By studying purple sulphur bacteria and green sulphur bacteria, Cornelius Van Niel was the first scientist to demonstrate that photosynthesis is a light-dependent redox reaction in 1931, in which hydrogen from an oxidizable compound reduces carbon dioxide to cellular materials. This can be expressed as: 2 H2A + CO2 $\to$ 2A + CH2O + H2O, where A is the electron acceptor. His discovery predicted that H2O is the hydrogen donor in green plant photosynthesis and is oxidized to O2.