James Chukwuma Ogbonna and Mark P. McHenry

Abstract The feasibility of using various culture systems incorporating heterotro­phic metabolism for biodiesel oil production was compared. Heterotrophic culture can be used to achieve high cell concentration, and depending on the strain and organic carbon source employed, the introduction of light (mixotrophic culture) can enhance cell growth and oil accumulation. However, mixotrophic cultures also face the problem of light limitation, and depending on the relative concentrations of the organic carbon source and light intensity, the interaction between the heterotrophic and photoautotrophic metabolic activities can have negative effects on cell growth and oil accumulation. Systems that separate the two metabolic activities in time or space, such as cyclic photoautotrophic-heterotrophic cultures, sequential hetero­trophic-photoautotrophic cultures, and sequential photoautotrophic-mixotrophic cultures, can all be used to improve oil productivity. However, the effectiveness of each system depends on the strain of microalgae and other culture conditions.

4.1 Introduction

Requirement for light, and the technical problems and costs of light supply and distribution inside photobioreactors are the major challenges facing large-scale microaglae production in photoautotrophic cultures (Ogbonna et al. 1995, 1996). Much work has been carried out to optimize photoautotrophic cultures in large- scale outdoor cultures, yet it remains difficult to increase cell growth and final biomass concentrations in open pond cultures (Borowitzka and Moheimani 2013). Although higher productivities can be achieved in closed photobioreactors, such

J. C. Ogbonna (H)

Department of Microbiology, University of Nigeria, Nsukka, Nigeria e-mail: james. ogbonna@unn. edu. ng

M. P. McHenry

School of Engineering and Information Technology, Murdoch University, Murdoch, WA 6150, Australia

© Springer International Publishing Switzerland 2015

N. R. Moheimani et al. (eds.), Biomass and Biofuels from Microalgae,

Biofuel and Biorefinery Technologies 2, DOI 10.1007/978-3-319-16640-7_4 photobioreactors are highly complex, and technically difficult and very expensive to construct and operate (Ogbonna et al. 1996; Ogbonna 2003). Many strains of photosynthetic cells can metabolize various organic carbon sources in the presence of light (mixotrophy) or in its absence (heterotrophy). Various types of efficient large-scale heterotrophic bioreactors are available and are currently used for large — scale cultivation of fungi, bacteria, and yeasts for production of various metabolites. Such bioreactors are also used for large-scale heterotrophic cultivation of micro­algae. Even in mixotrophic cultures, the limitation of light is not as critical since the algae can still grow efficiently by metabolizing organic carbon sources. However, for many strains of microalgae in mixotrophic cultures, there is interaction between the photoautotrophic and heterotrophic metabolic activities (Ogbonna et al. 2002a). This interaction can positively or negatively affect both the cell growth and metabolite accumulation. Regulation of these metabolic activities is therefore required for the efficient accumulation of the desired metabolites (Ogbonna et al. 2002b). These metabolic activities can be separated in space (by employing a different bioreactor for each activity), or in time (by switching from one metabolic activity to another in the same bioreactor). Thus, many culture technical designs have been investigated to exploit heterotrophic metabolic activities in microalgae cultures. Such culture systems include heterotrophic, mixotrophic, cyclic photo­autotrophic-heterotrophic, sequential heterotrophic-photoautotrophic, and sequen­tial photoautotrophic-mixotrophic cultures. Each of these culture systems can be used for efficient biodiesel oil production, but their effectiveness is dependent on the nature of the microalgae used, the media components, and other culture conditions. The potentials and limitations of these culture systems are reviewed in this chapter.