Heterotrophism is a mode of nutrition whereby microalgae utilize external substrates as sole carbon sources for their growth and lipid accumulation. The circumstances in which microalgae use organic molecules as primary energy and carbon sources is called heterotrophic nutritional mode (Kaplan et al., 1986). In heterotrophic nutrition, the simpler carbohydrates enter the cell and are subsequently converted to lipids and participate in other metabolic pathways such as respiration (Figure 8.7). Heterotrophic nutrition takes place both in the presence and absence of light. In photoheterotrophic nutrition, light acts as an energy source, but the source of carbon remains organic only. Heterotrophic growth in the dark condition is supported by a carbon source replacing the light energy. This unique ability is shared by several species of microalgae (Perez-Garcia et al., 2011). Glucose is the simpler carbon source for heterotrophic microalgae. Higher rates of growth and respiration are obtained with glucose than with any other substrate, such as sugars, alcohols, sugar phosphates, organic acids, and monohydric alcohols. This oxidative assimilation takes place in algae apparently through two pathways; i. e., the Embdenn Meyerhoff pathway (EMP) and the pentose phos­phate pathway (PPP) (Neilson and Lewin, 1974).

Carbon metabolism in heterotrophic growth of microalgae under dark condition occurs via a PPP pathway, whereas the EMP pathway is the main glycolytic process in light conditions (Lloyd, 1974; Neilson and Lewin, 1974; Yang et al., 2000; Hong and Lee, 2007). Both pathways are carried out in the cytosol and are functional in microalgae. However, the PPP pathway might have a higher flux rate than the other, depending on the carbon source and the presence of light (Perez-Garcia et al., 2011). Light is not required for the transport of glucose inside the

cell during dark heterotrophic operation. Glucose transport system in the algal cell become inefficient in the presence of light, because of higher availability of photosynthates inside the cell due to photosynthesis and down-regulation of hexose transport protein. The carbon is obtained from outside the cell and converted to the acetyl-CoA via pyruvate, which further converts to malonyl-CoA and subsequently enters the lipid biosynthetic pathway (Figure 8.7). In heterotrophic nutrition mode, because of abundant glucose availability, respiration and other metabolic processes do not compete with the lipid biosynthesis, unlike autotrophic mode. Moreover, microalgae can utilize organic carbon under dark conditions because of the ability of light-independent glucose uptake. Hence, the lipid productivity is high in het­erotrophic nutrition mode (Abeliovich and Weisman, 1978).

Heterotrophically it is possible to obtain high densities of microalgal biomass that provide an economically feasible method for large-scale mass production (Chen, 1996; Chen and Johns, 1996; Lee, 2004; Behrens, 2005; Perez-Garcia et al., 2011). Photoheterotrophic nutri­tional mode avoids the limitations of light dependency, which is the major obstruction to gaining high cell density in large-scale photobioreactors (Huang et al., 2010). Chlorella protothecoides showed higher lipid content (40%) during heterotrophic growth (Xu et al., 2006). Higher lipid productivity (3,700 mg/L/d) was also reported by using an improved fed-batch culture strategy in heterotrophic nutritional mode, where the lipid productivity was 20 times higher than that obtained under photoautotrophic cultivation (Xiong et al., in 2008). The major advantage of heterotrophic nutritional mode is the facilitation of wastewater treatment along with lipid productivity, which gives an edge to its application in the present state of increasing pollution loads. Moreover, cost effectiveness, relative simplicity of opera­tion, and easy maintenance are the main attractions of the heterotrophic growth approach (Perez-Garcia et al., 2011). However, heterotrophic systems suffer from contamination prob­lems (Abeliovich and Weisman, 1978; Olguin et al., 2012).