Progress of non-destructive hydrocarbon extraction technology of Botryococcus braunii

Coal, petroleum and natural gas have remained the premium energy sources driving the economy and sustaining population growth for years. Global human population has shown a 7-fold increase in the last two centuries and, consequently, an astronomical increase in the global economy and energy demand. Interestingly, rising global warming, climate change and destruction of the natural environment are trends parallel with the world popu-lation profile and continual dependence on fossil fuels. Actions to mitigate these challenges have ignited interest in autotrophic production of hydrocarbons from microalgae as an alternative to fossil-based fuels. The microalgal strain, Botryococcus braunii, offers an excellent platform for renewable hydrocarbons production. B. braunii has unusual and fascinating attributes, and by producing liquid hydrocarbons extracellularly, a classical non-invasive, non-destructive extraction technique known as milking can be applied. In the milking process, hy-drocarbons in the extracellular matrix of the algae are repetitively extracted without the need for cell harvesting, dewatering, disruption, and purification of the metabolites, thereby reducing downstream processing costs. In this way, the algae do not need to grow for cellular biomass production but can act only as light-driven catalysts to convert CO2 into hydrocarbons that accumulate externally in the medium. In this work, the different lipid extraction techniques from B. braunii are discussed in depth, taking critical analysis of the effect of each process on hydrocarbon recovery and purity as well as their economic viability. Moreover, hydrocarbon composition of B. braunii and the factors influencing the hydrocarbon production is comprehensively reviewed. The milking process is critically discussed, and the cultivation protocols suitable for milking as well as strategies to improve this process are highlighted. We indicate that switchable solvent-based milking has exceptional potential and biofilm-based cultivation holds great promise to reduce downstream costs. However, more research is needed to improve their large-scale implementation and feasibility.