With petroleum reserves dwindling, the search is on to replace gasoline with a cleaner, greener alternative. Though much eco-talk has centered on ethanol from corn and biodiesel from soybeans, the biofuel that looks more likely to replace petroleum on a large scale comes from a most unlikely place: pond scum.

Algae, like corn, soybeans, sugar cane and other crops, grows via photosynthesis (meaning it absorbs carbon dioxide) and can be processed into fuel oil. However, the slimy aquatic organisms yield 30 times more energy per acre than land crops such as soybeans, according to the U.S. Department of Energy. The reason: They have a simple cellular structure, a lipid-rich composition and a rapid reproduction rate. Many algae species also can grow in saltwater and other harsh conditions -- whereas soy and corn require arable land and fresh water that will be in short supply as the world's population balloons.

"If you replaced all the diesel in the U.S. with soy biodiesel, it would take half the land mass of the U.S. to grow those soybeans," says Matt Caspari, chief executive of Aurora Biofuels, a Berkeley, Calif.-based private firm that specializes in algae oil technology. On the other hand, the Energy Department estimates that if algae fuel replaced all the petroleum fuel in the United States, it would require 15,000 square miles, which is a few thousand miles larger than Maryland.

Another bonus: Because algae can be grown just about anywhere in an enclosed space, it's being tested at several power plants across the nation as a carbon absorber. Smokestack emissions can be diverted directly into the ponds, feeding the algae while keeping greenhouse gases out of the atmosphere.

Although processing technology for algae fuel -- a.k.a. "oilgae" in some environmentalist circles -- is improving, it's still years away from reaching your local gas pump. "It's feasible; it's just a question of cost, because no large-scale facilities have been built yet," Caspari says. Boeing and Air New Zealand recently announced a joint project with a New Zealand company to develop an algae-based jet fuel, while Virgin Atlantic is looking into the technology as part of a biofuels initiative. Watch this space for updates.

-- Eviana Hartman