Microalgae: Sustainable resource of carbohydrates in third-generation biofuel production

Microalgae have gained interest over the century due to numerous intrinsic attributes surpassing higher plants, making them a potential feedstock for third-generation biofuel production. The current state of art technologies produces biodiesel from microalgal biomass attributed to high intrinsic neutral-lipid contents. However, persisting hurdles in terms of techno-economic feasibility have impeded commercial-scale operations. The latent qualities of microalgae towards the accumulation of multiple high-value products ranging from animal feed to pharmaceuticals adds up to the economic feasibility. Alternatively, the high abundant carbohydrate contents of microalgal strains are used as low-cost substrates for the growth of commercially important microbes to synthesize biofuels. Novel carbohydrate enhancement strategies such as two-stage cultivation, phytohormones, starvation strategies, combinatorial stress strategies, etc., are frequently emerging to mitigate the challenges. Therefore, this study targets to review the recent trends in tuning the microalgal metabolism for increased carbohydrates and associated biofuel generation to attain process feasibility and sustainability. According to recent reports, nitrogen limitation, phosphate limitation, the optimal light intensity with reduced dissolved oxygen, limited inorganic carbon, optimal organic carbon levels, and indole-3-acetic acid augmented the carbohydrate productivity in different microalgal strains. Further analysis on different pretreatment methods highlighted electric-based physical treatment strategies with high efficiencies and less energy requirements of 13.3 kJ to 1.5 MJ per kg biomass. The production cost for microalgae-based bioethanol varies from US$ 1.67 to 31.36 per gallon for different process scenarios, which needs further attention.