthesis can be achieved in flashing light compared to continuous light. Such effects are more clearly seen at > 10 Hz, and similar phenomena are sometimes claimed for lower rates of flashing, although such effects are by no means well accepted. Also, there has been no practical application of these results especially with concentrated solar photon sources. Much of the work has been done outside of the United States, and even the research that has been performed in the United States has been in the basic science (biochemistry) of photoinhibition. Application of concentrated solar photon technology for algal biotechnology is unlikely to take place in the United States as there is no support for such work. Leadership in this area is with Japan with its strong research programs in photobioreactor design, application of concentrated solar photon intensities, and search for high-value bioactive products from algae. Efforts in Israel have been an early demonstration of the potential of this approach.