Explanation

Cornelius van Niel (1897-1985) who based on his studies on purple and green bacteria, demonstrated that photosynthesis is essentially a light dependent reaction in which hydrogen from a suitable oxidisable compound reduces carbon dioxide to carbohydrate. This can be expressed by $2H_{2}A + CO_{2}\overset{\ \ Light\ \ }{\rightarrow}2A + CH_{2}O + H_{2}O$ In green plants, $H_{2}O$ is the hydrogen donor and is oxidised to $O_{2}$ photosynthesis. Some organism do not release $O_{2}$ during photosynthesis. When $H_{2}S$ instead is the hydrogen donor for purple and sulphur bacteria, the oxidation product is sulphur or sulphate depending on the organism and not $O_{2}$. Hence, he inferred that $O_{2}$ evolved by green plant comes from $H_{2}O$, not from carbon dioxide