In 1930 C.B Van Niel proved that, sulphur bacteria use H2S and CO2 to synthesize carbohydrates as follows:
6CO2+12H2S→C6H12O6+6H2O+12S
This led Van Niel to the postulation that in green plants, water (H2O) is utilized in place of H2S and O2 is evolved in place of sulphur. He indicated that water is electron donor in photosynthesis.
6CO2+12H2O→C6H12O6+6H2O+6O2