Light-induced cell aggregation of Euglena gracilis towards economically feasible biofuel production

One of the most energy-consuming processes in conventional microalgal biofuel production is harvesting cells from dilute media. Cell harvesting by centrifugation or membrane filtration requires equal or more energy than is captured by photosynthesis, resulting in a negative net energy and CO2 balance. As a cost-effective alternative to this approach, we investigated the possibility of using the inherent motility and behavioral responses of the green microalgae Euglena gracilis to light stimuli, to promote cell aggregation. Irradiation of cells with light stimuli of different wavelengths and intensities revealed that E. gracilis cells are specifically attracted to green light. The cell aggregation rate for cultures irradiated with green light for 24 h was 8.7 fold and the cell collection rate reached 70%, which is comparable to the efficiency of centrifugal separation. Utilization of green light for cell aggregation does not compete with the light absorption by chlorophylls in photosystems I and II (PSI and PSII). Therefore, the findings in the present study offer the use of green light in solar radiation, which was originally wasted energy in photosynthesis, as the energy source for one of the most energy-intensive downstream processes in microalgal biofuel production.