Why Does Microalgae Biodiesel Not Work?

In recent decades, microalgae biodiesel has been repeatedly presented as a fundamental pillar of future bioenergy systems in relation to fossil diesel. This is largely due to the high photosynthetic efficiency of microalgae, their high growth rates, and their ability to accumulate neutral lipids—particularly triacylglycerols (TAGs)—which constitute the main raw materials for biodiesel production. However, this route has not yet become economically competitive with conventional fuels and vegetable oils. In this context, the simultaneous increase in biomass productivity and TAG content remains essential to reduce the cost difference, but achieving these goals depends on a detailed understanding of lipid metabolism and its regulation under different environmental and nutritional conditions—and on overcoming the intrinsic trade-offs between growth and storage. Thus, this article aims to critically analyze the viability of microalgae biodiesel, seeking to identify the main factors that explain why this route has not yet become competitive with conventional fuels after decades of research. In parallel, the growing trend of multi-product microalgae biorefineries is examined, highlighting bottlenecks in downstream processing and product purification, as well as the inherent trade-offs between production strategies. Practical limitations related to biomass productivity per area, culture dilution, intracellular lipid storage, and vital steps such as transesterification are also discussed, which together impose high energy and operational penalties throughout the production chain. Finally, emerging trends and integrated approaches are discussed, with emphasis on strain and process co-optimization, as well as greater integration between cultivation and downstream operations, aiming to enable more efficient and realistically consistent microalgae biodiesel concepts.