Pond scum is a hot topic in the renewable energy world. Green (and other colors) algae can be used to produce a combustible oil. That would be renewable and green in the environmental sense. But the question remains: could it produce green profits putting algae farmers in the black financially?
One issue, the algae-into-oil industry has a problem that's parallel to water in your gas tank, on a grand scale.
Meanwhile there's research on using algae to produce hydrogen. The U.S. Department of Energy is putting some energy into that work.