Glucose Accumulation Inside the Cell

Accumulation of energy-rich compounds is the primary step for microalgal lipid biosynthesis. However, this carbon accumulation varies with both autotrophic and hetero­trophic organisms. Autotrophs synthesize their own carbon (photosynthates) through photosynthesis, whereas heterotrophic organisms assimilate it from outside the cell. In photoautotrophs, the chloroplast is the site of photosynthesis where, light reaction takes place at the thylakoid followed by CO2 fixation to carbohydrates in the stroma of the chloroplast. These photosynthates provide an endogenous source of acetyl-CoA for further lipid biosynthetic pathways. Heterotrophic nutrition is again light-dependent and light-independent, where the carbon uptake will be through an inducible active hexose symport system from outside the cell (Perez-Garcia et al., 2011; Tanner, 1969; Komor, 1973; Komor and Tanner, 1974), and in this process the cell invests energy in the form of ATP (Tanner, 2000). However, carbon assimilation is more favorable in the case of light-independent processes (dark heterotrophic) over light-dependent ones (photoheterotroph). In dark heterotrophic algae, light inhibits the expression of the hexose/H+ symport system (Perez-Garcia et al., 2011; Kamiya and Kowallik, 1987), which decreases glucose transport inside the cell. Algae can also accumulate carbon in the presence of light through photoheterotrophic nutrition. Once carbon enters the cytosol, it follows cytosolic conversion of glucose to pyruvate through glycolysis and leads to the generation of acetyl-CoA, similar to photoautotrophs, followed by the pathway of lipid biosynthesis. In mixotrophic nutrition, both the biochemical process of autotrophs and het — erotrophs occur simultaneously, and the preference of substrate uptake depends on the substrate availability in addition to other environmental conditions.

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