Microalgal Neutral Lipid Accumulation: Cellular Mechanism and Ways to Improve Biodiesel Production

Microalgae-based biodiesel is attractive for its renewability, low toxicity, and biodegradability. Richness of saturated and monounsaturated fatty acids in microalgae makes them ideal for high-quality biodiesel production. Under favourable conditions, microalgae accumulate only a basal level of neutral lipids. When the conditions become adverse, cells undergo a rapid metabolic rearrangement that accelerates the neutral lipid accumulation. However, this rearrangement compromises the growth and thus affects the overall yield. To solve this bottleneck, it is important to understand the key mechanism. This review first analyses the cellular metabolic regulation associated with triacylglycerol (TAG) accumulation in microalgae during different stress conditions. From the analysis, it is clear that both growth-related pathways and neutral lipid biosynthesis share common precursors involving multiple pathways, and this makes the simultaneous achievement of high biomass and TAG yield challenging. Some of the strategies that have the potential to overcome this challenge, like genetic engineering, altering the mode of nutrition, and two-phase cultivation, are discussed. The efficiency of these strategies in achieving the goal, possible limitations, and suggestions to overcome the limitations are also included. The review also finds that merely maximizing TAG content within microalgae cells is insufficient for commercializing microalgae for biodiesel. Technoeconomic analysis reveals that high capital and operational costs associated with microalgae biodiesel production result in elevated production costs. So, utilizing wastewater as an alternative medium is promising to cut down the cost associated with culture nutrients, a major contributor to the operational cost. Additionally, the development of economically viable operating systems is crucial.