As previously discussed, mixotrophic cultures have very high potentials for oil production by many strains of photosynthetic organisms. However, optimization of mixotrophic cultures is difficult as the conditions that favor photoautotrophic metabolic activities may not favor heterotrophic metabolic activities. The relative contribution of these two metabolic activities depends on such factors as light intensity, the nature, and concentration of the organic carbon source, and carbon dioxide concentration (Ogbonna et al. 2002a, b). For instance, it has been observed that the assimilation of glucose in a certain strain of Chlorella is suppressed by light, even at low light intensities (Haass and Tanner 1974; Kamiya and Kovallik 1987); therefore, only photoautotrophic metabolic activities are observed under mixotrophic conditions. Sequential heterotrophic-photoautotrophic culture system can be used to overcome such limitations. In this system, the cells are first culti­vated to high densities in heterotrophic cultures, and the condition is then changed to photoautotrophic. This system ensures that the advantages of heterotrophic cultures (such as high cell densities) and those of the photoautotrophic cultures (such as light-induced synthesis of metabolites) are realized. The effectiveness of this culture system has been demonstrated for Chlorella (Ogbonna et al. 1997), Euglena (Ogbonna et al. 1999), and Haematococcus (Hata et al. 2001). Sequential heterotrophic-mixotrophic cultivation, in which light illumination is started before the organic carbon source is completely utilized, has also been investigated for biodiesel oil production (Mitra et al. 2012).