Many environmental factors affect the performance of the complex photosynthetic machinery in microalgae, reducing its efficiency to well below the maximum at which photosynthesis can perform. That maximum is dictated by the underlying mechanisms, biophysical constraints, and physiological adaptations. One objective of applied microalgal R&D would be to develop strains and techniques that achieve productivities as close as possible to the maximum.

However, somewhat surprisingly, there is still argument about the maximum limit for photosynthetic efficiencies. The arguments boil down to the mechanisms assumed and the many possible loss factors that may or may not be considered. Most researchers agree that an absolute minimum of eight quanta (photons) of light absorbed are required by the two-photosystem mechanism (Z-scheme) of photosynthesis to reduce one molecule of CO2 (and closer to 10 to 12 quanta if the energy needs for CO2 fixation and cell metabolism are considered). However, there have been many reports of higher efficiencies. For example, recently Greenbaum et al. (1995) reported that some algal mutants lacking one photosystem still fixed CO2 (and produced H2), suggesting less than 8 (and as few as 4) quanta per CO2 reduced. However, recent reports cast doubts on this interpretation, and the two-photosystem mechanism appears robust.

The maximum efficiency can be estimated at about 10% of total solar (Bolton 1996). Such efficiencies have been used in the projections for microalgae biodiesel production (see Section III. D.). However, high sunlight conversions are observed ony at low light intensities. Under full sunlight, typically one-third or less of this maximal efficiency, biomass productivity is obtained, because of the light saturation effect.

Light saturation is simply the fact that algae, like many plants, can use efficiently rather low levels of light, typically only 10% of full sunlight (and often even less). Above this level, light is wasted. In fact, full sunlight intensities can damage the photosynthetic apparatus, a phenomenon known as photoinhibition. Light saturation and photoinhibition result from several hundred chlorophyll molecules collaborating in light trapping, an arrangement ideally suited for dense algal cultures, where on average a cell receives little light. However, exposed to full sunlight, the photosynthetic apparatus cannot keep up with the high photon flux and most of the photons are wasted, as heat and fluorescence, and can damage the photosynthetic apparatus in the process. One possibility, suggested by Neidhardt et al. (1998), is that photosynthetic productivity and light utilization could be maximized in microalgae by reducing the size of the light-harvesting antenna through mutation or genetic engineering. This is an interesting idea that will be discussed further in the next section.

I Publications:

Bolton, J. R. (1996) “Solar photoproduction of hydrogen.” Report to the Int. Energy Agency, under Agreement on the Production and Utilization of Hydrogen, IEA/H2/TR-96.

Greenbaum, E.; Lee, J. W.; Tevault, C. V.; Blankinship, S. L.; Metz, L. J. (1995) “Carbon dioxide fixation and photoevolution of hydrogen and oxygen in a mutant of Chlamydomonas lacking photosystem I.” Nature, August 3rd, (1995).

Kok, B. (1953) “Experiments in photosynthesis by Chlorella in flashing light.” In Algal Culture: From Laboratory to Pilot Plant (Burlew, J. B., ed.), Carnegie Inst. of Washington, Publ. 600, pp. 63-75.

Kok, B. (1973) “Photosynthesis.” Proceedings of the Workshop on Bio Solar Hydrogen Conversion (Gibbs, M., et al., eds.), September 5-6, Bethesda, Maryland, pp. 22-30.

Melis, A.; Neidhardt, J.; Bartoli, I.; Benemann, J. R. (1998) Proc. Biohydrogen ’97.

Neidhardt, J.; Benemann, J. R.; Baroli, I.; Melis, A. (1998) “Maximizing photosynthetic productivity and light utilization in microalgae by minimizing the light-harvesting chlorophyll antenna size of the photosystems.” Photosynthesis Res., in press.