Assessment of light distribution model for marine red microalga Porphyridium purpureum for sustainable production in photobioreactor

Modelling Nannochloropsis gaditana Growth in Reactors with Different Geometries, Determination of Kinetic Parameters and Biochemical Analysis in Response to Light Intensity

Microalgae are unicellular and photosynthetic microorganisms which grow thanks to inorganic salts, CO2 and light, and find applications in several fields thanks to their variety. The industrial application of microalgae has not often been fully exploited because of a lack of information about how microalgae respond to inputs and to different growth environments. In the present work a model able to predict the microalgae growth in reactors with different geometries was developed. We combined a Monod-like model for the specific growth rate with the Lambert-Beer law of homogeneous light distribution in thick photobioreactors. Kinetic parameters related to the cultivation of the microalga Nannochloropsis gaditana were obtained, for the first time through batch cultivation under different photon flux densities inside a quasi-isoactinic photobioreactor, in order to obtain a practically homogeneous light distribution. The maximum specific growth rate and saturation constant resulted, respectively as µmax = 0.0256 h−1 and Ik = 15.28 µE s−1m−2. These parameters were applied to the model to obtain data on microalgae growth in different geometries. Model simulation results are presented and discussed. Furthermore, biochemical analysis was performed on the biomass obtained at the end of each batch cultivation, grown both under different light intensities and in reactors with different configurations. Results indicated that lipid content increases with increasing average photon flux density. The fatty acid and carotenoids profiles markedly shift when the average light intensity varies: the PUFA content decreases and the SFA content increases when the average light intensity rises, and an accumulation of carotenoids at lower photon flux densities is observed. In conclusion, the model resulted in a useful tool, able to predict the growth of the microalga Nannochloropsis gaditana in reactors with different configurations.

Controlling average number of photons received per biomass to promote the growth of Synechocystis sp. PPC 6803

To investigate the actually received light of cells in the photo bioreactor, a light attenuation model of Synechocystis sp. PCC 6803 was established. The relationship between the average number of photons received per biomass (APRPB) and the growth of cell was analyzed. The results demonstrated, Cornet model was accurately fitted with the light attenuation of Synechocystis sp. PCC 6803 and the cell growth rate was affected by APRPB. When the value of APRPB is 3.2 μmol g^-1 s^-1, the cell have the maximum light efficiency. A maximum specific growth rate of 0.05 h^-1 was achieved with APRPB from 3.2 to 12.8 μmol g^-1 s^-1. After 156 h cultivation, compared to cells cultured under constant light [light intensity: 100 and 1800 μmol/(m² s)], the DCW under controlled light intensity (light intensity increasing with the cell density) was higher by 79.1% and 20.0%, respectively. This study indicated that APRPB could be used as a light intensity regulation criterion to improve cell production despite different types of reactor and cell density, which provided a theoretical basis for improving the biomass yield of Synechocystis sp. PCC 6803 or other photosynthetic auto-trophic organism.