A biomass-based fuel needs to be cheap and energy dense. Gasoline sets a high standard.

http://www.nature.com/nature/journal/v474/n7352_supp/full/474S09a.html

The green slime that covers ponds is an efficient factory for turning sunlight into fuel, but growing it on an industrial scale will take ingenuity.

http://www.nature.com/nature/journal/v474/n7352_supp/full/474S015a.html