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Growth optimization of Synechococcus elongatus PCC 7942 in lab flask and 2D photobioreactor Kuan, David


One of the most promising mechanisms for the production of high value biologically active products is through the cultivation of microalgae. In addition to serving as a carbon capture system, this photosynthetic microorganism has demonstrated potential for recombinant protein expression as an approach towards sustainable development in biotechnology. Extensive studies on cyanobacterium Synechococcus elongatus PCC 7942 have assessed the dual function of carbon capture with product generation such as biodiesel and recombinant protein. In order to maximize CO₂ fixation and production rates of valuable product, a high cell growth rate needs to be achieved. Consequently, challenges in photobioreactor operation and cultivation need to be addressed, such as CO₂ mass transfer limitations, light availability, and minimizing energy consumption. Thus, the effects of the major growth factors need to be studied. In this research, the objectives were to optimize the specific growth rate and biomass concentration of S. elongatus by investigating the effects of medium composition, light intensity, temperature, and CO₂ concentration. Preliminary studies at the shake flask scale revealed that an optimization of components in BG-11 medium resulted in no significant improvements for the specific growth rate and biomass concentration. However, a maximum specific growth rate of 0.0519 1/h and a maximum biomass concentration of 0.496 g/L were achieved at 33⁰C and 120 μE/m²/s. A 1 L airlift photobioreactor was used to investigate the effects of light intensity, CO₂ concentration, and gas flow rate on the specific growth rate and biomass concentration. Additional experiments carried out in this photobioreactor revealed that air enriched with 5% CO₂ at 1 L/min, 33⁰C, and 120 μE/m²/s achieved a maximum biomass concentration of 1.006 g/L at a reduced specific growth rate of 0.0234 1/h. Further increases in CO₂ % and light intensity, as well as light/dark cycles, reduced the growth rate and biomass concentration. Mass transfer experiments also revealed that 5% CO₂ provided the best growth conditions, as growth was significantly limited by CO₂ when supplied with air, whereas 10% CO₂ reduced the pH and consequently reduced the specific growth rate.

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