A greenhouse near Aachen is dedicated not to growing plants,but to cultivating algae. Researchers use algae to consume carbon dioxide,a gas produced by coal-fired power plants. The small,green,single-cell life forms are cultured in plastic tubes filled with salt water and nutrients.As the gas bubbles through the cloudy liquid,the algae use the carbon dioxide for food and multiply. The result: reduced CO2 emissions,as well as organic matter that the chemical industry can use as valuable biofuel,free of charge.