Keeping the light energy constant - cultivation of Chlorella sorokiniana at different specific light availabilities and different photoperiods
Microalgae cultivation is strongly dependent on light availability. Sunlight is the natural energy source for algae cultivation processes, but its fluctuation during the day and the natural day-night-rhythm mean an increasing challenge for an efficient algae production. Additionally, different seasons lead to unequal photoperiods and their influence on the productivity has not been determined in detail yet. Therefore, we investigated the growth of Chlorella sorokiniana at different light intensities and illumination cycles of 24 h/0 h (light/dark), 16/8 h and 12/12 h. The light integral, more exactly the amount of photons per gram biomass and day (specific light availability) was kept constant during 24 h to ensure the comparability of the results of the different photoperiods. During the cultivation in repeated fed-batch mode, the productivity and biomass yield on light of Chlorella sorokiniana were determined to characterize the growth. During the night phase a biomass loss occurred, which was independent of the night temperature and the specific light availability on the day before, but increased with higher biomass concentration. A higher biomass yield on light during the shorter photoperiods led to an improved gross productivity compared to the continuous illumination, especially at a biomass concentration above 4 g L−1. This compensated for the night loss; therefore, the net productivity (during one day) was independent of the photoperiod. In order to determine an optimal process window biomass yield on light and productivity have to be considered. They are diametric with respect to the specific light availability and furthermore dependent on the biomass concentration. The optimal combination of biomass yield on light and productivity could be achieved with a biomass concentration below 4 g L−1 during 24/0 illumination and with a biomass concentration below 7 g L−1 during shorter photoperiods; the specific light availability should be as high as possible.