BIOCHEMICAL AND MOLECULAR FACTORS FOR ENHANCED LIPID ACCUMULATION IN MICROALGAE

Abstract

The rapid increase in global energy demand, the rise of climate change events, and fossil fuel shortages have led humankind to search for alternative greener and sustainable energy resources. With multiple advantages, bioenergy is a promising alternative to conventional energy; however, biofuel production from the first and second-generation sources may add concern mainly the problems in water scarcity and threats to food security. Microalgae have recently emerged as a promising renewable feedstock for biofuel production, mainly due to their rapid growth rate, ability to fix CO2, flexible adaptation, and capability to grow in nonarable lands. However, the real technological challenge is to mass produce microalgae with higher lipid content for the production of a low-value high-volume product to make it economically viable. To accomplish this, approaches to maximize biomass and lipid production that are crucial for improving the economics of microalgae-based biofuels were employed. This includes the bioprospecting of potential algal strains (preferably from indigenous habitats), derivation of pre-eminent nutritional parameters and exogenous supplementation of phytohormones to direct metabolic flux towards enhanced lipid accumulation. Among the nineteen-microalgal strains screened, Desmodesmus sp. JS07 appears to have maximum lipid accumulation (50.01±3.21%) and productivity (36.3±2.06 mg/L/d) in nitrogen-limited conditions. Besides, Desmodesmus sp. JS07 had shown improved levels of SFA and MUFA content under the nitrogen-limited condition that is desirable for achieving high-quality biodiesel. Enhanced biomass and lipid content were observed with exogenous application of Desmodesmus sp. JS07 with selected phytohormones. Among auxins, IBA led into improved biomass and lipid content up to 1.46±0.53 g/L and 43.12±3.87%, respectively whereas among cytokinins; BAP increased the biomass and lipid content up to 1.60±0.61 g/L and 38.99±3.11% respectively, in Desmodesmus sp. JS07. Further, the cumulative impact of IBA and BAP was evaluated, which showed their synergistic effect in stimulating biomass and lipid content up to 2.01±0.67 g/L and 45.1±2.11% respectively, in Desmodesmus sp. JS07 at IBA7.5mg/l+BAP7.5mg/l. Therefore, the application of phytohormones, which is an easy and scalable strategy, could be used for significant improvement in microalgal biomass and lipid content for commercial biofuel production. II To reveal the molecular mechanisms of lipid metabolism in response to nitrogen-limited conditions in an isolate Desmodesmus sp. JS07, lipidomics, and transcriptomic analysis were performed. Lipidome analysis demonstrated a diverse spectrum of lipids, revealing 8 lipid classes and 87 lipid species consisting of both polar and neutral lipids in Desmodesmus sp. JS07. Under N-limitation, TAG levels remarkably increased along with the concomitant decrease in polar lipids. This signifies a significant remodeling in the lipid pools through augmenting de novo fatty acid biosynthesis in chloroplast for enhanced TAG accumulation. Transcriptome profiling of Desmodesmus sp. JS07 revealed the up-regulation of potential genes related to fatty acid and TAG biosynthesis. Taken together, this provides a basis for improving our understanding of TAG synthesis and identifying the key enzymes involved in TAG production and for their overexpression in microalgae. Further, molecular modification of Desmodesmus sp. JS07 was taken up by heterologous and homologous expression of BnDGAT2 and CvGPD1 constructs respectively in order to augment the lipid accumulation. An integrated approach involving the overexpression and site-directed mutagenesis of the key regulatory gene CvGPD1 to engineer the microalgae for economic viability for biofuel production was also explored. Therefore, the present work is an endeavor to develop an engineered microalgal strain with enhanced lipid content for sustainable biofuel production.