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Algal biofuel
Algal biofuel

Pluses: The fastest-growing plants, algae theoretically can produce 30 times more energy per acre than other biofuel options. A particularly rich mix of byproducts can be made in algal-biofuels operations (everything from nutraceuticals to feedstocks for making plastics), potentially abetting their cost-effectiveness. This is the biofuels' dark horse.

Minuses: Unlike cellulosic ethanol, the biomass for making a lot of fuel from algae doesn't yet exist; it has to be grown from scratch. Harvesting is still expensive. Cost-effectively producing algal biofuels on a large scale may be many years away.

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Last updated April 22 2008: 7:09 AM ET