Scientists say they have found a way to make pond scum power your car, though it may take another couple decades to perfect. Two teams of researchers have discovered how to alter the process of photosynthesis in common green algae to produce hydrogen, a clean-burning fuel that many experts believe will be the power source of the future for cars and home generators. The discovery, announced yesterday at the annual meeting of the American Association for the Advancement of Science, could help solve one of the main problems with hydrogen: finding a way to produce it cheaply. Tasios Melis, a professor at the University of California at Berkeley, said a small pond full of algae could eventually produce enough fuel to power about a dozen cars for a month.