Microalgae are more efficient than higher plants with respect to photosynthesis, through which light, together with CO2, is converted to chemical energy. Aside from photo­autotrophy, some microalgae are capable of growing heterotrophically as well as mixotro — phically. Heterotrophy refers to the fact that microalgae utilize organic carbon as the solo carbon and energy source for their reproduction in the absence of light; mixotrophy is indic­ative of microalgae performing growth in the presence of light through use of both CO2

(photosynthesis) and organic carbon sources. A number of microalgae have been reported for heterotrophic growth, among which green algae, in particular, Chlorella, are the most studied (Table 6.1). Microalgae are capable of utilizing a wide range of organic carbon sources, includ­ing sugars, hydrolyzed carbohydrates, waste molasses, acetate, and glycerol, as well as or­ganic carbons from wastewater (Table 6.1). Regardless of the microalgal species and strains, sugar—in particular, glucose—is the most commonly used organic carbon for boosting heterotrophic growth of microalgae (Table 6.1).

The uptake of external glucose relies on a hexose/H+ symport system that has been char­acterized in Chlorella (Hallmann and Sumper, 1996). In the presence of glucose, the hexose/H+ symport system is activated and transports glucose and H+ (1:1) into cytosol at the cost of equal ATP molecules (Tanner, 2000). The catabolism of transported glucose starts with a phos­phorylation of the hexose to form glucose-6-phosphate, an important intermediate product for respiration, storage, and biomass synthesis. Two pathways that share the initially formed glucose-6-phosphate are proposed to be involved in the aerobic glycolysis in algae—namely, the Embden-Meyerhof-Parnas (EMP) pathway and the pentose phosphate (PP) pathway (Figure 6.1; Neilson and Lewin, 1974). Both pathways are present in cytosol and contribute to the glucose metabolism in algae of autotrophy, mixotrophy, and heterotrophy, though their contributions may vary largely (Yang et al., 2000, 2002; Hong and Lee, 2007). For instance, glucose is mainly metabolized via a PP pathway in heterotrophic Chlorella pyrenoidosa, which accounts for 90% of total glucose metabolic flux distribution (Yang et al., 2000). The dominant role of a PP pathway was also demonstrated in the heterotrophic culture of the cyanobacterium Synechocystis sp. PCC6803 (Yang et al., 2002). In contrast, the EMP pathway serves as the major flux of glucose metabolism in algae in the presence of light (Yang et al., 2000,2002), suggesting the regulation of light on glycolysis. Table 6.2 shows the central metabolic network of glucose in heterotrophic algae with stoichiometric reactions.