Metabolic pathway engineering towards enhancing microalgal lipid biosynthesis for biofuel application—A review

Microalgae have recently emerged as the most favorite feedstock for triacylglycerol (TAG), the storage neutral lipid, for renewable and sustainable production of biodiesel, mainly due to their comparable lipid contents, faster growth rates and lesser land requirements as compared to the non-conventional and non-edible oilseed crops. But the real technological challenge is to mass produce microalgae with much higher lipid content to make the production of a low-value-high-volume product like biodiesel economically viable and environmentally sustainable. Recent scientific achievements in TAG overproduction in higher eukaryotic systems may be leveraged upon to enhance lipid synthesis by manifold in microalgae. Since the available sequence homology information have been effectively used in case of the model unicellular green alga, Chlamydomonas reinhardtii to perform genome-scale metabolic reconstructions, the gained knowledge and the well established genetic engineering tools and techniques coupled with the modern system biology approaches may well pave the way for delineating and deciphering the TAG biosynthetic pathways in lipid accumulating microalgae as targets for metabolic pathway engineering. This review thus analyzes the trends and developments in the area of metabolic engineering of lipid synthesis in microalgae and discusses the vision based on some of the possible strategies that could be adopted to reconstruct a stable modified engineered microalga with enhanced lipid producing capabilities. The strategies include flux balance analysis for target gene identification, over expression of the target enzymes involved in lipid biosynthesis, over expression of the target gene under specific inducible promoters, constitutive expression of transcription regulators, diverting the flux of key metabolites, and integrated in silico based approaches. An integrated approach involving multiple gene targeting by applying the principles and knowledge of systems biology and bioinformatics would provide us with a holistic view and help derive some feasible solutions.