Few stories in the energy business are as seductive as that of algae biofuels. Using sunlight, CO2 and little else, many varieties of fast-growing pond scum, when starved of nutrients, quickly build up oil in their cells. They need no external sugar from corn or cane to grow, so they don't compete with food crops. Farmed in ponds or translucent reactors, microalgae can be raised on cheap, sun-splashed land that is unsuitable for crops or much of anything else. That was the idea, anyway, of a host of startups that launched into algae fuels over the past half decade. Often ignorant of algae's biology, these companies stumbled into major physical and engineering hurdles that can derail most of their lofty goals, industry and government experts say. Even the most promising approaches are a decade or more away, experts say. By then, many firms will have failed.