Depending on the strain and organic carbon source, a major disadvantage of het­erotrophic culture is that light is required for increased productivity. In some strains, for example, lipid productivity is higher under photoautotrophic cultures when compared with heterotrophic cultures. Furthermore, the cost of organic carbon source can be very high, making heterotrophic production of biodiesel oil uneconomical. Solar light energy is abundant and freely available in outdoor photoautotrophic cultures. Thus, it is desirable to use light from solar energy to reduce production costs. Depending on the location and season, however, only a few hours of the day have the high light intensity needed to support photoautotrophic growth. During the night, cells not only cease to grow, but they metabolize the already intracellularly stored energy for cell maintenance, thereby leading to a decrease in biomass con­centration (Ogbonna and Tanaka 1996). Cyclic photoautotrophic-heterotrophic culture seeks to overcome this problem by cultivating cells photoautotrophically during the day, while adding the required amount of organic carbon source to grow the cells heterotrophically at night. By taking advantage of both alternating photo­autotrophic and heterotrophic cultures, the cells grow continuously during both day and night, leading to increased productivity (Ogbonna and Tanaka 1998; Ogbonna et al. 2001). This is especially useful for the cultivation of some microalgae that are not truly mixotrophs, yet can switch between phototrophic and heterotrophic metabolisms, depending on environmental conditions (Kaplan et al. 1986). However, the technical challenge of minimizing contamination risk through the selection and control of the organic carbon source added during the night remains.