3.4.1 Optimization of Light Conversion Efficiency (LHCB)

Optimization of light conversion efficiency (LCE) is another way to make microalgae-based biofuels cost-effective. LCE is defined by Ghirardi et al. (2009) as the "fraction of the energy content of the incident solar spectrum that is converted into chemical energy by the organ­ism." It has been known that sunlight intensities are much higher than those required to saturate photosynthesis. To avoid overexcitation of the photosystem, plants and green microorganisms deal with excess light by dissipating heat and emitting fluorescence. As a consequence, the realistic LCE converts solar energy to biomass is much lower than the theoretical calculation (Dismukes et al., 2008; Melis, 2009; Wijffels and Barbosa, 2010).

Another energy issue dealing with light efficiency is uneven distribution of light in a high — density cultivation system. For cells directly exposed to sunlight, up to 80% of the absorbed photons could be wasted due to dissipation of excitation by nonphotochemical quenching and photoinhibition of photosynthesis (Melis, 1999; Melis et al., 1999). On the other hand, cells underneath the culture are shaded from sunlight and have reduced photosynthesis rates.

To improve solar illumination distribution of the microalgal culture, mutants with reduced light-harvesting chlorophyll antenna sizes that would allow for efficient utilization of light energy, and therefore would increase productivity, have been proposed. The rationale of this approach is to minimize light absorption by cells on the surface and to permit greater sunlight penetrance into the deeper layers of the culture. This concept was experimentally validated by isolation and characterization of truncated light-harvesting chlorophyll antenna size (tla) mutants (Lee et al., 2002; Polle et al., 2000; Polle et al., 2003). Reduction of photosystem chlorophyll antenna size in tla mutants has been demonstrated to improve solar energy conversion efficiency and productivity. The notion has also been verified independently by an RNAi approach. Reduction of the light harvest complex I(LHCI) and LHCII antenna complex system by knocking down light harvest complex B major proteins results in improved photon capture efficiency, enhanced growth rate, and reduced photoinhibition (Mussgnug et al., 2007).

In summary, accumulated experimental evidence indicates optimization of light-capture efficiency by genetic engineering can be very useful to improve culture productivity. Designs integrating growth optimization and fuel production will be important to making microalgae-based fuel cost effective.