A method of growing photosynthetic organisms comprising providing the organisms with flue gases from a fossil-fuel power plant, the gases being previously treated by desulfurization. The carbon dioxide (CO.sub.2) concentration of the flue gases may be increased over the CO.sub.2 concentration as released from the power plant. Also disclosed is a method for producing .omega. fatty acids and bio-fuels comprising growing microalgae by providing said microalgae with flue gases from a fossil-fuel power plant.