It may not be as noble as soybeans or corn, but pond scum is poised to supplant both as our next great national resource, thanks to the "bioreactors" of GreenFuel Technologies. Founded six years ago by an MIT engineer who had been working to grow algae in space (the microscopic plants eat carbon dioxide), GreenFuel is now building reactors big enough to cut carbon-dioxide emissions from your typical coal-fired power plant by as much as 45%. But wait, there's more. After digesting all that carbon dioxide, the algae produce soupy "biomass," a handy source of ethanol and biodiesel. A 1,000-megawatt plant hooked up to one of the bioreactors could theoretically produce more than 100 million gallons of fuel each year. The company now operates one U.S. bioreactor, but so far, most customers are overseas. Still, CEO Cary Bullock is convinced his technology will eventually earn a high enough return from biodiesel production to make it a no-brainer. "All we're doing is what Mother Nature does," he says, "only we do it a lot faster."