Truncated chlorophyll antenna size of the photosystems - a practical method to improve microalgal productivity and hydrogen production in mass culture

Unicellular microalgae hold the promise of commercial exploitation in mass culture for hydrogen and biomass production. In any microalgal production system, the achievable photosynthetic productivity and light utilization efficiency of the algae are the single most important factors in the determination of cost. Microalgal mass cultures growing under full sunlight have a low per chlorophyll (Chl) productivity since, at high photon flux densities, the rate of photon absorption by the Chl antenna far exceeds the rate at which photons can be utilized for photosynthesis. Excess photons are dissipated as fluorescence or heat. Up to 80% of absorbed photons could thus be wasted, reducing light conversion efficiencies and cellular productivity to fairly low levels. This shortcoming could possibly be alleviated by the development of microalgal strains with a limited number of Chl molecules in the light-harvesting antenna of their photosystems, i.e., strains that have a truncated Chl antenna size. It is expected that individually, such microalgae will not be able to saturate rates of photosynthesis and, thus, will not be subject to wasteful dissipation of excitation energy. In turn, the productivity of the mass culture will be improved. The method of choice to reach the objective of a "truncated light-harvesting Chl antenna" size (tla) employed DNA insertional and chemical mutagenesis of the unicellular green algae Chlamydomonas reinhardtii and Dunaliella salina, followed by a rigorous screening protocol to identify mutants with a smaller light-harvesting Chl antenna size. Molecular and genetic analyses of isolated tla strains were performed. Biochemical and physiological analyses in terms of photosynthetic productivity and light conversion efficiencies are presented. The results show that a truncated Chl antenna size of PSII is more important than that of PSI in terms of the photosynthetic productivity of a mass culture. A list of genes that confer a "truncated light-harvesting Chl antenna" size to green algae is being compiled for future application in algal hydrogen and biomass production. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.