Transition of Biofuels from the First to the Fourth Generation: The Journey So Far

Biofuel production can be classified into four major generations based on the type of feedstocks utilized. The first generation utilizes edible crops and involves benefits in terms of reduced CO2 emissions and secured rural energy supply; still it invites food versus fuel controversy. In this context, second-generation biofuels came to the fore, wherein the abundant lignocellulosic biomass are used, and hence it is considered as the most viable option for commercializing biofuels. However, these processes suffer from low conversion yield and long processing time. Thus, third-generation biofuels have been adopted, which have added advantage of carbon sequestration for biomass production amenable for conversion into biofuel, e.g., biofuels from microalgae. Still these technologies are not cost-competitive with their fossil counterparts owing to low biomass productivity. Thus, metabolic engineering approaches are being targeted (fourth generation) for improving the microbial strains/feedstock to make the overall process cost-competitive with fossil fuels. Metabolic engineering approaches have been targeted to improve the photosynthetic efficiency of microalgae, to enhance the light penetration, to reduce photoinhibition, and to increase the lipid content of oleaginous microbes.