A major disadvantage of mixotrophic cultures is contamination, especially when open systems are used. Many species of bacteria have much higher growth rates than photosynthetic microorganisms. Thus, they can easily outcompete and out­grow the photosynthetic cells. Although many antibacterial substances can be used to inhibit bacteria growth, their effectiveness depends on the nature of the micro­algae. To minimize the risks of contamination, the cells can be cultivated photo- autotrophically to a relatively high cell concentration before switching to mixotrophic culture by the addition of an organic carbon source. Thus, the high concentration of microalgae before the addition of the organic carbon source will enable the cells to outcompete the initial low concentration of the contaminant. Furthermore, a shorter period of the mixotrophic culture can be applied so that cultivation is terminated as soon as concentration of the contaminants reaches a critical level. There are many reports on sequential photoautotrophic-mixotrophic cultures; for example, Fernandez Sevilla et al. (2004) employed it for cultivation of P. tricornutum UTEX 640, using both bubble column and airlift photobioreactors. The photoautotrophic phase lasted until a cell concentration of 3.5 g/L was reached; it was then switched to mixotrophic condition by the addition of glycerol under limited nitrogen, during which the cell concentration increased to 25.4 g/L. Yen and Chang (2013) similarly reported higher biomass concentration in sequential pho — toautotrophic-mixotrophic cultures; however, the linoleic acid content (18:1) was the same when compared with photoautotrophic cultures. Das et al. (2011) studied intracellular lipid accumulation by Nannochloropsis sp. in sequential photoauto — trophic-mixotrophic cultures. Photoautotrophic culture was used for 7 days fol­lowed by 3 days of mixotrophic culture, using either glycerol, glucose, or sucrose as the organic carbon source, resulting in a 72 % increase in lipid productivity when compared with the photoautotrophic culture.

4.2 Conclusions

Despite efforts to improve the productivity of photoautotrophic cultures, biodiesel oil productivity is still very low because of the low innate light conversation effi­ciencies of the microalgae. Large-scale photobioreactors with high light supply efficiencies remain technically challenging from a design perspective, and thus, the cost of microalgae biodiesel remains very high. The photoautotrophic microalgae cultures have several productivity limits, thus making it difficult to further reduce the cost of biodiesel oil production in photoautotrophic cultures. One solution is to exploit the ability of some strains of microalgae to metabolize organic carbon sources both with and without light. Depending on the strain, facilities available, technical know-how, and other culture conditions, the various culture systems discussed in this chapter present a viable alternative to photoautotrophic cultures in biodiesel production. For example, culture systems employing heterotrophic metabolism can be used to improve both the productivity and quality of oils pro­duced by microalgae by increasing the relative composition of triglyceride oils with high oleic acid contents, which are a better substrate for the production of biodiesel.