Some organisms like green sulphur bacteria do not release oxygen during the process of photosynthesis. In purple and green sulphur bacteria, the hydrogen sulphide is the hydrogen donor, instead of water (which is usually the hydrogen donor in green plants) and sulphur or sulphate is the oxidation product depending on the organism performing photosynthesis and not on the oxygen. This also proves that oxygen that evolves from the green plants comes from water and not carbon dioxide.