Futuristic opportunities for pretreatment processes in biofuel production from microalgae

Microalgal biofuel is a promising solution to replace fossil fuel as a renewable and environmental-friendly energy source, thereby contributing to the United Nations (UN) Sustainable Development Goals (SDGs), in particular SDG-7, or Affordable and Clean Energy. Unlike energy crops (like oil palm and sugar cane), microalgae benefit from faster growth rate, higher lipid content, smaller land area required, ability to flourish using waste or brackish water, and posing zero competition with food crops. Microalgae-derived biofuels (like biodiesel, bioethanol, biomethane, and biohydrogen) are sustainable energy sources that can be produced using well-developed techniques (e.g., transesterification, fermentation, anaerobic digestion, and Fisher–Tropsch process). To prevent dire climate conditions resulting from the global temperature rise of 1.5°C and resolve worldwide energy security issue, our generation will need to establish and implement renewables on a global scale. To improve the industrial production of microalgal biofuel, the efficiencies of biomass and metabolite production to post-cultivation biofuel synthesis processes must be enhanced. For the cultivation step, there exist three key techniques that can directly change the traits, structure, and behavior of microalgal cells, and induce them to accumulate targeted metabolites rapidly and in large amounts. These techniques are genetic engineering, chemical modulation, and nanomaterial approach. Genetic engineering commonly alters the chloroplast DNA of microalgae to overexpress or down-regulate key genes in various metabolic pathways so that the cells accumulate more lipids. Chemicals can also be used to modulate microalgal growth and lipid accumulation by inducing oxidative stress or prevent conversion of lipid molecules. Nanomaterials and nanoparticles can also enhance microalgal lipid production by microenvironmental stress induction, vitamin supplementation, and light backscattering. Therefore, in this review, the recent progress as well as the pros and cons of genetic engineering, chemical modulation, and nanomaterial approach in achieving greater biofuel production from microalgae are comprehensively examined.