Photosynthetic Antenna Complexes: A Structure-Function Investigation of Light Harvesting and Photoprotection

Photosynthesis is the process that uses sunlight energy to convert inorganic CO2 into sugars. This process is carried out by both microalgae and higher plants. During evolution, photosynthetic organisms developed several strategies to optimize the capture of light and funnel solar energy to the reaction centers (RC) of the photosystems (PS), which instead, remained essentially unchanged. Both PS, PSII and PSI, are equipped with a Light Harvesting system made of the so-called antenna proteins or Light Harvesting Complexes (LHC) aimed to increase the capture of solar radiation and transfer excitation energy, through pigment molecules, toward the RC. In general, a large antenna system is an advantage for plants, allowing for harvesting a larger fraction of the incident solar radiation, however under excess light conditions or when plants (or algae) are cultivated in dense canopies this may lead to a general loss of productivity. Indeed, under excess light conditions photosynthetic organisms intercept an excess of light that can easily result in the production of dangerous reactive oxygen species (ROS), responsible for damaging cell structures possibly causing cell death. On the other hand, the intensive cultivations forces plants, or algae to grow extremely closed to each other, thus determining self-shading effects that reduce light penetration within cultures. Under these conditions, the top of the canopy (or the external layer of algae cultures) is reached by strong radiation that could induce photooxidation while leaves in the inner part of the canopy (or in the inner layers of algae cultures) are exposed to suboptimal irradiances. Taken together, all these factors influence net productivity of cultures that, in field conditions, ranges around 1-2% of solar energy conversion into sugars, despite a theoretical yield of about 12%. This low yield is one of the few traits of cultures that is still far from its theoretical upper limits and has become a promising target toward improving crop yields. The engineering of LHC is a promising strategy to improve light management in algae cultures. Indeed, algal strains with a reduced antenna system showed better growth performances under intensive cultivation conditions; this is mainly due to the increased light penetration within the medium, allowing for a more homogeneous distribution of the light in the culture layers. On the plants side, recent studies revealed that a possible strategy towards the improvement of crop yield could be the tuning of photoprotective mechanisms to increase the fraction of solar radiation converted into biomass and reducing the amount dissipated as heat. These approaches, in both microalgae and higher plants, have been useful to improve yields. Yet many aspects need to be further clarified and there is still room for improvement towards the production of more productive strains that will be fundamental in the future to cope with the increasing food demand worldwide.