Living organisms can be divided into two large groups, autotrophs and heterotrophs, according to the type of carbon source they utilize. Autotrophic organisms have the capability to convert physical (light) and chemical (CO2 and H2O) sources of energy into carbohydrates, which further form the base for the construction of all other carbon-containing biomolecules (Yoo et al., 2011). Mostly, the external energy is stored as a reduced form (carbohydrates) that is compatible with the needs of the cell. Autotrophic organisms are relatively self-sufficient and self-sustainable because they obtain their energy from sunlight (Nelson et al., 1994; Eberhard et al., 2008; Nelson and Yocum, 2006; Krause and Weis, 1991). On the contrary, het­erotrophic organisms utilize organic carbon produced by autotrophs as energy sources for their metabolic functions because they cannot utilize atmospheric CO2 as a carbon source. Oxidative assimilation of carbon begins with a phosphorylation of glucose/hexose, yielding phosphorylated glucose, which is readily available for storage, cell synthesis, and respiration (Figure 8.5). Nutritional modes significantly influence the carbon assimilation and lipid pro­ductivity of the microalgae (Xu et al., 2006). Three types of nutritional modes—autotrophic,

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Triacylglycerides

heterotrophic, and mixotrophic—are reported to produce algal fuel in the presence of light. In addition, the dark hetrotrophic nutrition mechanism is also found to be capable of lipid bio­synthesis by microalgae under specific conditions.