Macmillan Learning In the classic experiment of Ruben, et al. in 1941, they found that when photosynthesizing Chlorella cells were supplied with COâ‚‚ (carbon dioxide made with the heavy isotope of oxygen, O), they did not observe heavy O2 as a byproduct of photosynthesis. However, when they supplied Hâ‚‚O (water made with the heavy oxygen), they did observe an increase in Oâ‚‚. These results provided primary evidence that: oxygen produced in photosynthesis comes from both carbon dioxide and water. oxygen produced in photosynthesis comes from carbon dioxide, not water. although Chlorella cells can use Hâ‚‚O, they cannot use COâ‚‚. molecular oxygen is a byproduct of photosynthesis. oxygen produced in photosynthesis comes from water, not carbon dioxide.