When algae itself is a source of fuel, it's similarly environmentally friendly. Even if used as biodiesel feedstock or biomass—essentially any living or dead organic material that can be used as a renewable energy source—it still produces "carbon negative" energy. In other words, the net carbon it releases during energy production is less than the carbon it removes from the solar biofuel process. And, unlike other first-generation biofuels, like sunflowers or corn, made from food crops, scientists believe algae is more agriculturally-friendly. The residue left after they have been pressed for biodiesel, for instance, can come in handy as mineral-rich fertilizer.