What was the contribution of Cornelius van Niel to the study of photosynthesis?

(N/A) $\Rightarrow$ $A$ milestone contribution to the understanding of photosynthesis was made by the microbiologist Cornelius van Niel ($1897$-$1985$),whose studies were based on purple and green bacteria.
$\Rightarrow$ He demonstrated that photosynthesis is essentially a light-dependent reaction,in which hydrogen from a suitable oxidizable compound reduces carbon dioxide to carbohydrates.
$\Rightarrow$ General equation: $2H_{2}A + CO_{2} \xrightarrow{\text{light}} 2A + CH_{2}O + H_{2}O$.
$\Rightarrow$ In green plants,$H_{2}O$ is the hydrogen donor and is oxidized to $O_{2}$.
$\Rightarrow$ Some organisms do not release $O_{2}$ during photosynthesis. For purple and green sulfur bacteria,$H_{2}S$ acts as the hydrogen donor.
$\Rightarrow$ The 'oxidation' product in these bacteria is sulfur or sulfate,depending on the organism,and not $O_{2}$.
$\Rightarrow$ Conclusion: He inferred that the $O_{2}$ evolved by green plants comes from $H_{2}O$ and not from carbon dioxide. This was later proven using radioisotopic techniques.
$\Rightarrow$ The correct equation representing the overall process of photosynthesis is: $6CO_{2} + 12H_{2}O \xrightarrow{\text{Light}} C_{6}H_{12}O_{6} + 6H_{2}O + 6O_{2}$.
$\Rightarrow$ Here,$C_{6}H_{12}O_{6}$ represents glucose. The $O_{2}$ released originates from water.