In 1941, Reuben, Camen and Randall of the University of California in the United States used radioactive oxygen (18O2) in photosynthesis to prove that the source of O2 released in the photosynthesis process is water. They observed the photosynthesis of Chlorella algae by identifying the radioactive isotope 18O2 in water. See the released oxygen has a radioactive isotope (H218O).
6CO2+12H218O→ C6H12O6+6H2O+618O2
Again, he observed the photosynthesis of Chlorella algae by labeling the oxygen of CO2 with the radioactive isotope 18O2. They found that sugar has a radioactive isotope of oxygen (C6H1218O6). Emitted oxygen does not contain radioactive isotopes.
6C18O2 +12H2O→ C6H1218O6+6H2O+6O2
It is clear from the experiments that water is the source of O2 released in the photosynthesis process.