Algae are diverse group of organisms that inhabit a vast range of ecosystems, from the ex­tremely cold (Antarctic) to extremely hot (desert) regions of the Earth (Guschina and Harwood, 2006; Round, 1984). Algae account for more than half the primary productivity at the base of the food chain (Hoek et al., 1995). Lipid metabolism (the biosynthetic pathways of fatty acids and triacylglycerol, or TAG synthesis), particularly in algae, has been less stud­ied than in higher plants (Fan et al., 2011). Based on the sequence homology and some shared biochemical characteristics of a number of genes and/or enzymes isolated from algae and higher plants that are involved in lipid metabolism, it is generally believed that the basic path­ways of fatty acid and TAG biosynthesis in algae are directly analogous to higher plants (Fan et al., 2011). The de novo synthesis of fatty acids in algae occurs primarily in the thylakoid and stromal region of the chloroplast (Liu and Benning, 2012). Algae fix CO2 during the day via photophosphorylation (thylakoid) and produce carbohydrate during the Calvin cycle (stroma), which converts into various products, including TAGs, depending on the species of algae or specific conditions pertaining to cytoplasm and plastid (Liu and Benning,

2012) . Microalgae are proficient at surviving and functioning under phototrophic or hetero­trophic conditions or both. A schematic illustration of algal-based lipid biosynthesis by a pho­toautotrophic mechanism is given in Figure 8.1. The biosynthetic pathway of lipid in algae occurs through four steps: carbohydrates accumulating inside the cell, formation of acetyl — CoA followed by malony-CoA, synthesis of palmitic acid, and finally, synthesis of higher fatty acid by chain elongation.

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