The experiment material used by Van Neil, to prove that ${\mathrm{O}}_2$ comes out from water was studied on

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: $2{\mathrm{H}}_2\mathrm{A} + {\mathrm{CO}}_2 \to 2\mathrm{A} + {\mathrm{CH}}_2\mathrm{O} + {\mathrm{H}}_2\mathrm{O}$, where A is the electron acceptor. His discovery predicted that ${\mathrm{H}}_2\mathrm{O}$ is the hydrogen donor in green plant photosynthesis and is oxidized to ${\mathrm{O}}_2$.