Algae have been called grass of the waters and lungs of the earth. they are among the smallest and the largest organisms on earth. They inhabit the most extremes of temperatures and are present in such vast numbers that they harvest more energy from sunlight than all the world's plants. Products derived from algae have many uses. But now scientists around the world are researching new ways of exploiting algae to reduce water pollution, to recycle human waste on board spacecraft, and even to produce petroleum.