Cornelius van Niel demonstrated that in purple and green sulphur bacteria, the hydrogen donor is H2S instead of H2O. Consequently, the oxidation product is sulphur or sulphate depending on the organism, and O2 is not evolved. In contrast, Nostoc (cyanobacteria), Cycas (gymnosperm), and Chara (green alga) perform oxygenic photosynthesis where water is the hydrogen donor and O2 is released.