CHAPTER 14. Improving Photosynthetic Solar Energy Conversion Efficiency: the Truncated Light-harvesting Antenna (TLA) Concept

Application of the Truncated Light-harvesting Antenna (TLA) concept in photosynthesis entails a determined genetic engineering effort to minimize the light-harvesting antenna size of the photosystems in order to increase solar energy conversion efficiencies and the photosynthetic productivity of cyanobacteria and microalgae in mass cultures under bright sunlight cultivation conditions. Evidence in the literature supports the notion that TLA technology can significantly improve sunlight-to-product (e.g., biomass or hydrogen) energy conversion efficiencies. Approaches to generate TLA-strains in cyanobacteria and microalgae and their functional properties are summarized and discussed in this chapter. TLA-cyanobacteria were generated upon genetic deletion of the cpc operon, encoding phycocyanin and associated components of the phycobilisome peripheral rods. In the case of microalgae, recent advances on the assembly mechanism for the peripheral chlorophyll a-b light-harvesting complex enabled application of genes and proteins of the chloroplast Signal-Recognition Particle (CpSRP) pathway toward the attainment of green microalgal TLA-strains. These advances have expanded the molecular tool box by which to generated TLA-microalgae and, recently, TLA-diatoms. Substantial mechanistic and functional differences between the CpSRP pathway in microalgae and higher plants are outlined and discussed, showing that the CpSRP pathway cannot be equally applied for TLA strain generation in green microalgae and higher plants. The recent application of TLA-technology in green microalgae and cyanobacteria showed an interim increase in the sunlight-to-biomass energy conversion efficiency in the range of 1.5-1.6-fold over that measured with the corresponding wild type. However, it was estimated that the theoretically maximum potential of the TLA technology is an improvement in the solar-to-biomass energy conversion efficiency of microalgae by up to 3-fold over what can be achieved with wild type strains. In summary, current TLA-technology has shown significant improvement in the sunlight-to-biomass or sunlight-to-H2 energy conversion efficiency. Nonetheless, there is room for further improvement in solar energy conversion efficiency and yields, moving the process of photosynthesis from the current best 2-3% solar-to-biomass measured with green microalgae up to the theoretical maximum of 8-10% for the process of photosynthesis.