Look at this.

This is from http://money.cnn.com/2009/03/25/smallbusiness/algae_oil_fuel.fsb/index.htm if you want to read more.

SAN DIEGO (Fortune Small Business) -- Step into the greenhouse at Sapphire Energy, a small biofuel company in San Diego, and you might expect to be accosted by rows of exotic tropical orchids or at least a few tomato plants. But the only thing growing here is algae - lots of it.
Inside 1,000-liter bags of salt water, each one bubbling with carbon dioxide, live some of the fastest-growing plants on earth: algae that have been genetically modified to produce an oil much like underground crude. If Sapphire can grow enough algae, the oil generated could replace petroleum and the organisms themselves could help save the planet by eating excess carbon dioxide in the atmosphere. Not bad for pond scum.
The concept of algae as fuel is not new. A federal energy project created in 1978 studied pond scum for nearly two decades. It examined 3,000 strains and concluded that algae with a high oil content and a taste for harsh living conditions just might help mankind kick its fossil fuel addiction.
Once popular with environmentalists, ethanol and biofuel made from vegetable oil have fallen out of favor, largely because they take more energy to make than they produce. But there's been a tiny bloom of algae startups in recent years. There are 60 of them in the U.S. today, according to the Algal Biomass Organization, 50 more than existed five years ago.
"All of them are small companies," says Thomas Byrne, the industry group's secretary. "And many right now just have a concept."

This is a really good idea to replacing fossile fuels, eh?