Algae as a source of renewable energy: opportunities, challenges, and recent developments

Sustainable production of renewable biofuels is being examined globally. First- and second-generation biofuels are not able to meet the demand. Third-generation biofuels use microalgae as the fuel feedstock and can, at least in principle, meet global demand. Microalgae can also become a carbon sink to minimize or even reverse fossil-carbon emissions. Despite their benefits, microalgae-to-fuel technologies have been stalled due to three barriers: the relatively low solar-energy-conversion efficiency of photosynthesis, substantial energy inputs for process operation, and the costs of harvesting and down-stream processing of microalgae biomass. Here, we describe the unique advantages that microalgae present based on their superior biochemical composition as fuel precursors, compared to first- and second-generation feedstocks. We review recent advancements in microalgal cultivation, harvesting, biofuel quality, and metabolic engineering. We also review non-fuel products that are promising for improving the economics of microalga cultivation via a biorefinery strategy. Taken together, these advancements lead to updated economic and environmental considerations that improve prospects for microalgal commercialization. In any case, moving microalgal-based biofuels and by-products to a large scale will require continued research and development to lower the cost of the produced feedstock and to ensure that algal biomass is acceptable for human consumption.