The Current Prospect of Drop-in Biofuel Production Through Biochemical Routes

The depletion of fossil fuels and its adverse effects on the environment, such as climate change and global warming, have promoted the search for sustainable alternative energies. One alternative is the use of drop-in biofuels, which are defined as liquid bio-hydrocarbons functionally equivalent to petroleum-derived fuels and are fully compatible with the oil infrastructure and the engines and turbines of existing transportation systems. Drop-in biofuels are produced using three main technologies and their interactions: oleochemical, thermochemical, and biochemical, and hybrids incorporate elements of the three technologies. Biochemical technology has been investigated by exploring various biosynthetic pathways in conventional microorganisms such as Escherichia coli and Saccharomyces cerevisiae, as well as unconventional ones such as cyanobacteria and algae. Currently, five main biosynthetic pathways have been proposed for the generation of drop-in biofuels: the isoprenoid pathway, the fatty acid pathway, the α-keto acid pathway, the non-decarboxylative Claisen condensation pathway, and the polyketides pathway. The natural production of drop-in biofuels through these pathways is still insufficient to be used for commercial purposes, and thus various strategies have been investigated to increase their productivity and yields. These strategies include metabolic and genetic engineering, upregulating pathway enzymes, deleting or downregulating competing pathways, engineering transcriptional factors, increasing the robustness of host cells, enhancing cell viability, and regulating cell populations. In this chapter, the five biochemical routes for generating drop-in biofuels are described, as well as some strategies implemented to increase titers and yields.