Higher efficiencies of solar energy capture and conversion innate to microalgae, translate to rapid growth rates, enhanced oil contents and reduced land requirements for cultivation. The largest losses in photosynthetic efficiency are associated with non-photochemical quenching and energy dissipation under full sunlight conditions when the light-harvesting/reaction center complexes are saturated and as much as 80% of the photons absorbed by the antenna can be wastefully dissipated and not utilized for the production of chemical energy or lipids.

It has been demonstrated that optimization of the peripheral light-harvesting antenna size associated with Photosystem II (PSII) is sufficient to obtain higher photosynthetic electron transfer rates. In this study we have altered the size of the peripheral antennae complex by altering the amount of chlorophyll (Chl) b accumulated by algae. This objective has been achieved by targeting the Chl a oxygenase gene (CAO), that is responsible for the synthesis of Chl b from Chl a, using RNA interference (RNAi). In the absence of Chl b, the PSII peripheral light harvesting antenna cannot assemble. We have shown that the resulting RNAi mutants have less than the full complement of peripheral PSII antenna, higher rates of photosynthetic oxygen evolution and quantum yields comparable to the wild-type (WT). In addition, under laboratory conditions the transgenic algae have faster growth rates than a knockout mutant that completely lacks Chl b, suggesting that the complete loss of Chl b might have an impact on growth at sub-saturating light intensities. Competition growth experiments against a WT strain conducted in the green house in ponds of different depths, showed improved culture productivities for a Chl-b less mutant in the deepest of the ponds tested during the brightest times of year.