Airlift photobioreactors comprise two interconnecting zones called the riser, where the gas mixture is sparged, and the downcomer, which does not receive the gas. Generally, an airlift photobioreactor exists in two forms: internal loop and external loop (Chisti, 1989; Miron et al., 2000). In an internal loop reactor, regions are separated either by a draft tube or a split cyl­inder; in an external loop reactor, the riser and downcomer are separated physically by two different tubes. Mixing in the system is done by bubbling the gas through a sparger in the riser tube, with no physical agitation. A riser is similar to a bubble column, where sparged gas moves upward randomly and haphazardly, which decreases the density of the riser, mak­ing the liquid move upward. Gas held up in the downcomer significantly influences the fluid dynamics of the airlift reactor. Increasing the gas hold-up, the difference between a riser and a downcomer, is an important criterion in designing airlift reactors (Chisti, 1989; Kaewpintong et al., 2007). Airlift reactors have the characteristic advantage of creating circular mixing pat­terns in which liquid culture passes continuously through dark and light phases, giving a flashing-light effect to algal cells (Barbosa et al., 2003). The biomass growth pattern of Nanochloropsis occulata and Scenedesmus quadricauda was studied inside two vertical airlift photobioreactors suitable for indoor operation, with both salt and freshwater and different lighting systems. Results depicted that the biomass productivity of the cultures was found to depend on the light regimes and the duration of operation.