Antenna size reduction as a strategy to increase biomass productivity: a great potential not yet realized

A major limitation in achieving high photosynthetic efficiency in microalgae mass cultures is the fact that the intensity of direct sunlight greatly exceeds the photosynthetic capacity of the cells. Due to the high pigment content of algal cells, the light absorption rate surpasses the much slower conversion rate to biochemical energy. The excess of light energy is predominantly dissipated as heat, decreasing the light use efficiency of the culture. Algae with a truncated antenna system could substantially increase biomass productivity of mass cultures because oversaturation of the photosystems and concomitant dissipation of light energy, are minimized. In this study, we measured the areal biomass productivity of wild-type strain cultures and four promising antenna size mutant cultures of Chlamydomonas reinhardtii. This was performed under simulated mass culture conditions. The strains were cultivated in turbidostat controlled lab-scale panel photobioreactors at an incident light intensity of 1500 μmol photons m-2s-1. The mutant cultures did not exhibit the expected higher productivity. The greatest mutant culture productivity values were approximate to those of the wild-type productivity of 1.9 g m-2h-1. The high sensitivity to abrupt light shifts indicated that the mutant cultures experienced reduced fitness and higher susceptibility to photodamage. This can possibly be explained by impaired photo protection mechanisms induced by the antenna complex alterations, or by unintended side effects of the genetic modifications. Still, if these effects could be eliminated, the principle of antenna size reduction is a promising strategy to increase productivity. Selection criteria for the future creation of antenna size mutants should, therefore, include tolerance to high light conditions.