Bubble column PBRs are more widely used than other reactors. In them, mixing and CO2 mass transfer are carried out through spargers with an external light supply (Nigar et al., 2005; Doran, 1995). Photosynthetic efficiency depends on gas flow rate, which further de­pends on the light and dark cycle as the liquid is circulated regularly from central dark zone to external photic zone at higher gas flow rate (Janssen et al., 2003). Photosynthetic efficiency can be increased by increasing the gas flow rate (>0.05 m/s), leading to shorter light and dark cycles. Degen et al., 2001 used a bubble column photobioreactor to improve light utilization efficiency of the strain Chlorella vulgaris through a flashing-light effect in batch mode opera­tion and achieved 1.7 times higher productivity of biomass (Degen et al., 2001).