Optimization of photosynthetic light energy utilization by microalgae

Over 50% of the energy losses associated with the conversion of solar energy into chemical energy during photosynthesis are attributed to kinetic constraints between the fast rate of photon capture by the light harvesting apparatus and the slower downstream rate of photosynthetic electron transfer. At full sunlight intensities, energy flux from the light harvesting antennae to the reaction centers may be 100-folds greater than the overall linear electron flow resulting in the dissipation of up to 75% of the captured energy as heat or fluorescence. One possible means to couple energy capture and photosynthetic electron transfer more efficiently is to reduce the optical cross-section of the light harvesting antennae. We show that by partially reducing chlorophyll b levels in the green alga, Chlamydomonas reinhardtii, we can tune the peripheral light harvesting antennae size for increased photosynthetic efficiency resulting in more than a two-fold increase in photosynthetic rate at high light intensities and a 30% increase in growth rate at saturating light intensities. Unlike chlorophyll b-less mutants which lack the peripheral light harvesting antennae; transgenics with intermediate sized peripheral antennae have the advantage that they can carry out state transitions facilitating enhanced cyclic ATP synthesis and have robust zeaxanthin–violaxanthin cycles providing protection from high light levels. It is hypothesized that the large antennae size of wild-type algae and land plants offers a competitive advantage in mixed cultures due to the ability of photosynthetic organisms with large light harvesting antennae to shade competing species and to harvest light at low flux densities.