Q.State Van Niel's hypothesis regarding the true source of the O2 released during oxygenic photosynthesis, and name the kind of experimental evidence that later confirmed it.

[!TLDR]

Van Niel hypothesised, from comparing plant and bacterial photosynthesis, that the O2 released in green-plant photosynthesis comes from water, not CO2 -- later confirmed using isotopically labelled water (H2^18O).

[!ANSWER]

Van Niel's hypothesis: the O2 evolved in photosynthesis originates from the splitting of water, not from CO2; confirmed by isotope-labelling experiments using H2^18O, which yielded labelled ^18O2.

By comparing sulfur-bacteria photosynthesis (which uses H2S and releases sulfur, not O2) with green-plant photosynthesis, van Niel reasoned that water (or, more generally, the hydrogen donor) is the source of the released gas in both cases. This prediction, made before it could be directly tested, was later confirmed using water labelled with the heavy oxygen isotope ^18O, which was shown to yield ^18O2 as the evolved gas -- directly demonstrating that photosynthetic O2 comes from water, not from carbon dioxide.

[!ANSWER]

Van Niel's hypothesis: the O2 evolved in photosynthesis originates from the splitting of water, not from CO2; confirmed later using isotopically labelled water (H2^18O) that yielded labelled ^18O2.

Recall the comparative logic: sulfur bacteria use H2S and release S; green plants use H2O and release O2 -- the released gas comes from the electron/hydrogen donor in both cases.

Assuming the released O2 comes from CO2, since CO2 also contains oxygen -- the isotope evidence specifically rules this out.