Developing fourth-generation biofuels secreting microbial cell factories for enhanced productivity and efficient product recovery; a review

•Medium/long-chain biofuel molecules can be produced in engineered microbial platforms.•Low titer, cytotoxicity, and difficult product recovery are major challenges.•Biofuel secreting systems can perform better as compared to the accumulating systems.•Membrane efflux pumps can be employed for easier recovery and enhanced productivity. Fourth generation (4G) biofuels have been found compatible with engines, storage systems, and transport facilities. Due to advances in synthetic biology and genetic engineering tools, considerable progress has been made in producing 4G biofuels including high-carbon alcohols, long-chain hydrocarbons, terpenoids-based, and/or fatty-acid derived biofuels using Escherichia coli, Saccharomyces cerevisiae, Pichia pastoris, Yarrowia lypolytica, and Zymomonas mobilis as microbial cellular factories. However, these microbial platforms face various challenges including low product titers, low product yields, difficult product recovery incurring intensive energy consumption, and discharge of large amounts of wastewater. All these issues make the production and recovery of these biofuels expensive and commercially non-competitive. However, various genetic manipulation approaches including overexpression of heat-shock proteins, enhanced production of precursor molecules, regulating redox-balance, and membrane engineering have been employed to cope with these challenges. This review discusses the progress made in the molecular approaches for the enhanced biosynthesis and easier recovery of these biofuels through employing the secreting microbial cell factories. Besides, based on the data published on membrane transporters in previous fifteen years, selected fungal and bacterial membrane efflux pumps are studied to evaluate their biofuel secretion potential as future targets to be employed in the biofuel secreting microbial cell factories to achieve commercial robustness in the future.