Green‐lighting biofuels from microalgal lipids: A review of strategies to integrate stress into continuous cultures

Abstract The vision of using microalgae to obtain organic chemicals in a carbon‐neutral manner is intriguing. It promises to produce various substances including biofuels by photosynthesis, without competing with agricultural land. Processes have been established whereby microalgae undergo a growth phase followed by a stress phase, which induces lipid synthesis (two‐phase‐cultivation). The neutral lipid triacylglycerol is finally extracted to produce biodiesel. However, improvements to the microalgal culture and subsequent processes are needed to allow large‐scale biofuel production competitive to mineral oil fuels. Here, we recapitulate approaches from current literature studies which increase the process efficiency by converting the classical two‐phase batch into a continuous cultivation. Three process categories were identified and evaluated, with a focus on productivity, applicability, and research gaps. The first approach makes use of continuous transfer between two separate reactors for biomass and lipid production, where a higher equipment demand is compensated for by maximal productivity. In the second approach, a trade‐off is generated by applying relatively low stress conditions in a single bioreactor, allowing simultaneous cell growth and lipid production, albeit at a lower productivity. The third approach is a repeated‐batch variant, that alternates between growth phases to high cell density and stress phases, followed by a partial harvest and cell recovery. This yields a high product concentration using a single reactor. A direct comparison between these three approaches allows selection of the most productive strategy for future microalgal processes, depending on requirements and application field.